The Raven Kernel: a Microkernel for Shared Memory

The Raven Kernel: a Microkernel for Shared Memory
The Raven Kernel: a Microkernel for Shared Memory Multiprocessors 1
Duncan Stuart Ritchie
[email protected]
Department of Computer Science
University of British Columbia
Vancouver, B.C., Canada V6T 1Z2
Technical Report TR 93-36
April 30, 1993
The Raven kernel is a small, lightweight operating system for shared memory multiprocessors. Raven is characterized by its movement of several traditional kernel abstractions into user
space. The kernel itself implements tasks, virtual memory management, and low level exception dispatching. All thread management, device drivers, and message passing functions are
implemented completely in user space. This movement of typical kernel-level abstractions into
user space can drastically reduce the overall number of user/kernel interactions for ne-grained
parallel applications.
This work was made possible by grants from the Natural Sciences and Engineering Research Council of
Canada.
1
1
Table of Contents
List of Tables
5
List of Figures
6
1 Introduction
1
1.1 Motivation : : : : : :
1.2 Raven kernel overview
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2 Run-time environment overview
2.1 Hardware Overview :
2.2 Software environment
5
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3 Kernel level implementation
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
3.9
3.10
3.11
3.12
2
3
5
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The interrupt model : : : : : : : : : : : :
Low level mutual exclusion : : : : : : : :
List and Queue Management : : : : : : :
Low level console input/output : : : : : :
Memory management : : : : : : : : : : :
Physical memory management : : : : : :
Memory mapping and cache management
Virtual memory management : : : : : : :
Task management : : : : : : : : : : : : :
Kernel management for global semaphores
Interrupt management : : : : : : : : : : :
User/Kernel Shared memory regions : : :
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4 User level kernel implementation
4.1
4.2
4.3
4.4
4.5
4.6
4.7
Upcall handling : : : : : : : : : : :
User level spin-locks : : : : : : : :
Thread management : : : : : : : :
Semaphore management : : : : : :
Interprocess communication : : : :
The global nameserver : : : : : : :
User/User Shared memory regions
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5 Performance Evaluation
5.1
5.2
5.3
5.4
5.5
5.6
5.7
5.8
123
Benchmark tools : : : : : : : : : : : : : :
Function calls vs. System calls : : : : : :
Thread management performance : : : : :
Interrupt handling performance : : : : : :
Task signalling performance : : : : : : : :
Interprocess communication performance :
Memory management performance : : : :
Ethernet driver performance : : : : : : : :
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6 Related Work
137
6.1 Low-level mutual exclusion
6.2 Threads : : : : : : : : : : :
6.3 Interprocess communication
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7 Conclusion
7.1 Summary : :
7.2 Future Work
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Appendices
148
3
A Kernel system call interface
148
A.1 System calls provided to the user level kernel :
A.2 System calls provided for application programs
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B User kernel library call interface
B.1
B.2
B.3
B.4
B.5
B.6
B.7
151
Thread management : : : : : : : : : : : : :
Synchronization primitives : : : : : : : : : :
Asynchronous Send/Receive port IPC : : :
Synchronous Send/Receive/Reply port IPC
Nameserver : : : : : : : : : : : : : : : : : :
User level memory management : : : : : : :
Interrupt and exception management : : : :
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C Unix version
154
Bibliography
156
4
List of Tables
2.1 GNU C computer and kernel register usage.
5.2
5.3
5.4
5.5
5.6
5.7
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4 parameter user level function call vs. kernel system call. : : : : : : : : : : : : : 125
Thread management performance. : : : : : : : : : : : : : : : : : : : : : : : : : : 125
Interrupt service routine invocation latencies. : : : : : : : : : : : : : : : : : : : : 128
Task signalling invocation latencies. : : : : : : : : : : : : : : : : : : : : : : : : : 130
Performance of asynchronous and synchronous local ports, 4 byte data message. 132
IPC performance for asynchronous and synchronous global ports, 4 byte data
message. : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 133
5.8 IPC primitive breakdown. : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 134
5.9 Virtual memory operation execution times. : : : : : : : : : : : : : : : : : : : : : 135
5.10 Time to transfer 10MB of data over Ethernet. : : : : : : : : : : : : : : : : : : : : 136
5
List of Figures
1.1 High level system organization.
: : : : : : : : : : : : : : : : : : : : : : : : : : : :
2.2 88200 CMMU address translation table format. : : : : : : : : : : : : : : : : : :
2.3 Hypermodule physical address map, showing DRAM, VMEbus, and device utility
space. : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :
3.4
3.5
3.6
3.7
3.8
3.9
3.10
3.11
3.12
3.13
3.14
3.15
3.16
3.17
3.18
Kernel source code organization. : : : : : : : :
kprint module system call summary. : : : : : :
Memory management system decomposition. :
Virtual memory system call summary. : : : : :
Typical user level memory space. : : : : : : : :
Initialized supervisor memory space. : : : : : :
task.c module system call summary. : : : : : :
Destroying a remote task. : : : : : : : : : : : :
Task state transitions. : : : : : : : : : : : : : :
Task ready queue structure. : : : : : : : : : : :
Kernel level global semaphore support routines.
Interrupt handler registration system calls. : :
Register r28 usage. : : : : : : : : : : : : : : : :
Kernel interrupt dispatching process. : : : : : :
User/Kernel shared memory regions. : : : : : :
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4.19 User level kernel source code organization. : : : : : : : : : : :
4.20 User level upcall dispatching routines. : : : : : : : : : : : : :
4.21 Summary of upcall events and the user level upcall handlers.
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95
Chapter 1
Introduction
One of the goals of a multitasking operating system is to present the illusion of parallelism to
users. To the naked eye, programs running in such an environment appear to run concurrently.
At a conceptual level, the system can use the notion of threads and processes to present a parallel
model for constructing programs. On uniprocessor hardware, the parallelism seen in these two
cases is indeed illusion. It is the relative speed of microprocessors and operating system design
techniques such as time-slicing and cooperative scheduling that make this illusion possible.
The recent proliferation of low-cost multiprocessor hardware has made it possible to change
this illusion into reality. The parallelism that a user or application developer sees is no longer
an illusion { it is real. With multiple processors, separate threads of execution can execute
concurrently. While the amount of true parallelism is bounded by the number of processors in
the system, the illusion of a general purpose parallel environment can be enhanced by using
uniprocessor techniques such as time-slicing and cooperative scheduling.
However, while scheduling techniques can be borrowed from uniprocessors and applied to
multiprocessors, their implementation cannot. Traditional uniprocessor operating systems often
lack desirable features that are possible with parallel hardware. Also, they can be dicult
to adapt to a multiprocessor environment, leading to inecient operation [JAG86]. There
are two main factors which contribute to inecient operation: the increased requirement for
concurrency control within the kernel; and new scheduling possibilities.
This report presents a new operating system kernel which addresses the above factors for
a shared memory multiprocessing environment. This design is geared specically towards uniformly shared memory architectures, and not non-uniform architectures (NUMA) or distributed
1
Chapter 1. Introduction
2
memory architectures.
This report is organized as follows. The remainder of this introduction chapter introduces
the reader to motivation of this work, and describes the overall design of the kernel. Chapter
2 describes the hardware and software run-time environments. Chapter 3 and 4 discuss the
implementation of the supervisor kernel and user level kernel, respectively. Chapter 5 presents
performance gures for the various kernel services. Chapter 6 provides a look at previous
work that inuenced the design of the system. Finally, chapter 7 concludes the report with a
summary of future work.
1.1 Motivation
The availability of low cost multiprocessor hardware has opened up new avenues for providing
higher performance programming environments for applications. However, the availability of
general purpose operating system software to take advantage of new techniques is slow to
appear.
Traditional microkernel architectures, such as Mach 3.0 and Chorus, are designed with
the view that the kernel should only provide a minimum set of primitive abstractions (tasks,
threads, virtual memory, device management, etc). These primitives are then used by user
space modules to provide additional services to the operating system environment, such as
networking protocols and le servers.
When used as general purpose computing environments, this architecture is sucient. Robustness is the most important requirement, with performance being second on the list. The
system is secure against malicious or errant user programs, and performance is adequate.
Recently, however, the speed of network and other input/output devices has increased by
an order of magnitude or more. The traditional kernel mediated operating system now exposes
its performance problems more than ever. For many applications running under such an environment, the cost of crossing user/kernel boundaries for each primitive abstraction becomes a
concern. Typical system call overhead is likely 10 times greater than a procedure call [ALBL91].
Chapter 1. Introduction
3
By moving several of the high-use kernel services into user space, less time is spent invoking
operations. The general motivation is to reduce the overall number of user/kernel interactions.
Several techniques can be employed to do this:
User level thread scheduling. Rather scheduling threads in the kernel, move the scheduling
code into the user space.
User level interrupt handling. Allow interrupt handlers to upcall directly into the user
space. Device drivers can be implemented completely in user space, eliminating the costs
of moving data between the user and kernel.
User level interprocess communication. By making extensive use of shared memory between client and server address spaces, data copying through the kernel is eliminated.
Low level synchronization primitives. Provide a simple mechanism to allow an event to be
passed from one address space to another. With appropriate hardware, remote processor
interrupts can be implemented completely at the user level.
1.2 Raven kernel overview
The Raven kernel is a small, lightweight microkernel operating system for shared memory multiprocessors. The kernel executable compiles into less than 32KB of code. The most intriguing
part of the Raven kernel design is the dislocation of many traditional kernel services into user
space.
Two main abstractions are provided by the kernel: tasks and virtual memory. All other
services are provided by the user level: threads, semaphore synchronization, interprocess communication, and device management1. Extensive use of shared memory allows disjoint address
spaces to eciently communicate scheduling information and interprocess communication data.
Figure 1.1 demonstrates the supervisor kernel in relation with the user level.
1
The supervisor kernel initially handles interrupts, but they can be dispatched to the user level.
Chapter 1. Introduction
4
User program
standard C
library
thread
scheduling
Interprocess
communication
upcall and interupt handling
kernel upcall
events
system
calls
Task management
and scheduler
shared
memory region
Memory
management
Exception and interrupt handling
Figure 1.1: High level system organization.
The system is implemented for a four processor, Motorola 88100 shared memory multiprocessor [Gro90]. A special gdb-based kernel level debugger, known as g88 [Bed90], allows the
development environment to be hosted under a friendly Unix account, and provides downloading
features to run programs.
Several sample applications have been developed to demonstrate the kernel operation. A
user level serial port device driver implements a le server and terminal tty server to a Unix
host. An ethernet device driver demonstrates network data throughput through the kernel IPC
services.
Chapter 2
Run-time environment overview
The main goal for an operating system is to provide a reliable and convenient work environment.
Hidden beneath the operating system is the raw physical hardware, an environment far too
exacting for higher level users to deal with. Just as users are given a run-time environment
to work in, the operating system has its own run-time environment: main memory, exceptions
and interrupts, and the various devices that make up a general purpose computer. As well, the
operating system uses tools such as compilers and debuggers. This chapter provides an overall
view of the hardware and software environment available to us. The rst part of this chapter
provides an overview of the current hardware platform. The second part then describes the low
level software environment and development tools.
In the spirit of eciency and simplicity, the Raven kernel makes assumptions about the
hardware architecture model: the kernel is designed for shared memory multiprocessor platforms, where each processor has the same view of memory and devices. Non-uniform memory
access (NUMA) machines and distributed memory machines require dierent considerations in
terms of memory management, scheduling, and input/output. Some systems, such as Mach
[ABB+ 86], provide hardware compatibility layers to isolate porting details, and this can alleviate porting to dierent classes of hardware architectures. But this generality has its costs in
terms of additional code size and complexity.
2.1 Hardware Overview
The hardware platform used for the implementation of the kernel is known as the Motorola
MVME188 Hypermodule [Gro90]. The Hypermodule is a general purpose shared memory
5
Chapter 2. Run-time environment overview
6
multiprocessor, based on Motorola's 88100 RISC architecture [Mot88a]. The Hypermodule
contains four 88100 processors, each with a uniform view of 32MB of shared memory. In
addition, the Hypermodule contains many devices such as timers, interrupt management, a
VMEbus controller, and serial ports to make up a general purpose computer. This section will
examine many of the relevant aspects to this hardware, as it pertains to the kernel.
2.1.1 The MC88100 RISC Microprocessor
Each of the four 88100 processors runs at a clock speed of 25MHz. Most instructions execute
in a single clock cycle. Using four processors, the Hypermodule can theoretically achieve a
performance rating of about 60 MIPS1 .
The instruction set is typical of many 32-bit RISC microprocessors: a simple set of single
cycle instructions used to build higher level constructs. All instructions are represented in
memory as 32-bit words, simplifying the decoding phase. A delayed branching feature can be
used to alleviate pipeline stalls when executing branch instructions.
There are 32 general purpose user level registers, r0 | r31, each 32-bits wide. Register
r0 is read-only and always holds a value of zero. In supervisor mode, the 88100 contains 21
additional control registers: cr0 to cr20. These registers reect the state of the processor mode,
data unit pipeline, integer unit pipeline, and general purpose scratch registers. The oating
point execution unit contains 11 more registers: fcr0 to fcr8, and user registers fpcr62 and
fpcr63.
2.1.2 The MC88200 Cache/Memory Management Unit
The 88200 CMMU augments the 88100 processor by providing instruction and data caches,
as well as adding memory management. Each processor on the MVME188 uses two 88200's,
giving each CPU 16KB of code and 16KB of data cache. A memory bus snooping protocol
allows cache coherency between all caches in the system.
1
VAX MIPS
Chapter 2. Run-time environment overview
7
The memory management section implements a two level page table scheme with a page
granularity of 4096 bytes. Figure 2.2 shows the layout of this page table scheme. Up to 4GB
can be mapped to a single address space. Each page in that address space can have various
combinations of the following attributes: no translation, cache-disable, writethrough, write
inhibit (for read-only pages), and snoop enable. The detailed settings for these attribute bits
can be found in the 88200 technical documentation.
Map descriptor
Segment table
Page table 0
Physical
Memory
segment 0
page 0
4K page
segment 1
page 1
4K page
segment 2
page 2
4K page
.
.
.
.
.
.
segment 1023
page 1023
4K page
Page table 1023
.
.
.
.
page 0
4K page
page 1
4K page
page 2
4K page
.
.
.
.
page 1023
4K page
Figure 2.2: 88200 CMMU address translation table format.
Access to the 88200 registers for programming is done through the Hypermodule utility
space (discussed below). Each of the 88200's in the system can be accessed separately, allowing
cache ushes and page table management to be performed by the host processors.
2.1.3 System Controller
In addition to the processor, CMMU's and memory, the Hypermodule has a system controller
board that contains all of the additional functionality and glue that make up a general purpose
computer. Components such as timers, serial ports, and the VMEbus controller are found on
this system controller board.
Chapter 2. Run-time environment overview
8
The system controller board maps all of its devices and memory into the processor's physical
address space. To access a device on the controller board, the processor executes load/store
operations to the memory-mapped device registers. Other devices and resources can be added
to the physical address space through the VMEbus interface.
Figure 2.3 shows the default physical address space that the Hypermodule resources reside
in. The remainder of this section will be devoted to examining each portion of this address
space, and noting the features pertinent to kernel operation. For additional detail, consult the
hardware manuals [Gro90] and [Mot88a].
VMEbus 16-bit short I/O
0xffff0000
4MB
utility
address
space
MVME188 device and CMMU
registers
0xfff88030
0xfff00000
128Kbyte SRAM (tek188mon)
0xffe20000
0xffe00000
512Kbyte EPROM (188Bug)
0xffc80000
0xffc00000
VMEbus 32-bit address space
0x02000000
32MB
physical
address
space
DRAM memory (60 ns)
188Bug local storage
0x00010000
0x00000000
Figure 2.3: Hypermodule physical address map, showing DRAM, VMEbus, and device utility
space.
Chapter 2. Run-time environment overview
9
Utility address space
All onboard devices and resources are located in an upper region of memory called the utility
space. The utility space address map spans a 4MB region from 0xffc00000 to 0xffffffff.
Each resource within the utility space are given their own section of address space to reside in,
padded by null memory space.
Access to the utility space by the operating system kernel is a necessity. While the default
base address of the utility space can be modied by switching jumpers, it cannot be made to
go away completely. In fact, the address translation hardware of the 88200's contain hardwired
entries to always make the utility space available in the supervisor memory map. This is done
so that the operating system can nd the utility space in the event of translation table errors
or software malfunction.
The following devices are made available inside the utility address space. Some of these
devices are shared by user level memory maps. For instance, in order to implement a user level
serial port driver, the DUART registers must be mapped in user space.
MC68681 DUART/Timer
The MC68681 provides the system software with two RS-232C compatible serial ports and a
programmable interval timer. The serial port speeds can be programmed to support varying
data rates, but also support 18 preprogrammed rates from 50bps to 38.4kbps. The kernel uses
one serial port in polled mode for low level debugging support, and another interrupt driven
port to connect with attached terminals or Unix hosts.
The kernel also uses this chip's interval timer. Using the 3.6864MHz clock on the DUART,
an interrupt can be generated with a period anywhere from 540 nsec to 563 msec. The kernel
allows this period to be congured at boot time, in terms of ticks per second. This value is
then used as the system clock tick.
Chapter 2. Run-time environment overview
10
Z8536 Counter/Timer
In addition to the interval timer provided by the MC68681, the Z8536 Counter/Timer device
serves as an enhanced timer service. The Z8536 contains three individual 16-bit timers, each
using a clock rate of 4MHz. The timers can be programmed separately, or cascaded together to
provide higher resolutions. The kernel cascades two of these timers, allowing intervals from 1
usec to 4295 sec (71 minutes) to be timed with 1 usec accuracy. This resolution is good enough
to benchmark relatively small sections of code with reasonable accuracy.
The third timer on the Z8536 can be enabled as a watchdog timer for the Hypermodule
board. When programmed to do so, the timer will trigger a reset sequence which reboots the
machine. This feature allows the system to self-recover from fatal system crashes.
MK48T02 Time-of-Day Clock
As if two timers were not enough, the Hypermodule also contains a time-of-day clock. This
clock can be used to maintain the correct wall clock time (it is battery backed and will correctly
maintain the time when power is o). Currently, the kernel doesn't use the notion of wall clock
time anywhere, so this device is not used.
MVME6000 VMEbus Controller
Access to VMEbus peripherals is provided by the MVME6000 \VMEchip" controller [Mot88d].
This device manages the interface between the Hypermodule memory bus (master) and the
VMEbus, and the attached devices (slaves). It allows regions of the VMEbus address spaces
to be mapped into the Hypermodule's local physical address space. For example, the Ethernet
device driver uses the VMEchip to map in the Ethernet board's device registers at location
0x10000000.
In addition to providing access to slave devices, the VMEchip contains features that facilitate
operation in a multiprocessor environment. If more than one Hypermodule board is installed
on the VMEbus, the VMEchip on each board can be used for coordination and synchronization
Chapter 2. Run-time environment overview
11
services.
128KB SRAM
The Hypermodule board contains 128KB of non-volatile (battery backed) SRAM, which can
be used to retain data and program instructions while the power is turned o.
Currently, the kernel uses this section of memory to store the g88 debug monitor. The
debug monitor code rarely changes, so once it is downloaded into the SRAM, it is there for
good. The next time the Hypermodule is started, the debugger checksums the monitor area to
see if it is complete. If the monitor checksum passes, there is no need to download a fresh copy
(which can take a while at serial-port speed).
512KB EPROM and 188BUG Debugger
An EPROM chip module contains a standalone, onboard monitor/debugger known as 188Bug
[Mot88c]. This is a low level interactive debugger which can be accessed via the serial port using
a dumb terminal. The debugger is non-symbolic but full featured, with the ability to perform
raw operations on disk, tape and serial port devices. This allows code such as operating systems
to be easily bootstrapped from disk, tape, or serial port.
In addition to the software debugging features, a full suite of diagnostics are provided to
test and exercise the hardware.
Since 188BUG is non-symbolic, it can be dicult to debug programs written in high level
languages like C. Instead, the kernel uses a gdb-based debugger called g88 to run and test the
system. g88 allows kernel code to be downloaded and interactively debugged using the familiar
gdb environment. Breakpoints can be set, data examined, just like debugging a regular user
level process under Unix.
Chapter 2. Run-time environment overview
12
DRAM address space
The Hypermodule can be congured to hold up to 64MB of 60ns DRAM, in 16MB increments.
All of this physical memory is symmetrically visible and available to all processors in the system.
Peak memory bandwidth is 44.4MB/sec for reads, and 66.7MB/sec for writes.
The onboard debugger, 188Bug, reserves the lowest 64KB region for its own use (exception
table, local variables, code, and stack). This region must be preserved for the 188Big to operate
properly. Sometimes 188Bug is useful, so the kernel doesn't intrude on its territory. Useable
memory begins at 0x10000 and grows upwards.
VMEbus address space
The Hypermodule VMEbus chipset supports 32-, 24-, and 16-bit VMEbus addressing modes.
The 16-bit VMEbus SHORTIO space is hardwired to live in the upper 64KB region of memory,
so that master and slave devices always know how to locate this region. All other addresses
within the physical address space, between the end of DRAM and the beginning of utility space,
can be used for mapping in portions of the 32-bit and 24-bit VMEbus address spaces.
As a convention, the kernel uses 32-bit VMEbus address mapping for external devices.
Currently only the Ethernet board is mapped in this fashion, so this convention is certain to
change for other kinds of devices.
2.1.4 Interrupt management
In many earlier multiprocessor systems, one special processor was designated as the I/O processor. Doing so would help simplify an already complex memory and interrupt bus. This special
processor is specically wired to accept all interrupts from external devices. While this does
leave other processors to do useful work for non-I/O bound operations, a processor that requires I/O must always go through the special I/O processor. This can introduce a throughput
bottleneck in the system, and can complicate I/O and device driver code.
The Hypermodule interrupt management scheme is fully symmetric. Any processor in the
Chapter 2. Run-time environment overview
13
system can respond to any particular interrupt by setting appropriate bits in the processor's
interrupt enable register (IEN). In a system with four processors, there are 4 interrupt enable
registers named IEN0 to IEN3.
The hardware supports up to 32 dierent interrupt sources. Each bit in the IEN registers
corresponds to one of the possible interrupt sources. Some of the common interrupt sources are:
DUART timer, DUART serial port, VMEbus IRQ0 { IRQ7, and software interrupts (SWI).
Individual interrupt sources can be set to occur on any combination of the processors in
the system. It is possible to congure the IEN registers such that multiple processors receive
the same interrupt. In this case, all interrupted processors will halt execution and branch to
the interrupt handler when the particular interrupt occurs. This could generate a great deal
of contention if spin-locks are used within the interrupt handler. Therefore, certain interrupts
should be enabled on only a single processor at a time. The kernel interrupt architecture
manages the setting of the interrupt enable bits to minimize latency to interrupt handlers.
For example, the Ethernet device driver is a user level task that requires to be executed
every time an interrupt is generated on the Ethernet board. Switching in the task can involve
cache ushes and address space changes and is therefore an expensive operation. This task
switch is avoided if by setting Ethernet interrupt enable bit properly on the processor which
is currently executing the Ethernet task. Doing so localizes the interrupt to a single processor
that has the correct address space activated.
If there is more than one processor allocated to an interrupt driver task, only one processor
at a time has the device interrupt enable bit set. Which processor it is, is determined by the
kernel. If the interruptable processor is relinquished, another processor allocated to the device
task is assigned the interrupt bit.
When a processor receives an interrupt exception, one of the rst things it needs to do is nd
out exactly which device caused the exception. The interrupt status register (IST) allows this to
be easily accomplished. The IST is bit-for-bit similar to the IEN registers, except that it reects
the status of all interrupts in the system. If a bit is set in the IST, then the corresponding
Chapter 2. Run-time environment overview
Register
r0
r1
r2 { r9
r10 { r13
r14 { r25
r26
r27
r28
r29
r30
r31
14
Compiler and system usage
Read-only 0 constant
Function call return address
Function call parameters
Function temporary registers
Function preserved registers
Temporary scratch register
Temporary scratch register
Locking and upcall status bits
User thread context save pointer
Stack frame pointer
Stack pointer
Table 2.1: GNU C computer and kernel register usage.
device is requesting an interrupt.
Note that there is only one IST, and not one for each processor. The IST gives the global
status of all interrupting devices, and the IEN registers give the enable mask for each processor.
So to check whether a particular processor received a particular interrupt, the processor must
AND the IST its IEN register: status = ien reg[cpu] & ist reg.
2.2 Software environment
In the hardware overview section, various features of the hardware runtime environment were
discussed. In this section, the software development environment is examined. This environment includes the g88 debugger and simulator, compiler tools, and software conventions such
as register usage. Other factors which inuence the runtime environment are the processor
supervisor/user state settings, and supervisor register conventions.
2.2.1 The gcc compiler and tools
The C compiler used is the ANSI-compliant GNU gcc version 1.37.29. All other tools used for
compiling and linking executable code, such as the linker and assembler, are also GNU tools.
Chapter 2. Run-time environment overview
15
The C compiler uses a standard format for processor register allocation. Table 2.1 shows
this register convention. Functions can always rely on registers r10 { r13 to be available for
temporary scratch purposes. Registers r14 { r25 can also be used for general purpose storage,
but they must be preserved by the called function if they are to be used. The C stack frame
contains the frame pointer, saved registers of the previous function, function return address, and
local variable storage for the function. This information is useful when interfacing C programs
with handcoded assembler.
2.2.2 The g88 kernel debugger
The g88 cross-debugger/simulator [Bed90] is a GNU gdb [Sta89] based kernel debugger. Implemented as an extension to gdb 3.2, g88 allows Hypermodule users to download code and
interactively debug their programs from within a standard gdb terminal session. g88 can set
breakpoints, step through code, examine variables, etc. { just about everything one expects
from a Unix version of gdb.
g88 runs on a Unix workstation and connects to the Hypermodule hardware via two serial
ports. One serial port is used for the gdb command protocol, console input/output and downloading code. The other is used as a software controlled interrupt and reset line. This line is
connected to the Hypermodule reset and interrupt logic, and is used to reboot and interrupt
the target. For example, when control-C is pressed within g88, the interrupt logic is toggled,
generating an ABRT interrupt on the Hypermodule. The onboard g88 monitor program recognizes this interrupt, suspends kernel execution, and returns control back to the g88. This allows
users to interactively debug their code running on the Hypermodule, with the same control as
if it were a regular Unix process.
When starting a session, g88 downloads a monitor program to the Hypermodule memory.
This monitor, known as g88mon, handles the g88 serial port communication protocol between
the Unix host and Hypermodule. It manages all the gdb features such as breakpoints, single
Chapter 2. Run-time environment overview
16
stepping and memory examination. The monitor is designed and implemented to be as unobtrusive as possible. It is downloaded into a portion of the Hypermodule non-volatile SRAM,
and resides there until it is erased. The program being debugged does not have any knowledge
of g88mon existence in the system.
2.2.3 The g88 Hypermodule simulator
g88 also contains a complete instruction level simulator of the MVME188 Hypermodule. The
simulator provides a virtual environment that is a clone of the Hypermodule: four 88100 processors, eight 88200 CMMUs, and all the system controller devices. An environment variable
controls the size of the simulated physical memory size. Programs running in the simulator are
virtually oblivious to the fact that they are running in a simulated environment. g88 provides
the customary gdb interface to the simulator, so programs can be downloaded, executed, and
debugged.
When the simulator is used to execute something, no physical serial links are necessary
because g88 simulates the hardware directly in its address space on the Unix host. The overhead
associated with sending commands across the serial link is eliminated. Thus the downloading
of code and overall communication with the \physical" hardware is much faster. This property
makes the simulator ideal for debugging and development, where a fast edit/compile/test cycle
is desirable. It takes about 60 seconds to download the Raven kernel to the real hardware,
compared to a fraction of a second for the simulated hardware. Similarly, overall debugging
commands on the simulator are much faster.
Raw execution speed of the simulator is of course much slower than the real hardware (about
100 times slower on a SPARC 1+). However this does not impact the usability of the Raven
kernel on the simulator for debugging purposes. The kernel itself and most user level programs
at this point are not compute bound, so most executions are fast enough.
The current version of g88 has one major limitation: it can only be used to debug supervisor
code. This means that g88 cannot debug user level programs. This limitation stems from the
Chapter 2. Run-time environment overview
17
fact that gdb 3.2 is only able to manage one execution context (i.e., one program) at a time.
However, g88 is still under development by its author, in cooperation with Horizon Research,
for Mach 3.0 work. An upgrade to g88 which will allow user level debugging and performance
improvements will soon be available.
Chapter 3
Kernel level implementation
The Raven kernel is split into two distinct entities: the supervisor kernel, and the user level
kernel. Each of these entities is implemented as separate executable programs. This chapter
discusses the design and implementation of the supervisor kernel part.
Task scheduling
management
Memory
management
Miscellaneous
task.c
vm.c
kprint.c
task_timer.c
map.c
loader.c
queue.h
readyq.c
rawpage.c
lock.s
interrupt.c
exception.c
Exception handling
and dispatching
intr_handler.s
excp_handler.s
excp_vects.s
Figure 3.4: Kernel source code organization.
Figure 3.4 shows the modular breakdown of the system, and the source code les involved.
The rst section in this chapter discusses the overall programming model of the kernel, and the
following sections describe the code modules in detail.
Several of the kernel modules export a system call interface to the user level. These system
18
Chapter 3. Kernel level implementation
19
calls are broken into two categories: calls available for general purpose user applications, and
calls available for user level kernels only. For modules which export such a system call interface,
each description in this chapter will contain a table summarizing the interface provided by that
module.
System calls, and internal kernel functions, often return a success or failure condition to
the caller. Throughout the implementation of the kernel, the following convention is used
for function return values. Functions that complete successfully always return the value OK.
Functions that fail in some manner during their invocation return the value FAILED.
3.1 The interrupt model
The implementation of the kernel is based on the interrupt model, as opposed to the process
model. In the process model, a kernel is composed of several cooperating processes, each of
which has their own stack and local state variables. Interrupts and preemption are normally
allowed during execution. The processes must be scheduled by the kernel, and special cases for
preemption locking and concurrency locking must be explicitly coded.
In the interrupt model, the kernel can be viewed as one big interrupt handler. All kernel
invocations, including system calls, device interrupts, and exceptions, enter into the kernel
though the exception vector table, excp vects.s. From this point, the low level exception
handler routines allocate a stack for the processor and dispatches the event.
Any number of processors can be executing within the kernel at the same time. While a
processor is executing within the kernel, interrupts and preemption is disabled for that processor. This simplies the implementation quite substantially, because there is no need for special
purpose preemption locking and protection. All calls to the kernel are non-blocking: except for
the processor relinquishment call, all kernel calls return immediately to the user after executing.
When control returns to user space, interrupts and preemption is re-enabled.
Each processor in the system uses its own dedicated kernel stack. In a four processor
system, there are four dedicated kernel stacks. The kernel stacks are located at well-known xed
Chapter 3. Kernel level implementation
20
locations in the supervisor memory space, and are allocated at very beginning of initialization
time. Each time the kernel is invoked, the processor stack pointer is set to the top of its kernel
stack. The system can assume that there is only one execution context per processor while
running inside the kernel. This substantially simplies implementation issues.
3.2 Low level mutual exclusion
In a shared memory parallel environment like the Hypermodule, many algorithms require that
sections of their code have atomicity. These critical sections of code must be executed atomically
in mutual exclusion with their neighbours, or risk leaving their state inconsistent. In the kernel,
these algorithms range from simple enqueue/dequeue operations on a shared queue, to ensuring
sequential access to device registers.
Several techniques exist to ensure mutual exclusion. One technique is to avoid critical
sections altogether by implementing data structures as lock-free objects [Ber91] and optimistic
synchronization [MP89]. In this case, a compare-and-swap operation allows data structures to
be concurrently accessed with consistency. However, the lack of proper hardware support for
compare-and-swap on the Hypermodule hardware does not make this algorithm practical.
Other techniques range from the simple spin-lock to the higher level semaphore. The latter
technique relies on the operating system to \schedule around" critical hot spots by relinquishing
the processor to another thread when such a spot is reached. This technique requires some
cooperation with the operating system, commonly in the form of semaphore data structures,
which in themselves require mutual exclusion. Hence the requirement for a more primitive
mutual exclusion technique.
3.2.1 Spin locks
The spin lock is a brute force method of providing mutual exclusion around sections of code.
The algorithm tests a shared lock variable to see if the lock is free or used. If the lock is free, the
lock is claimed by setting its state to \locked", and execution continues. If the lock is not free,
Chapter 3. Kernel level implementation
21
continually check until it is free. A lock is freed by storing a \free" value to the lock variable.
While waiting for a lock to become free, the processor cannot do anything else. In a system
with many processors, this property can become a bottleneck when frequently accessing a shared
resource.
3.2.2 lock wait() implementation
On many processors, a test-and-set instruction is used to ensure atomicity of the lock variable
test and set stage. The test stage is broken into two parts: one instruction loads the value of the
lock variable, another instruction tests its value. If the value of the lock variable is altered after
the load instruction but before the test, the algorithm will fail. To prevent this problem, the
88100 instruction set includes the xmem instruction. The xmem instruction atomically exchanges
the contents of a register with the contents of a memory location. The load and store accesses
of the xmem instruction are indivisible: the instruction cannot be interrupted part-way through
its execution.
Using the atomic exchange instruction, the lock acquire routine can be safely implemented
in the following manner:
_lock_wait:
or
lw: xmem
bcnd
jmp
r10, r0, 1
r10, r2, r0
ne0, r10, lw
r1
;
;
;
;
;
r2 <- lock variable addr
set a lock value
atomic exchange
try lock again if not zero.
return to caller
begins by putting a \locked" value into register r10. This register is then
exchanged with the lock variable stored in memory at the address r2. As a result of the
exchange, r10 is loaded with the previous lock value. If this value is non-zero, then the bcnd
instruction branches to the top of the routine, where the test starts again. This sequence is
repeated until a \free" value is found in the lock value. The memory bus transactions generated
by the repeated accesses to the lock variable can quickly saturate the memory bus, hindering
other processors in the system from doing useful work.
lock wait()
Chapter 3. Kernel level implementation
22
An optimization can be achieved by relying on the data cache to maintain a coherent copy
of the lock variable. In this case, initially the processor spins on a cached copy of the lock,
generating negligible memory bus accesses. If the lock is freed, the processor performs an xmem
to grab the lock. If this xmem fails, then return to spinning on the cached copy. This algorithm
is implemented as follows, and can be found in lock.s:
void lock_wait(int *lock_addr);
_lock_wait:
ld
bcnd
or
xmem
bcnd
jmp
r10,
ne0,
r10,
r10,
ne0,
r1
r2, 0
r10, _lock_wait
r0, 1
r2, r0
r10, _lock_wait
;
;
;
;
;
;
;
r2 <- lock variable addr
read the lock value
loop if lock is busy.
set a lock value.
atomic exchange
try lock again if nonzero
return to caller
3.2.3 lock free() implementation
Freeing a lock is trivial. All that needs to be done is to store a \free" value to the lock variable.
This can be accomplished in a single st instruction on the 88100.
void lock_free(int *lock_addr);
_lock_free:
jmp.n
r1
st
r0, r2, 0
; r2 <- lock variable addr
; return to caller
; store "free" value
In this routine, the jmp.n r1 instruction demonstrates the use of delayed branching on the
88100. The instruction cycle immediately after a control transfer instruction, such as jump,
is known as the delay slot. This is where the processor gures out where execution should
continue. While it does this, another instruction can be executed. In this case, the st r0, r2,
0 instruction is executed in the jmp r1 delay slot.
3.2.4 Lock initialization
Before using a spin lock, a lock variable must be allocated and initialized. The lock variable is
of type int and can be allocated in any appropriate fashion, such as statically at compile time.
Chapter 3. Kernel level implementation
23
Initializing a lock variable is as simple as assigning zero to it:
int my_lock = 0;
lock_wait(&my_lock);
....
lock_free(&my_lock);
3.2.5 Summary
The simple spin lock can become surprisingly complex, as shown by [And89] and [KLMO91].
These more complex techniques arise from dierences in hardware characteristics, such as the
number of processors, memory bus architecture, and instruction sets.
In a shared memory system such as the Hypermodule with only four processors, spinning
on a cached copy of the lock is sucient to attain good performance. This algorithm is implemented by the routines lock wait() and lock free(). Modules throughout the kernel use
these routines to provided mutual exclusion for shared data structures and hardware devices.
3.3 List and Queue Management
List and queue data structures are primitive and fundamental building blocks for operating
systems. Much of the information stored within the kernel, such as task control blocks and
memory regions, are kept track of using linked lists. The operations involved are insertions,
removals, and traversals. This section describes these operations implemented in the kernel and
the data structures involved.
All of the queue management routines are implemented as C macros, so they are easily
inlined to avoid procedure call overhead. The routines could be coded in assembler, but inlining
ability and portability would be sacriced.
The queue macros are designed to be type-exible. This allows a small set of macro routines
to be general enough to handle any queue node structure. To do this, the macros require that
the caller specify the node type. When the macros are expanded at compile time, the proper
Chapter 3. Kernel level implementation
24
type casting is performed using the supplied type. This technique is similar to the queue macros
seen in the Mach kernel.
There are some algorithms where an ordered list of items is a basic requirement. For
example, the system clock timer maintains an ordered list of task control blocks. The list is
sorted by the wake time key. This allows the clock timer to examine only the head node on
the list to check for a timer expiry.
The queue macro package does not provide any macros for ordered list or priority queue
management. It is felt that such a structure can be eciently constructed using the supplied,
more primitive macros.
In a multiprocessor environment, concurrency control is required to protect against multiple
accesses to common data structures. Many of the system queues are shared amongst all the
processors. The queue management library does not implement locking, leaving it to be done
explicitly by the user. This option can reduce overhead when locking is not required.
The queue and linked list structures are simple enough to allow the use of the spin lock
library seen in the previous section. The use of spin locks to manage queue structure access is
demonstrated in the code fragments below.
3.3.1 Single linked lists
Most of the data that is kept on the linked lists are arrays of statically allocated control blocks.
Some control block data structures are very simple and don't even need the exibility provided
with a double-linked list. For example, the rawpage table[], which keeps track of physical
pages in the system, uses a single-linked list to remember all the free pages. To get a new page,
rawpage alloc() dequeues the head pointer from the free list. To free a page back to the pool,
rawpage free() enqueues the page onto the free list head. The rawpage routines don't need
to be able to remove elements from the middle or end of the list, so a backward link is not
required.
Simple linked lists as in the above example are used throughout the system whenever a
Chapter 3. Kernel level implementation
25
simple allocate/free operation of some data object is required. Since the lists are so simple,
the queue management library doesn't provide any help { it's up to the kernel programmer
to provide the head and next pointers for the list structure. This allows the list types to be
tailored for use within a code module.
3.3.2 Queues
The most common type of list used in the kernel is the double-linked list. These lists are
used extensively as FIFO queues for task, thread, and semaphore management, to name a few.
Since the queue operations are so basic to the operation of the kernel, a set of macros and data
structures are provided to simplify the job and make the code look a bit cleaner.
All queues have a master handle of type QUEUE which maintain the head and tail pointer
for the linked list. The other data structure that is used is the QUEUE LINK structure. The
QUEUE LINK structure contains the next and previous pointers, and is used as the \link" for
each node in the list. Each node in a queue must have at least one of these structures. Using
more than one link per node allows nodes to be linked to several dierent queues at once.
The task control block table, task table[], provides a good example of how these structures
are used. Each TASK structure contains several elds of data, two of which are the QUEUE LINK
structures:
/* next/prev link for queue structures */
QUEUE_LINK sched_link;
/* scheduler queue */
QUEUE_LINK timer_link;
/* timed event queue */
These links are used to connect the task control block to the scheduler queue and timer
queue1 .
Enqueue operations
There are three possible enqueue operations: enqueue at head of list, enqueue at the tail of list,
and enqueue before a given node. For queues that need to pay attention to FIFO ordering, the
1
The timer queue is used by the system clock timer to notify tasks of timed events.
Chapter 3. Kernel level implementation
26
enqueue/dequeue operation always occur on opposite ends of the queue. The kernel follows the
standard convention that nodes are always enqueued at the tail and dequeued from the head,
as seen in the following example from task create():
/* queue the task on the suspended queue */
lock_wait(&task_susp_q_lock);
ENQUEUE_TAIL(task_susp_q, task, TASK, sched_link);
lock_free(&task_susp_q_lock);
The following macros can be used for enqueue operations:
ENQUEUE HEAD(queue, node, node type, node link)
ENQUEUE TAIL(queue, node, node type, node link)
ENQUEUE ITEM(node, next node, node type, node link)
This macro inserts a node directly before the supplied next node. This macro can not be
used when next node is either the head or tail of the list { in those cases, the other two
enqueue routines should be used.
Dequeue operations
The following two dequeue operations are provided.
DEQUEUE HEAD(queue, node, node type, link)
DEQUEUE ITEM(queue, node, node type, link)
Note that these macros do not check for empty queues. It's an error to try and dequeue a
node from an empty list. To protect against this error, the QUEUE EMPTY() macro can be used
to check for the empty/non-empty condition. For example, task create() uses the following
code to get a new task control block:
lock_wait(&task_free_q_lock);
Chapter 3. Kernel level implementation
27
if ( QUEUE_EMPTY(task_free_q) )
{
lock_free(&task_free_q_lock);
kprint("task_create: no free tasks!\n");
*task_id = -1;
return(FAILED);
}
/* get a free task descriptor */
DEQUEUE_HEAD(task_free_q, task, TASK, sched_link);
lock_free(&task_free_q_lock);
Miscellaneous queue operations
As the previous example illustrates, the following queue operations can sometimes be useful:
QUEUE EMPTY(queue)
This macro evaluates to non-zero if the specied queue is empty.
QUEUE HEAD(queue)
This macro returns the rst node of the queue.
QUEUE NEXT(queue, node, link)
This macro returns the next node on the link after the specied node.
Allocating and initializing a queue
Before a queue list can be created, a master queue handle must be allocated and initialized.
The kernel uses the policy of static allocation wherever possible, so the handles are normally
allocated at compile time by declaring a variable of the QUEUE type. The following code from
task.c demonstrates this:
QUEUE task_free_q; /* queue of free tasks */
QUEUE task_susp_q; /* queue of suspended tasks */
int task_free_q_lock; /* spin locks for the above queues */
int task_susp_q_lock;
Chapter 3. Kernel level implementation
28
Once the master queue handle is allocated, it must be initialized to contain the value of
an empty list. The QUEUE INIT() macro performs this task, as demonstrated in the following
initialization code from task init():
/* create task system queues. */
QUEUE_INIT(task_free_q); task_free_q_lock = 0;
QUEUE_INIT(task_susp_q); task_susp_q_lock = 0;
3.3.3 Other linked list schemes
The Xinu operating system [Com84] uses an interesting technique to implement double-linked
lists for its process queues. In this scheme, the node links are stored in an array of links,
completely separate from the process descriptors. There is a one-to-one mapping between links
in the array and process descriptors. Both the links array and process table are indexed by an
integer. So, if you know which link you are, you automatically know which process descriptor
you belong to. Next/previous pointers in the links allow for double-links, and an extra key eld
allows ordered lists to be constructed.
This way of structuring a queue can make the enqueue/dequeue operations very ecient.
However, it does have the limitation that a node can only reside on one queue at a time. The
Raven kernel task scheduler and system clock timer require that a task be queued on two queues
at once, so this scheme cannot be used.
3.3.4 Summary
Queues provide a means of ordering and organizing data. They are a basic component in
operating system kernels. Both single linked and doubly linked lists are used throughout the
kernel to organize resources such as task descriptors and memory pools. The routines used
in the kernel to manage these queues are implemented as C macros which are portable and
ecient.
Chapter 3. Kernel level implementation
29
3.4 Low level console input/output
During the development stages of an operating system, one of the greatest aids is a low level
console output routine. Sometimes, the only way to debug at the kernel level is to output
debugging strings to the console. The output routine should be simple enough that it can be
executed from anywhere, independently of the rest of the system. This allows information to be
printed out whether or not the kernel is functioning properly, or from within interrupt handlers
and other delicate routines.
Using polled output in the presence of interrupt drivers can have negative consequences.
So rather than always sending console output through the serial port, a dedicated portion of
memory is set aside for console messages. This section of memory is known as the kmsg buer.
Synpopsis
Interface availability
Description
kprint()
user application
Prints out a string to the console (polled output).
kgetstr()
user application
Waits for a string from the console (polled input).
Figure 3.5: kprint module system call summary.
The kprint.c module implements the console polled input/output driver. The table in
Figure 3.5 summarizes the system call interface exported to the user level by this module.
3.4.1 Console output
void kprint(char *str);
This routine is the lowest level output routine. It takes a null-terminated string of characters
as its argument, and uses polled output to send the string to the console.
void kprintf(char *fmt, ...);
This is the kprint() routine for formatted printing. It passes the format string and arguments to sprintf() for formatting, and then calls kprint() to output the resulted string.
kprintf() understands the following format sequences: %c, %s, %d, and %x.
Chapter 3. Kernel level implementation
30
Special care should be taken when calling kprintf(): the output string should not be
greater than 200 bytes, or the stack will be damaged. kprintf() allocates a temporary 200
byte output buer on the caller's stack.
On the real hardware, the g88mon monitor controls communication across the serial port
to provide access to the g88 console under Unix. In the simulated environment, the simulator
contains a character console device at 0xffff0000. Access to both interfaces uses strictly polled
I/O.
void kprint_mode(int mode);
This routine controls the suppression of console output strings to the serial port device. In
some cases, the blocking nature of polled I/O has negative consequences. Outputting a string
across the serial port can take a long time in relation to other devices. For the Ethernet device,
the time taken to output a debug message may result in lost packets.
kprint mode() can help prevent this problem by allowing console output to be shut o under
program control. This is useful when debugging code where interrupt activity is necessary, such
debugging an Ethernet protocol stack. Passing a non-zero value for the mode argument allows
output to the serial port. Passing a zero value for the mode argument suppresses serial port
output. In either case, the kmsg buer logs all output strings, which can be viewed at a later
time.
3.4.2 Console output buer
The console output buer, or kmsg buer, is a large buer in the kernel memory space which
logs all strings that have been outputted to the console using kprint(). Even strings that are
sent with the mode disabled are placed in the kmsg buer. This allows all console output to be
viewed at a later time using the debugger.
The kmsg base variable points to the beginning of the kmsg buer. The g88 debugger can
be used to print out console strings starting at the kmsg base address. For example:
Chapter 3. Kernel level implementation
31
[0] (gdb) x/4s kmsg_base
Reading in symbols for kprint.c...done.
0x1e80000:
(char *) 0x1e80000 "CPU 0 started\n"
0x1e8000f:
(char *) 0x1e8000f "Executing on real hardware.\n\n"
0x1e8002d:
(char *) 0x1e8002d "total system memory: 8192 pages\n"
0x1e80065:
(char *) 0x1e80065 "avail user memory:
7751 pages\n"
[0] (gdb)
The size of the kmsg buer is controlled at compile time by the KMSG BUF SEGS constant.
This buer is allocated in segment sizes of 512KB, to facilitate memory mapping.
3.4.3 Console input
int kgetstr(char *buf, int buf_size);
In addition to providing a means to output information, the kernel also has a primitive
way to input information. This can be useful to ask conrmation questions at boot time, for
instance. The caller supplies the preallocated buer to place the inputted string in buf and the
maximum length of the string in buf size. The size of the inputted string is returned.
3.4.4 Console I/O initialization
Before doing any low-level console I/O, the structures must be initialized. This is done at boot
time by the kernel initialization routine.
int kprint_init();
This routine sets the console output mode to 1, enabling output to the serial port device,
and clears the kmsg buer.
3.5 Memory management
During the normal operation of an operating system, memory allocation and deallocation are
common tasks. Beneath the operating system lies a contiguous area of physical memory, pieces
Chapter 3. Kernel level implementation
32
of which are parcelled o for various uses, and later returned. User level tasks do not work
directly with these raw regions, however. The memory management system provides a protected
linear address space for user level programs to work in.
The memory management system divides its work into three distinct modules: virtual memory management, memory mapping and cache management, and physical memory management.
Figure 3.6 shows the layered relationship of each module. This section describes each module
in detail.
Virtual memory layer
vm.c
Mapping and cache
management
map.c
Physical memory
management
rawpage.c
DRAM and 88200 CMMU hardware
Figure 3.6: Memory management system decomposition.
The virtual memory module provides the user level system call interface to the memory management system. Routines are provided to allocate, deallocate, and share regions of memory.
A region is a contiguous, page aligned portion of memory in a virtual address space.
The virtual memory module relies on the memory mapping module to manage the 88200
CMMU page tables. Regions of memory can allocated at specic addresses and with various
page protection statuses. The processor instruction and data caches must also be maintained
throughout memory allocation/deallocation operations. The mapping module also provides
user level address space switching functions.
The physical memory module provides a simple and ecient interface to the virtual memory
module for allocating and deallocating physical pages of memory. Pages are allocated and
deallocated on a page granularity.
Chapter 3. Kernel level implementation
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3.6 Physical memory management
The physical memory management module is very simple. Its job is to allocate physical pages
from the raw memory pool, and return physical pages when the virtual memory layer is nished
with them. While the virtual memory layer manages contiguous regions of an address space, the
physical memory layer works on a simple page-by-page basis. The allocation and deallocation
routines always work with single page units.
The rawpage table[] keeps track of each physical memory page in the system. The table
is statically allocated at compile time to contain one entry for each page in the system. When
more physical memory is added to the system, more elements need to be allocated in this
table. The static allocation strategy is used mainly for simplicity. However, in the future, this
static table could easily be replaced with a table that is allocated at runtime, after probing the
hardware for the true physical memory size.
The following data structures are used to manage the physical memory pages:
typedef struct raw_page_s
{
int count;
int lock;
struct raw_page_s *next;
} RAW_PAGE;
/* page reference count */
/* spin-lock to protect access */
/* next free page */
RAW_PAGE rawpage_table[RAWPAGE_TABLE_SIZE];
RAW_PAGE *pages_free_head;
int pages_free_lock;
/* linked list of free pages */
/* spin-lock to protect list access */
The RAW PAGE structure provides a handle for each physical memory page in the system.
The count eld implements a reference count for the page: when a page is allocated or shared,
its reference count is incremented. When a page is freed, its reference count is decremented.
This allows the system to keep track of which pages are used and which pages are unused.
There are three routines provided by the raw page module. All of these routines are available
within the kernel only, they are not exported to the user level.
Chapter 3. Kernel level implementation
34
3.6.1 Allocating a physical page
To allocate a free physical page, the following routine is used.
void *rawpage_alloc();
No parameters are required. The base address of the physical memory page is returned. If
there are no free pages, the system panics (something more elegant could be done in future).
Finding a free page is very simple. First, the free list lock is acquired. Then the rst entry
on the list is dequeued, and the lock is freed. The reference count is initialized. The address of
the physical page is computed from the position of the page entry relative to the head of the
raw page table. This address is returned to the caller.
3.6.2 Freeing a physical page
Freeing a physical page is equally as simple. Using the page address, the raw page table entry
is computed. The entry lock is acquired and the reference count is decremented. If the new
reference count is 0, the page entry is queued to the beginning of the free page list.
int rawpage_free(void *page);
The rawpage free() call decrements the reference count for the given page, and returns it
to the raw page pool if the count reaches 0.
3.6.3 Sharing a physical page
A reference count allows pages to be shared by dierent processes, keeping them from being
freed and reused while they are being used elsewhere. If two processes are sharing a page, and
one process frees the page from its address space, the page is not returned to the free memory
pool. The page is only freed when the last process discards the page.
int rawpage_reference(void *page);
The
page.
rawpage reference()
call increments the reference count for the specied physical
Chapter 3. Kernel level implementation
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3.6.4 Raw page initialization
At initialization time, the rawpage table[] is initialized and the free pages linked list is created.
Even at boot time, many pages are already allocated for the kernel data and code areas. These
pages are removed from the free list and their reference count is marked for use. Access to the
list of free pages is protected using the pages free lock spin-lock. The allocate/free routines
must acquire the lock before pages can be removed or added from the free list.
int rawpage_init();
The rawpage init() function is called by the boot processor to initialize the physical memory pool. It returns the total number of free physical pages in the pool.
3.7 Memory mapping and cache management
The map module is responsible for controlling the 88200 memory management unit [Mot88b].
All aspects to do with address translation and cache management are encapsulated in this
module. Changes in the memory management hardware would only require changes to this
module.
Routines are provided to create and manage the hardware translation tables and caching
parameters. These routines are used by the virtual memory layer to provide the user and
supervisor space with virtual addressing. These address spaces include physical memory, as
well as access to hardware device control registers. None of these routines are user-accessible,
they are only used locally within the kernel.
3.7.1 Translation tables
As described in the hardware overview section, the 88200 uses a two-level translation table to
allow mapping of a 4GB address space. The rst level segment table contains 1024 entries that
point to page tables. Each page table contains 1024 entries that provide the translation for a
single page of 4096 bytes. Refer to Figure 2.2 for a description of the translation table format.
Chapter 3. Kernel level implementation
36
In addition to address mapping information, the translation table entries contain attribute
bits that control cache and protection status. Dierent settings of these bits are used for dierent
types of memory pages. For example, since code pages are not modied during execution,
code pages are marked read-only/cache-enabled. Device control registers are marked readwrite/cache-inhibited. These attribute bits, as well as the 88200 control register osets, can be
found in the registers.h le. Refer to the 88200 user manual [Mot88b] for detailed information
on the attribute bits.
Each 88200 maintains two registers called area pointers: the user area pointer (UAPR), and
the supervisor area pointer (SAPR). The UAPR and SAPR contain a master pointer to the
translation table for the user and supervisor address spaces, respectively. Context switching an
address space requires setting the area pointer from one map to another. Throughout execution
of the system, the supervisor area pointer remains constant, while the user area pointer changes
for each running task. The map module keeps track of area pointers using the MAP structure:
typedef struct
{
int map;
int lock;
} MAP;
/* 88200 area pointer */
/* spin lock for this memory map */
The MAP structure contains the area pointer and a spin-lock to protect accesses to the translation table. The lock must be acquired by any map routine before changes to the translation
table are allowed.
Each processor in the system uses two 88200 units: one for the code caching and translation,
and one for data caching and translation. Each 88200 can use their own set of translation tables,
or they can share a common single table. The Raven kernel opts to have two tables: one for
code, and one for data. Thus each task descriptor in the system contains two map descriptors:
MAP code_map;
MAP data_map;
Chapter 3. Kernel level implementation
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3.7.2 Translation lookaside buer (TLB)
In addition to the instruction and data caches, the 88200 units also contain translation table
caches, or translation lookaside buers (TLB). The TLB stores frequently used translations,
so that translation table searches are not required for each memory access. On the 88200, the
TLB is termed the physical address translation cache (PATC), and contains 56 entries.
But unlike the instruction and data caches, the TLB caches are not automatically coherent
across multiple cache units. Therefore, modications to an activated translation table in the
form of attribute changes or freed pages require TLB ushes if the address space is enabled
on more than one processor. If this is not done, then remote TLB caches will not contain the
correct translation table information.
Another type of translation lookaside buer in the 88200 is the block address translation
cache (BATC). There are ten entries in this cache which can be set programmatically. The
entries remain until explicitly reprogrammed. Rather than mapping single 4KB pages, the
BATC entries maps contiguous 512KB blocks. The operating system software can use these
entries to provide mappings for high-use code or data regions.
3.7.3 Translation table storage area
A special region of the physical memory space is set aside at boot time for translation table
storage. The kernel allocates segment and page translation tables for each task in the system
from this common storage area. This area is located at the very top of physical memory. Its size
is dened at compile time, in a granularity of 128 pages, by the PAGE TABLE SEGS constant in
the le kernel.h. This allows the 88200 BATC entries to map the whole storage area into the
kernel space at all times. Using the programmable BATC entries help avoid the chicken-and-egg
problem that occurs when an unmapped page is allocated to store a page table.
Two macro routines are used to manage the allocation and deallocation of translation table
pages. These routines are very ecient: they simply keep track of a free list of pages in the
page table storage area. The PTE ALLOC() macro allocates a page table by dequeuing the next
Chapter 3. Kernel level implementation
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free one. The PTE FREE() macro returns a page to the storage area by enqueuing it back on
the free queue.
3.7.4 Allocating a translation map
When a user task is created, two address maps are allocated to create the tasks code and data
address spaces. The virtual memory layer uses the following routine to allocate these maps:
int map_alloc( MAP *map, int attrb );
The caller passes in a pointer to a preallocated, empty MAP structure and an attribute
setting. An empty translation table is allocated using PTE ALLOC(), and using the address of
this table along with the attributes, the map area descriptor is created.
3.7.5 Freeing a translation map
When a user task is destroyed, the virtual memory layer frees its memory, and then calls this
routine to free the tasks' translation tables. Since there are two translation tables for each user
task, this routine is called twice to completely free a user task:
int map_free( MAP *map );
The caller passes in a pointer to the MAP structure for the table to be freed. The routine
traverses the translation table structure and uses PTE FREE() to release each table back to the
free pool.
3.7.6 Enabling an address space
Before a user level task can execute, its address space must be activated. Enabling an address
space is simply done by assigning the map area pointer to the 88200 UAPR register. But before
this can be done, the previous user level TLB entries must be ushed. Otherwise, TLB entries
from the previous address space would pollute the translations of the new address space.
Chapter 3. Kernel level implementation
39
int map_enable_task(int cpu, MAP *data_map, MAP *code_map);
The rst parameter species the physical processor number to enable the address space on.
This cpu number is used to calculate the appropriate 88200 register addresses to assign the
provided map descriptors to (there are eight 88200 units in the system, two for each processor).
The proper TLB entries are ushed and the new area pointers are assigned to the UAPR
registers.
3.7.7 Mapping pages in an address space
When the virtual memory layer allocates physical memory to an address space, a new entry in
the address space's translation tables is created. The allocation of memory to an address space
includes the operation of allocating new memory regions, in addition to sharing pages between
address spaces. The map module provides two routines to allows page mapping on single or
multiple contiguous pages.
int map_page( MAP *map, void *page, void *logical_addr, int attrb );
The map page() routine eciently maps a single physical page to a logical address in an
address space. The map parameter species the address space to map the page. The page
parameter species the physical address of where the page to map is located. The logical addr
parameter species the address in the address space to map the page at. attrb species the
attribute bits for the page.
int map_pages( MAP *map, void **addr, void **hint, int num_pages, int attrb );
This routine allows multiple contiguous pages to be mapped into an address space. Unlike
the simpler map page() call, this routine also allocates new physical pages from the rawpage
module. map pages() is most commonly used by the virtual memory layer to eciently allocate
contiguous regions of memory for a task. By supplying *addr = NULL, the routine will choose
the next available free memory space and allocate num pages pages starting there. The hint
is supplied to provide a good starting location for the free memory search.
Chapter 3. Kernel level implementation
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3.7.8 Unmapping pages from an address space
When the virtual memory layer removes physical pages from an address space, the associated
translation table entries must be removed. For active address spaces, the TLB caches of the
associated cache units must be notied of the changes (otherwise remote TLB caches may
continue to contain the unmapped translation table entry). Every TLB in the system that is
potentially caching the changed entries must be ushed. This includes remote processors that
are executing the same address space.
On many multiprocessor systems, ushing a TLB on a remote processor involves sending
the remote processor an interrupt and stalling the remote processor until the ush is complete
[BRG+ 88]. This is because some machines do not allow ush operations on TLBs other than
their own. Fortunately, the 88200 does allow ush operations to be invoked from remote
processors. All the kernel needs to do is write an invalidate command to every 88200 unit in
the system that is using the address space. Flushing the whole TLB when only one page table
entry is changed can be wasteful. Fortunately, the 88200 allows system software to specify
the ush granularity on a page basis. Before issuing the ush command, the page entry to
invalidate is specied.
int unmap_page( MAP *map, void *logical_addr );
The
space
unmap page()
routine removes the specied logical page from the specied address
int unmap_pages( MAP *map, void *logical_addr, int num_pages, int free );
The unmap pages() routine is the plural form of the above unmap page() routine. In this
case, multiple contiguous pages can be removed from an address space, starting at the specied
logical address. If the free ag is non-zero, then each logical page is also returned to the
physical memory pool using rawpage free(). This allows large regions of an address space to
be eciently freed in one call.
Chapter 3. Kernel level implementation
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If a page table becomes empty after the last logical page is removed from it, that page table is
not returned to the page table pool (normally done by the PTE FREE() macro). This could leave
several page tables consumed for apparently no purpose. Curing this problem would require
some form of garbage collection in the unmap routines. However, this problem is not as bad as
it may seem. Any subsequent memory allocations will likely consume the next available entry
in these empty page tables. So in fact, not garbage collecting these tables would allow them
to be used directly without having to allocate new ones. Thus, an PTE FREE()/PTE ALLOC()
iteration is saved.
3.7.9 Finding free logical addresses
Before the virtual memory layer can allocate memory to an address space, it needs to know
where to place the memory in that address space. For example, when a user program calls
malloc(), the user does not specify the location to allocate the memory. Instead, the next
available address is chosen by the malloc library. Likewise, user level calls to the virtual
memory allocator do not always specify the address to place the memory at. The virtual
memory layer must decide where to allocate the memory. It uses the following routine to do
this:
int map_find_free(MAP *map, void **addr, void **addr_hint, int num_pages);
Given a address map map, map find free() will traverse the map starting at address
*addr hint, looking for a free contiguous region of pages. The size of the region is specied by
num pages. The found address is returned in *addr. The *addr hint address is updated to the
next page beyond this contiguous region. Returns OK if successful, or FAILED if a contiguous
free region could not be found.
3.7.10 Sharing and moving translations
Another common memory operation that the supervisor and user level tasks require is the
ability to share and move memory between address spaces. Sharing memory allows multiple
Chapter 3. Kernel level implementation
42
address spaces to read and write the contents of a single region of memory. The move operation
allows contiguous memory regions to be passed from one address space to another, removing
the mapping from the source. The following map share() routine provides the virtual memory
layer support to easily do this.
int map_share(MAP *src_map, void *src_addr, MAP *dest_map,
void **dest_addr, void **dest_hint, int num_pages,
int attrb, int share);
The routine takes a source map, a source logical address, and the number of contiguous
pages to map into a destination map. If the destination address is not known, then *dest addr
= NULL is passed, and map find free() nds a suitable address. Passing the dest hint address
gives the search algorithm a good place to start. Passing a non-zero value for share retains
the mapping in the source address space, passing a zero value causes the source region to be
unmapped.
3.7.11 Map module initialization
The only state maintained by the map module are the translation tables. Storage for these
tables is allocated at initialization time at the top of physical memory. The size of the storage
area is congured at compile time using the PAGE TABLE SEGS constant in kernel.h. The
pte init() routine builds a free list to manage the allocation and deallocation of translation
tables from this storage area.
void map_init();
This routine is called by the virtual memory initialization code. First, it calls pte init()
to initialize the translation table storage area. Then, it programs the 88200 BATC entries to
map in the kmsg buf and page table storage areas. These areas will then be available to the
kernel when the supervisor address space is enabled.
Chapter 3. Kernel level implementation
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3.8 Virtual memory management
All address spaces, user and supervisor, are created and managed by the virtual memory layer.
The virtual memory layer provides the kernel and user level with an interface to allocate,
deallocate, and share contiguous regions of memory within an address space. It uses the map
module for setting up address translation, and the rawpage module for managing physical pages.
The table in Figure 3.7 summarizes the routines exported to user level by this module.
Synpopsis
Interface availability
vm_alloc()
Description
user application
Allocates a region of virtual memory.
vm_free()
user application
Deallocates a region of virtual memory.
vm_share()
user application
Shares a region of memory between tasks.
vm_move()
user application
Moves a region of memory between tasks.
vm_map_device()
user application
Maps in a region of device registers.
vm_unmap_device()
user application
Unmaps a region of device registers.
Figure 3.7: Virtual memory system call summary.
Figure 3.8 shows a typical memory space for a user level task. A user level memory space
contains four basic components: an executable code segment; a data segment for global variables
and constants; dynamically allocated heap space; thread stacks; and hardware device mappings.
The dark areas denote regions of memory that are mapped. The code and data regions are
always contiguous, they contain the executable image as loaded from a le. The heap area
starts at USER HEAP START ADDR and continues to USER HEAP END ADDR (currently 0x700000
and 0xffbf0000 in context.h). This heap area gives user programs approximately 3.9GB of
virtual address space to work in.
Memory regions in the heap space are allocated and deallocated on demand throughout
execution of the user level program. Thus, as Figure 3.8 illustrates, the heap space can become
fragmented. A sparsely populated memory space can consume many translation tables, so
allocation routines try to minimize this fragmentation by allocating space close to neighbours.
However, this feature can be easily overridden, in the event that a specic address is required.
Chapter 3. Kernel level implementation
44
0xffffffff
Interrupt ctrl
0xfff84000
0xffbf0000
3.9GB
Heap
space
Heap regions
Ethernet device
Heap regions
0x00700000
Heap regions
Data region
0x00400000
0x0005c000
Code region
0x00010000
0x00000000
Figure 3.8: Typical user level memory space.
User level device drivers use the virtual memory layer to map in hardware device registers.
While hardware devices reside at very specic physical addresses, their logical address can
appear anywhere within the user's virtual space. Therefore, devices can be dynamically mapped
anywhere within the heap storage area.
3.8.1 Region descriptors
The virtual memory module manages memory in objects called regions. A region is a contiguous,
page aligned portion of memory in a virtual address space. The following VM REGION structure
describes a region:
typedef struct
{
int id;
int task_id;
MAP *map;
void *addr;
int size;
int attrb;
/*
/*
/*
/*
/*
/*
this region descriptor id */
task that this region belongs to */
page table mapping descriptor */
virtual address of region start */
size of region */
page attributes for region */
Chapter 3. Kernel level implementation
45
/* next/prev link for VM region queue structures */
QUEUE_LINK link;
} VM_REGION;
VM_REGION region_table[REGION_TABLE_SIZE];
QUEUE region_free_q;
int region_free_q_lock;
A region entry contains address mapping and ownership information. The start address,
size, attributes, and map specify the mapping information for the region. The link eld allows
region entries to be queued together into lists. For example, the region free q keeps track of
all unused region descriptors. The statically allocated region table maintains a global pool of
region entries. To allocate a new region entry, the VM GET REGION() macro dequeues the next
available region. Old regions are recycled by enqueuing them back on the free list.
3.8.2 Allocating a region of memory
All user level tasks allocate heap memory through one single system call, vm alloc(). To
allocate memory, a new region entry is dequeued, and its values are initialized. Then, the
map pages() routine performs the physical memory allocation and mapping for the region.
Finally, the region entry is enqueued onto the tasks heap list.
Each task maintains a list of heap regions that are allocated in its address space. When a
task is destroyed, its memory must be returned to the system. The heap list allows each region
to be kept track of, which can be eciently freed all at once.
int vm_alloc(int *region_id, int task_id, void **addr, int size, int attrb );
species the task to allocate memory into. *addr is set to contain the starting
address to allocate the region of memory at. However, if the caller passes *addr = NULL, then
the next available contiguous free region of memory is chosen. This address is then returned to
the caller in *addr. The size parameter species the size of region to allocate, in bytes. All
sizes are rounded up to the nearest page.
task id
Chapter 3. Kernel level implementation
46
The attrb parameter species the page attributes for the 88200 memory management
unit. These attribute settings are discussed in detail by the 88200 technical manual. The
registers.h le contains all possible bit settings for this parameter. For example, to allocate
a page of memory that can be shared amongst all threads in an address space, use attrb =
GLOBAL BIT | VALID BIT.
The newly allocated region identier is returned in *region id. Further operations on this
region of memory are specied by supplying this region identier to the virtual memory system
calls. For example, to free the region, call vm free() with the returned region id.
3.8.3 Freeing a region of memory, vm free()
int vm_free(int region_id);
To free a region of memory, the vm free() call is used. The call examines the supplied
region entry, and uses the unmap pages() routine to remove the address mapping and free the
physical pages.
3.8.4 Sharing memory
Sharing memory provides a convenient and ecient way for address spaces to communicate.
Memory regions can be shared between any number of address spaces.
int vm_share( int src_region_id, int *dest_region_id, int dest_task_id,
void **dest_addr, int dest_attrb );
The caller supplies a source region identier in src region id, and a memory address
*dest addr in the destination task to map the region at. If the destination address is not
known ahead of time, then passing *dest addr = NULL will automatically choose the next
available free location. The chosen memory address will be returned in *dest addr. The
dest attrb parameter species the page attributes for the destination address mapping. For
example, a server may wish to provide a read-only page of memory for clients to examine. A
region identier for the newly created region is returned in dest region id.
Chapter 3. Kernel level implementation
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3.8.5 Moving memory between tasks
A common operation throughout execution of device drivers and client/server interactions is
the movement of data between address spaces. For example, initially a device driver will read
a chunk of data from a device, and give it to a server for processing. The server will then send
the data to clients. Each step could involve the movement of data across address spaces.
Sometimes, the amount of data passed between clients and servers is small, and therefore
an explicit memory copy operation is the most ecient way to pass memory between address
spaces. But for high speed devices, and especially block-mode devices with large block sizes, a
memory mapping technique can eliminate the data copy altogether. The vm move() system call
provides a convenient interface to do this. Memory pages in one address space can be mapped
out and mapped in to another address space.
int vm_move(int src_region_id, int dest_task_id, void **dest_addr,
int dest_attrb);
The src region id species the source region to move. The destination task for the memory
region is specied by dest task id. *dest addr should be set to contain the starting logical
address for the region of to appear in the destination address space. If the destination address
is not known ahead of time, then passing *dest addr = NULL will automatically choose the
next available free location. The chosen memory address will be returned in *dest addr. The
dest attrb parameter species the page attributes for the destination address mapping. The
region identier src region id remains the same in the destination task.
3.8.6 Mapping hardware devices
On the 88100 Hypermodule, access to hardware peripherals and on-board devices is accomplished through memory mapped registers. These registers appear at well-known memory locations in the physical address space. Many of these physical memory locations are hardwired,
such as the 88200 registers, while others such as the VMEbus devices, can be congured via
jumper settings or programmable registers.
Chapter 3. Kernel level implementation
48
User level device drivers must know exactly where in the physical address space their device
registers reside. Using that information, the vm map device() can map in the appropriate
physical memory locations to provide access to these device registers.
int vm_map_device( int *region_id, void *phys_addr, void **addr, int size );
The caller must specify the physical address of the device in phys addr, and the size of the
memory region in bytes (rounded up to PAGE SIZE boundaries). The caller can also use *addr
to specify the desired logical address to map the physical region. If the logical address is not
known ahead of time, then passing *addr = NULL will automatically choose the next available
free location. The chosen memory address will be returned in *addr. A region identier for
this newly created region is returned in region id.
The previous memory mapping and allocation routines allowed the caller to specify a page
attribute for each memory page. Device register mappings have more stringent requirements:
they must be mapped as cache-inhibited to make sure that accesses truly hit the device, and
not just the cache. vm map device() sets all of its page attributes to CACHEINHIBIT BIT |
VALID BIT.
int vm_unmap_device( int region_id );
The vm unmap device() system call allows device register mappings to be removed from the
callers address space. The region identier region id species the memory region to remove.
3.8.7 Virtual memory initialization
When the kernel boots, initial execution of the system happens within the physical memory
space, no memory translation is done. When the virtual memory module gets a chance to
initialize, it initializes the lower layers, and then begins to setup the supervisor memory space.
During this setup phase, translation is disabled until the whole supervisor translation table is
built.
Chapter 3. Kernel level implementation
49
The lower layers of the virtual memory system include the rawpage module and the map
module. These modules are simply initialized with the rawpage init() and map init() calls.
MVME188
on-board devices
0xffffffff
0xff000000
0xffe1b000
g88mon
0xffe13000
Free VMEbus
32-bit space
0x02000000
translation tables
0x01f00000
32MB
physical
memory
kmsg buffer
0x01e80000
kernel stacks
0x01e7e000
Free physical
memory
0x00036000
Data region
0x00019000
Code region
188Bug area
0x00010000
0x00000000
Figure 3.9: Initialized supervisor memory space.
To begin building the supervisor memory space, the map alloc() call is used to create the
supervisor code and data address space maps: kernel code map and kernel data map. The
address space is then built using the map page() call. Figure 3.9 shows the layout of a fully
initialized supervisor memory space.
3.9 Task management
A task encapsulates the virtual memory space and processor allocation functions for user level
programs. The virtual memory space for a task includes program code, data and heap regions,
as well as shared regions and memory mapped device registers. The virtual memory module
manages most of a task's address space requirements: address space allocation and deallocation.
All other memory operations, such as page-wise dynamic allocation and mapping, are handled
directly by the virtual memory module.
Chapter 3. Kernel level implementation
50
The task.c module implements the bulk of the task management functionality. This module
works in cooperation with all of the other main system components, and ties them together to
form a basis for user level development: memory management, task scheduling, interrupt and
exception handling, and system call handling. The table in Figure 3.10 summarizes the system
call interface exported to the user level by this module.
Synpopsis
Interface availability
task_create()
user application
Creates a task.
Description
task_destroy()
user application
Destroys a task.
task_suspend()
user application
Suspends the execution of a task.
task_resume()
user application
Resumes the execution of a task.
task_info()
user application
Returns task state information
task_signal()
user kernel
Sends an asynchronous signal message to a task.
task_timer_event()
user kernel
Registers a task for a timer event.
task_request_cpu()
user kernel
Requests a processor for the task.
task_relinquish_cpu()
user kernel
Relinquishes control of the cpu to another task.
task_intr_relinquish()
user kernel
Relinquishes control to an interrupt driver task.
task_cleanup()
user kernel
Cleans up an exiting task.
Figure 3.10: task.c module system call summary.
This section describes the task management module, and how it interacts with the rest of
the system. Before describing the details behind task management, we rst present an overview
of the task scheduling environment. This discussion is intended to give the reader a global view
of how and why scheduling decisions are made.
3.9.1 Task management overview
All tasks are created using the task create() system call. Each task in the system is allocated
its own task descriptor from a global descriptor table, task table[]. The task descriptor
contains all the vital information which describes the task, such as name, entry point, and
virtual memory space. In addition, the task descriptor contains two pieces of information
which assist in the task processor allocation: num cpus, the number of processors currently
Chapter 3. Kernel level implementation
51
allocated to the task; and num ready threads, the number of ready threads in the task waiting
for a processor.
When a task receives a processor, the num cpus eld is incremented. When a processor
is taken away from a task, num cpus is decremented. Likewise, when the user level thread
scheduler places a thread on its ready queue, num ready threads is incremented. When a
thread is removed from the ready queue, num ready threads is decremented.
The task scheduling mechanism uses both of these variables to help decide how a task
should be scheduled. There are seven main sources of control in the system which invoke the
task scheduler to make a scheduling decision:
1. Processor requests from user programs. When a user level thread scheduler nds itself with
ready threads on hand (i.e., num ready threads > 0), then it will request a processor.
2. Processor relinquishment from user programs. When a user level thread scheduler runs out
of runnable threads (i.e., num ready threads == 0), then it will relinquish its processor.
3. Hardware interrupt handlers. An interrupt may cause an upcall event to be directed at a
specic task, in which case the task must be given a processor.
4. Hardware exception handlers. As with interrupt handlers, an exception may occur that
requires the attention of a specic task2.
5. When creating a new task. A newly created task begins with num ready threads
num cpus == 0.
== 1,
6. Resuming a suspended task. A suspended task, when resumed with num ready threads
> 0, is in need of at least one processor.
7. After a task is destroyed. When the currently running task is destroyed, another task is
scheduled in its place.
2
For example, an external pager task would be invoked whenever a task generates a page fault.
Chapter 3. Kernel level implementation
52
Tasks that are in need of a processor, when num ready threads > 0, are either placed on the
task ready queue to be given a processor in the future, or are given a processor immediately.
The task ready queue is a centralized priority queue with 32 levels. All processors in the
system share the same queue through a simple three routine interface: enqueue ready task(),
dequeue ready task(), and remove ready task().
The main scheduling loop within the kernel is in the routine sched(). When invoked,
sched() dequeues the next available ready task and runs it. If there are no ready tasks, then
the processor drops into the idle() loop. The idle loop simply runs a forever-loop, with interrupts
enabled. So rather than having idle processors poll system queues looking for work, work is
delivered to idle processors using the software interrupt service (SWI). When work becomes
available, an interrupt is delivered to the next available idle processor. For example, if a task
is in need of a processor, then one of the idle processors is delivered a TASK RUN SWI interrupt.
(A global structure, idle cpus[], keeps track of which processors are idle.)
One of the main task scheduler routines that is responsible for delivering work to idle
processors is the enqueue ready task() routine. Whenever a task is found to require an
additional processor, enqueue ready task() is called to place the task on the ready queue.
However, if an idle processor is available, enqueue ready task() will deliver the processor a
TASK RUN SWI interrupt.
3.9.2 The KERNEL INFO structure
The kernel maintains three important data structures to manage the scheduling of tasks:
the KERNEL INFO structure; the task descriptor, TASK; and the shared region structure,
SHARED REGION. This section focuses on KERNEL INFO; the following section discusses the latter
two.
The KERNEL INFO structure, shown below, contains a number of elds that are used throughout the kernel, and at the user level. At boot time, a physical memory page is allocated to
store the structure. The page is mapped into the kernel space with read/write privileges, and is
Chapter 3. Kernel level implementation
53
accessed through the global variable kernel info. When a task is created, the page is mapped
read-only into the tasks address space. All user level code has read access to the information,
but it cannot be overwritten.
typedef struct
{
int simulator;
/* nonzero when executing under the 88k simulator */
unsigned long timer_ticks;
/* clock ticks since boot time */
char idle_cpus[NUM_CPUS];
/* indicates which cpus are idle */
unsigned long idle_time[NUM_CPUS];
/* idle time counters for each CPU */
unsigned long kernel_time[NUM_CPUS]; /* average time in kernel recently */
unsigned long user_time[NUM_CPUS];
/* average time in user space */
int
int
int
int
int
page_table_entries;
physical_pages_free;
num_free_regions;
num_tasks;
num_ready_tasks;
/*
/*
/*
/*
/*
/*
number of free pte entries */
usable free memory */
free VM regions */
tasks created in system */
number of ready tasks (note: */
protected by ready_q_lock) */
/* stores which cpus tasks are running on. */
/* given a task id, it's easy to find which cpus it's running on */
char run_cpus[TASK_TABLE_SIZE][NUM_CPUS];
int lock;
} KERNEL_INFO;
KERNEL_INFO *kernel_info;
/* allocated and mapped at boot time */
The structure contains many miscellaneous elds, and a few important ones. simulator is
set at boot time to signify whether the system is running under the g88 simulator, or on the real
hardware. Sometimes, as when dealing with devices, it is important to know this dierence.
timer ticks is a counter that is incremented at each system clock tick. This can be used to
give programs a notion of time.
Many of the elds in this structure are used for resource usage accounting. A user level
Unix-style ps command could display this information, or log it to a le. The idle time[],
Chapter 3. Kernel level implementation
54
kernel time[],
and user time[] elds are used to record system activity for each processor.
The next ve elds are used to count other system resources, such as free memory and the
number of tasks running in the system.
The idle cpus[] eld is used to record the idle state of each processor in the system. When
a processor enters its idle loop, it sets the associated entry in the idle cpus[] array. Another
processor in the system can read this eld and immediately know which processors are idle
and are eligible for work. For example, if a new thread is created at the user level, the thread
scheduler can read idle cpus[] and quickly spawn the thread to a remote idle processor by
delivering a software interrupt message to the idle processor (via intr remote cpu().
The run cpus[][] eld records the running status of each task in the system. When a
processor is given to a task, the task's run cpus entry is set for that processor. This allows the
scheduling code to quickly nd out which processors are running a specic task. This can be
used when destroying a task, for example. The task destroy() code consults the run cpus
entry for the task, and broadcasts an interrupt to the processors running that task. As another
example, the IPC mechanism can easily target its messages to processors that are running the
destination task.
3.9.3 Task descriptors
The task descriptor structure, type TASK dened in kernel.h, contains most of the bookkeeping
data that comprises a task. The structure is divided into several main components, each of
which is maintained and shared between the various kernel modules. The descriptors are private
to the kernel memory space; it is not directly shared by user level programs.
typedef struct
{
int
int
void
int
id;
lock;
*stack_page;
/* 0 to TASK_TABLE_SIZE-1 */
/* spin lock for this descriptor */
/* kernel address of user's upcall stack*/
interrupts;
/* bit field of enabled user interrupts */
Chapter 3. Kernel level implementation
int
int
exceptions;
pending_intr;
TASK_INFO
info;
TASK_VM
vm;
SHARED_REGION *sr;
QUEUE_LINK sched_link;
QUEUE_LINK timer_link;
unsigned int wake_time;
} TASK;
55
/* bit field of enabled user exceptions */
/* remembers pending interrupt vector
*/
/* task statistics and other info */
/* task virtual memory spaces */
/* user/kernel shared region pointer
*/
/* scheduler queue */
/* timed event queue */
/* time to wake this task (task_timer) */
The following paragraphs and subsections describe each component of this structure. After
starting with the smaller miscellaneous elds, the larger component structures are described.
The id eld identies the task descriptor. This number is the index value of the descriptor
in the task table[] array. It is initialized at boot time, and remains constant throughout the
life of the system.
The lock eld is the spin lock used to provide mutual exclusion in routines that manage
the task descriptor. Normally this lock is acquired before any system call or scheduling action
is performed on a task. However, there are cases where locking is not required. These cases are
dened by their specic purposes.
The stack page eld points to the physical page of memory that is allocated for the tasks
upcall stack. When a task is created, a user level upcall stack is allocated and mapped into the
user level memory space at location TASK STACK ADDR. When the task is destroyed, the upcall
stack is returned to the physical memory pool.
The interrupts and exceptions elds are maintained by the kernel interrupt and exception dispatchers. interrupts and exceptions are bit elds which describe the interrupt and
exception handling capability for the task. For example, when a task registers an interrupt
handler in its address space, the corresponding bit is set in the interrupt eld. The kernel interrupt dispatcher can use this information for deciding what to do when a particular interrupt
arrives.
Chapter 3. Kernel level implementation
56
The interrupt dispatcher uses intr pending to remember a pending interrupt. When a
hardware device generates an interrupt, the kernel interrupt dispatcher looks up in its table
where to locate the handler for the interrupt. If the handler resides in a task that is currently
not active, it must be switched in. The intr pending eld is set in the old task to remember
the pending interrupt vector. When the old task is switched out and the interrupt handler task
is switched in, the vector is examined before upcalling to the new task.
The sched link eld is the main scheduler queue link for the task. This link is used to
place the task on the task descriptor free queue and ready queue. The timer link eld is used
by the task event timer system. A task enqueues itself on the task timer q using this link,
and will be issued a timer upcall event when the wake time interval expires.
TASK INFO
structure
The rst main component of the task descriptor, info, contains general information about the
task such as scheduling state, priority, an ASCII string name and resource consumption
statistics. The reason why these data elds are grouped in their own structure is so that they
can easily be marshalled to user level programs. A special system call, task info(), allows user
level programs to query this structure for information collecting. For example, the console
program implements a Unix-style ps command for displaying task scheduling and resource
usage. Currently, the resources kept track of are memory usage, but this could be expanded to
include other interesting information such as interrupt and context switching rates.
typedef struct
{
int state;
int priority;
char name[TASK_NAME_SIZE];
/* task scheduling state */
/* 0 to NUM_READYQ_PRIORITIES-1 */
/* ASCII string name for executable */
/* some memory statistics */
int code_size;
/* size of the program code (bytes) */
int data_size;
/* size of the program data (bytes) */
int bss_size;
/* size of the program bss (bytes) */
int virtual_size;
/* pages of memory mapped by this task */
} TASK_INFO;
Chapter 3. Kernel level implementation
TASK VM
57
structure
The next component, vm of type TASK VM, maintains information about the tasks' address space.
This structure is managed by the memory management system. The code and data address
map descriptors, and allocated virtual memory region descriptors are stored here.
typedef struct
{
MAP code_map;
MAP data_map;
int code_region_id;
int data_region_id;
QUEUE device_regions;
QUEUE heap_regions;
int heap_hint_addr;
} TASK_VM;
/*
/*
/*
/*
/*
/*
/*
map descriptor for code map */
map descriptor for data map */
region id for code memory */
region id for data memory */
region list for mapped devices */
region list for allocated memory */
hint address for next heap allocation */
The map descriptors code map and data map are used by the map module to perform address
translations for the task. When a task is created, the maps are allocated, and region descriptors
are allocated for the executable code and data areas. When a task is scheduled to run, the map
descriptors are passed to the address map activation routine.
The two region queues, device regions and heap regions, are used to remember region
descriptors that are dynamically allocated by the user throughout the life of the task. This
provides an easy way to free all of a tasks memory when it is destroyed: the region lists can be
eciently traversed and the descriptors freed. The heap hint addr eld is used to remember
to next free location in the tasks virtual address space for a heap or device memory allocation
to occur. The hint helps reduce the amount of searching through the task translation table
structures to nd free contiguous regions.
SHARED REGION
structure
The sr eld is a pointer to the tasks' user/kernel shared memory region. The shared region
structure, SHARED REGION, contains information that is commonly accessed by both the user
Chapter 3. Kernel level implementation
58
and supervisor address spaces for scheduling decisions and other operations. This helps reduce
communication costs between the user and kernel spaces.
When a task is created, a physical memory page is allocated for the shared region structure
using rawpage alloc(). The page is mapped read/write into both the user and supervisor
address spaces. The memory location of the page in the supervisor address space is remembered
by the sr eld. In the user address space, the page is mapped at the well known address,
USER SHARED REGION ADDR. The user level kernel code can use this constant to refer to the
structure.
typedef struct
{
/* parameter passing area for system calls */
int syscall_parms[NUM_CPUS][SYSCALL_PARMS_SIZE/4];
int num_cpus;
int num_ready_threads;
/* number of cpus assigned to this task */
/* number of ready threads in task */
void *upcall_addr;
int upcall_status[NUM_CPUS];
int intr_status[NUM_CPUS];
int excp_status[NUM_CPUS];
/*
/*
/*
/*
user upcall dispatch address */
upcall dispatcher events */
interrupt dispatcher events */
exception dispatcher events */
int num_signals;
/* number
int signal_head;
/* signal
int signal_tail;
/* signal
int signal_lock;
/* signal
SIGNAL signals[NUM_SIGNAL_ENTRIES];
of signals in list */
list head index */
list tail index */
list spin lock */
/* global semaphore links */
GLOBAL_SEM_LINK sem_links[NUM_GLOBAL_SEMS];
int argc;
char *argv[NUM_ARGV_VECTORS];
char argv_buf[ARGV_BUF_SIZE];
} SHARED_REGION;
/* number of command line arguments */
/* array of argv vectors */
/* buffer for storing parameters */
The syscall parms eld provides a buer for passing out-of-line parameters through system
calls. For example, when a user calls kprint(), the string parameter is copied into the system
Chapter 3. Kernel level implementation
59
call buer. The kernel kprint() routine can then access the string parameter from the buer.
Ideally, the kernel should be able to access the user's parameters directly. However, most of the
kernel system calls are implemented in C, a language which does not easily permit references
to remote address spaces. So kernel system calls which pass parameters by reference must use
this indirection. Each processor running a task uses its own parameter buer, so that system
calls can safely execute in parallel. Thus, the syscall parms eld allocates enough buers for
every processor in the system.
The num ready threads eld is maintained by the user level threading kernel to count
the number of runnable threads available. num cpus is maintained by the kernel to count the
number of processors currently allocated to the task. These elds are used by both the thread
and task schedulers as the basis for processor allocation decisions.
The upcall addr eld is used by the kernel upcall dispatcher as the address to call in the
user space for upcall events. Initially, when a task is created, this value is set to contain the
entry point for the executable program. This entry point corresponds to the user level kernel
initialization code. When the task runs and initializes itself, it can easily change this value to
any address within its address space, such as the user level upcall dispatcher.
The upcall status, intr status, and excp status are per-processor elds that are used
by the interrupt, exception, and task upcall dispatching mechanism to signal various events to
the user kernel. Each eld maintains a bit eld of possible ags, each ag is set to specify a
certain event, or combination of events.
The set of signal elds belong to the asynchronous task signalling mechanism. Any task
can send a simple signal message to another task using the task signal() system call interface.
These elds are used to implement a FIFO queue for a xed number of signals destined for a
task.
The sem links eld stores an array of queue links for the global semaphore service. Each
link in the array corresponds to a global semaphore entry. When a user thread blocks on a
global semaphore, the thread's task is queued on the global semaphore's queue. When the
Chapter 3. Kernel level implementation
60
semaphore is signalled, the next task in the queue is dequeued and issued a signal.
The arg elds are used to supply Unix-style command line argument information for user
programs. When a task is created, one of the parameters to task create() is a user argument
structure. task create() copies the user arguments into these elds. When the user level
initialization routine executes, the argv vector can be built and passed to the user's main()
thread.
3.9.4 Task descriptor table
Following the general policy of static allocation where possible, the task table[] array stores
all of the system task descriptors. The size of the table is dened at compile time. The table
is declared in the following fashion in task.c:
TASK task_table[TASK_TABLE_SIZE];
QUEUE task_free_q;
int
task_free_q_lock;
/* free descriptor queue */
/* spin lock for free queue */
The table and task free q are initialized at boot time. Each of the task descriptors are
linked together into a free queue. The free queue allows for O(1) allocation and deallocation of
task descriptors.
3.9.5 Upcalling to a task
A task is given a processor for a number of reasons. Each reason is termed an upcall event,
and there are eight dierent upcall events dened in upcall.h. These upcall events are used
in various places throughout the kernel:
#define
#define
#define
#define
#define
#define
RUN_UPCALL
0
TICK_UPCALL
1
RELINQUISH_UPCALL 2
TIMER_UPCALL
3
SIGNAL_UPCALL
4
KILL_UPCALL
5
/*
/*
/*
/*
/*
/*
task can execute now
timer tick upcall
task relinquish request
timed event has expired
task signal is pending
task destroy upcall
*/
*/
*/
*/
*/
*/
Chapter 3. Kernel level implementation
#define EXCP_UPCALL
#define INTR_UPCALL
6
7
61
/* a user exception occurred */
/* a user interrupt occurred */
Each of these upcall events correspond to a bit in the upcall status[] eld in the task's
shared region. These events are described in greater detail in Section 4.1. When an upcall
is issued to a task, the particular event is bit-wise or'ed into the upcall status[] entry for
the processor. Previous bits in the entry are therefore retained, and can be accumulated if
many upcall events are directed to a task before the task actually gets a chance to execute.
At the user level, the upcall dispatcher reads this bit-eld and executes the appropriate upcall
handlers, clearing the event bits as the handlers complete.
For example, when sched() nds a ready task to run, it issues a RUN UPCALL to the task in
the following manner:
curr_task->sr->upcall_status[cpu] |= 1 << RUN_UPCALL;
upcall_now(curr_task->sr->upcall_addr);
/* no return */
The upcall now() routine, from excp dispatch.s, is an assembler code routine that performs the upcall action into user space. Given the user level destination address, upcall now()
sets up the processor context, and executes a \return from exception" rte instruction.
Before a task can be issued an upcall, its memory space must be activated on the local
processor. Also, if the task resides on the ready queue, it may have to be dequeued. The
task upcall() routine performs all of the bookkeeping chores to enable a task and upcall
into its address space. This routine is used in various places in the kernel to facilitate upcall
dispatching.
void task_upcall(int cpu, TASK *task, int upcall_vec);
The task to activate is specied by task, and the upcall event is specied in upcall vec.
The task will be removed from the ready queue if num ready threads <= 1, and its state set to
TASK RUNNING. A task is not removed from the ready queue if num ready threads > 1, which
means that the task is eligible for more processors than the one it is currently receiving.
Chapter 3. Kernel level implementation
62
The task's num cpus eld is incremented and its run cpus[][] entry is set to indicate which
processor the task is running on. Finally, the map enable task() call activates the code and
data memory space, and upcall now() is invoked to pass execution to the user level.
3.9.6 Creating a task
The task create() system call is used to create a task. First, a free task descriptor is dequeued
from the task free q. Then, virtual memory call vm task alloc() is used to allocate the
task's address space and create the shared region storage area. The shared region elds are
then initialized. The syscall parms buer is used to pass the task's id and the region identier
for the nameserver to the user initialization routine. The task is either placed on the ready
queue to be run, or left in the suspended state to be later resumed.
int task_create(int *task_id, int priority, int ready, TASK_EXEC_HDR *hdr,
int code_region, int data_region, TASK_ARGS *args);
The priority parameter species the task's run priority level, an integer between 0 and
31. Priority 0 is low priority, priority 31 is high priority. A high priority task always receives
processors before a low priority task. If ready is passed a nonzero value, the new task will
be immediately readied and executed on an idle processor, or placed on the ready queue if all
processors are busy. If ready is zero, the task will be placed in the TASK SUSPENDED state, to
be later resumed by task resume(). task id returns the created task's descriptor identier.
task create() returns OK if task creation was successful, or FAILED if an error occurs.
The hdr parameter passes the executable header information. This structure contains the
executables code, data, and bss segment addresses and sizes. It also includes the executable
entry point. The caller is responsible for querying the lesystem for this information.
The code region and data region parameters pass the executable code and data regions.
Before the user calls task create(), the executable code and data must be read from the lesystem into two regions. These regions, plus the hdr information, are used by the vm task alloc()
routine to allocate the memory space for executable. In vm task alloc(), once the address
Chapter 3. Kernel level implementation
63
maps are created, vm share() is used to map the code and data regions to the tasks address
space. The caller to task create() can then free the code and data regions from their address
space, or use them for additional calls to task create().
The args parameter points to an argument structure of the following format. The user
must properly initialize this structure before calling task create():
typedef struct
{
int nameserver_id;
int tty_id;
int argc;
int argv_len;
char argv_buf[ARGV_BUF_SIZE];
} TASK_ARGS;
/*
/*
/*
/*
/*
region id of nameserver */
port id of the tty connection */
user argument count */
length in bytes of argv_buf */
user parameters */
The nameserver id eld is used to pass the nameserver region identier to the newly
created task. When the task initializes, it can use this identier in vm share() to map in the
nameserver. The tty id is used by the user level tty driver.c server to pass on the tty
connection identier. When the tty server creates a task, a stdin/stdout port connection is
created for the task to communicate with the server. When the task initializes, it uses the
tty id port to communicate with the server.
The argc eld is set to specify the number of command line parameters. argv len is set to
specify the length in bytes of the parameters in the argv buf eld. argv buf is built to contain
a contiguous list of null-terminated parameter strings. When the task gets a chance to run,
the user level initialization routine uses this information to construct an appropriate Unix-style
argv list to pass to the user's main().
3.9.7 Destroying a task
int task_destroy(int task_id);
Whether a task simply nishes its own execution, or whether a remote task must be killed,
task destroy() does the job. task destroy() is responsible for locating the specied task,
killing it, and returning its resources back to the system.
Chapter 3. Kernel level implementation
64
On a multiprocessor system, the eort required to kill a task is greater than on a uniprocessor system. The additional diculty arises when the task to destroy is executing on remote
processors. In this case, the remote processors must be interrupted and notied that the task
they are executing will be destroyed:
/* check if there are other cpus running the task */
if ( dead_task->sr->num_cpus > 0 )
{
/* interrupt each cpu currently running this task */
intr_remote_cpus(kernel_info->run_cpus[task_id], TASK_DESTROY_SWI,
task_id);
}
The remote interruption facility is provided by the intr remote cpus() call in
interrupt.c, which in this case, delivers a TASK DESTROY SWI software interrupt to all processors executing the specied task. The SWI interrupt handler on the remote processors recognize
this interrupt and discards the dying task.
However, stopping execution and cleaning up the task's memory space and kernel data
structures is not enough to completely remove a task from the system. There are also data
structures at the user level which must be cleaned up. For example, if a task has created and
is the owner of user level IPC ports, the ports must also be properly terminated. Otherwise,
the user level port data structures will be left with stale entries in their descriptor tables. As
another example, suppose a task has established a communication channel with a server. The
task must be given the chance to gracefully sever the connection with its server.
To oer a graceful shutdown for tasks which are to be destroyed, before the kernel frees the
tasks resources, a KILL UPCALL is delivered to the task. The KILL UPCALL handler executes at
the user level, and performs any cleanup that is necessary before returning to the kernel to be
ocially destroyed. While this is taking place, the task's state is set to TASK DYING, indicating
that the task should not be scheduled by remote processors. Figure 3.11 demonstrates this
sequence of events.
Chapter 3. Kernel level implementation
65
task id: 1
task id: 2
kill_handler()
task_destroy(2);
task_cleanup(2);
KILL_UPCALL
sched()
Kernel
Figure 3.11: Destroying a remote task.
When the KILL UPCALL handler has nished cleaning up the user level state, the system call
task cleanup() is used to return control to the kernel. task cleanup() frees all of the task's
memory, returns its descriptor to the free list, and jumps into the scheduler loop sched() to
continue processing other tasks.
If a task is voluntarily killing itself, then task destroy() assumes that the user level code
has already cleaned up its data structures, and therefore dispatching a KILL UPCALL is not
necessary.
3.9.8 Suspending a task
The act of suspending a task in a multiprocessor system has the same problem as destroying
a task: the task may be executing on a remote processor. As with destroying a task, the
intr remote cpus() call can deliver a TASK SUSPEND SWI interrupt to the appropriate processors. Upon receiving the interrupt, the SWI handler invokes a RELINQUISH UPCALL event into
the task. At the user level, the RELINQUISH UPCALL handler places the currently executing
thread back on the thread ready queue, and relinquishes control of the processor back to the
kernel, where another task is scheduled in its place.
Chapter 3. Kernel level implementation
66
int task_suspend(int task_id);
The task suspend() call suspends execution of the specied task. If the task is currently
executing on a remote processor, the processor will be interrupted and forced to schedule
another task. The tasks state is set to TASK SUSPENDED. Returns OK if successful, or FAILED if
the specied task is invalid.
3.9.9 Resuming a task
int task_resume(int task_id);
call resumes execution of a previously suspended task, specied by
task id. If an idle processor is available, a TASK RUN SWI interrupt will be delivered to it, and
the resumed task will be executed there. If all processors are busy, then the task is placed on
the task ready queue where it will be scheduled sometime in the future. Returns OK if successful,
or FAILED if the specied task is invalid.
The
task resume()
3.9.10 Task states
The task descriptor state eld, info.state, maintains the scheduling state of a task. There
are six possible scheduling states, dened in kernel.h:
/* possible states for a task descriptor */
#define TASK_FREE
0
#define TASK_SUSPENDED 1
#define TASK_READY
2
#define TASK_RUNNING
3
#define TASK_IDLE
4
#define TASK_DYING
5
Figure 3.12 summarizes the possible transitions between each state. The use of each state
is described as follows:
TASK FREE:
Is the initial state for all tasks. The task descriptor is queued on the free
queue, task free q, using the tasks sched link.
Chapter 3. Kernel level implementation
67
TASK_DYING
TASK_IDLE
TASK_SUSPENDED
TASK_RUNNING
TASK_READY
TASK_FREE
Figure 3.12: Task state transitions.
TASK READY:
The task currently resides on the ready queue. The task also may be executing on at least one processor.
TASK RUNNING:
TASK IDLE:
The task is executing on at least one processor.
The task is not executing anywhere, and does not require any processors.
TASK SUSPENDED:
The task has been suspended. It will receive no processors.
TASK DYING:
The task is being removed from the system. The state will progress to
TASK FREE when the tasks resources have been properly cleaned up.
3.9.11 Task ready queue
Tasks that require a processor are placed on the task ready queue. The le readyq.c provides a simple interface to a globally shared round-robin priority queue with 32 levels. All
accesses to the task ready queue are done through three routines: enqueue ready task(),
Chapter 3. Kernel level implementation
68
dequeue ready task(), and remove ready task().
As long as this interface remains the same,
the implementation of the ready queue can change.
The ready queue enforces priority levels between 0 and 31. Priority level 0 is low, level 31
is high. A higher priority task will receive a processor before a lower priority task.
Enqueue and dequeue operations are O(1) in the most frequent case. This eciency is
achieved by the use of 32 independent FIFO queues, rather than a single sorted priority list.
Figure 3.13 displays a typical ready queue layout. Using a sorted priority list would guarantee
an O(1) dequeue operation because the rst element on the list is always the highest priority
task. However, the enqueue and remove operation requires a sort operation to rebuild the
list, which for a heapsort structure, requires O(n log(n)) access time (not including extra code
complexity overhead).
readyq_table[]
priority 0
task 3
priority 1
hint
priority 2
task 5
task 1
priority 30
task 0
task 6
task 9
30
priority 31
Figure 3.13: Task ready queue structure.
QUEUE readyq_table[NUM_READYQ_PRIORITIES];
int hint;
int ready_q_lock;
The readyq table[] maintains a statically allocated array of FIFO queues. Each queue is
used to store task descriptors of a single priority level. A single spin lock, ready q lock, is used
to protect the queue from concurrent accesses. The hint integer is used to cache the highest
priority non-empty FIFO queue, and is used to speed up the dequeue operation.
Chapter 3. Kernel level implementation
69
TASK *dequeue_ready_task();
The dequeue ready task() operation dequeues the next available ready task and returns
its descriptor. The routine consults the hint variable to nd the highest level priority with a
non-empty queue. If the hint level queue is empty, a linear search counts down the hint until
the next non-empty queue is reached. The task is dequeued and returned to the caller.
void enqueue_ready_task(TASK *task);
The enqueue ready task() operation tries to nd an idle processor to execute the specied
task. If all processors are busy, then the task descriptor's state is set to TASK READY and is placed
on the end of the appropriate queue. If the task's priority is higher than the hint variable,
then the hint is updated to this priority value.
void remove_ready_task(TASK *task);
A task that resides in the ready queue structure may need to be removed for reasons other
than a processor becoming available to run the task. Sometimes a task must be removed out of
order, from the middle of the queue. For example, a task that is being destroyed or suspended
must be removed from the ready queue. The task descriptor to remove from the ready queue
is specied by the task parameter.
3.9.12 Task signalling
The task signalling mechanism provides a primitive building block for the construction of user
level communication services. A signal is a simple non-blocking one-way message delivered
from one task to another. The task signal() system call performs two operations: wakes up
a destination task, and delivers it a message. The following structure, from shared region.h,
denes a signal message:
Chapter 3. Kernel level implementation
typedef struct
{
int signal;
int data;
} SIGNAL;
70
/* signal message type */
/* message type-defined data */
Any task in the system can be the recipient of multiple signal messages from multiple
sources. To ensure the non-blocking, reliable delivery of a signal message, a FIFO queue of signal
descriptors is allocated to buer messages. The queue is stored as signals[], in the shared
region structure of each task descriptor. Signals are enqueued by the kernel, and dequeued by
the user level signal dispatcher as a result of a SIGNAL UPCALL.
In the event that many senders bombard a single task with signals, the signal queue of
the destination task may become lled. Further signal messages will be discarded. Thus the
delivery of signals is not totally reliable. However, reliability can be ensured for a bounded set
of interactions given a reasonable queue size. The signal queue ts within the task's shared
region page, where it has room for about 400 entries, at 8 bytes each.
Each task contains a set of signal handlers as part of its user level upcall dispatch tree
(c.f. Figure 4.20). One signal handler routine is used for each signal message type. Originally,
the signal mechanism was designed to allow handlers to be added and removed dynamically,
allowing user programs to add their own signal handlers at run-time. However, a static scheme
was implemented for simplicity. The system currently supports six dierent signal message
types:
#define
#define
#define
#define
#define
#define
GLOBAL_SEM_SIGNAL_SIGNAL
GLOBAL_SEM_DESTROY_SIGNAL
GRPC_PORT_SEND_SIGNAL
GRPC_PORT_REPLY_SIGNAL
GRPC_PORT_DESTROY_SIGNAL
GLOBAL_PORT_SEND_SIGNAL
0
1
2
3
4
5
/*
/*
/*
/*
/*
/*
global
global
global
global
global
global
semaphore signal */
semaphore destroy */
rpc send handler */
rpc reply handler */
rpc destroy handler */
port send handler */
These particular signals are used by the user level IPC and semaphore library to synchronize
their cross-address space communication events. For example, when a client invokes an RPC
Chapter 3. Kernel level implementation
71
message to a server, the server must be informed of the waiting message. In this case, the client
RPC library would issue a GRPC PORT SEND SIGNAL, specifying the port identier in the signal's
data eld.
Delivery of a signal
The user level can deliver a signal to a task by two means. The user level kernel determines
which of these two methods is most appropriate:
1. Issue a TASK SIGNAL SWI interrupt to a processor:
intr_remote_cpu(cpu_list, TASK_SIGNAL_SWI, task_id,
GRPC_PORT_SEND_SIGNAL, port_id);
If the destination task is executing on a remote processor, or if there is an idle processor,
then deliver an interrupt to that processor. The SWI service() handler on the remote
processor will queue the signal and dispatch an upcall to the destination task. It is possible
that the remote processor can become busy before the interrupt is actually sent. In this
case, the SWI service() handler will properly dispatch the signal to the destination task
and return to the existing work.
2. Issue a task signal() system call:
task_signal(task_id, GRPC_PORT_SEND_SIGNAL, port_id);
If the destination task is not executing anywhere, and there are no idle processors, then
the kernel level system call task signal() must be invoked on the local processor to
manually queue the signal and wake the destination task. The destination task is woken
by placing it on the ready queue. When the task eventually acquires a processor, its
upcall dispatcher is executed.
In either case, when the destination task obtains a processor, the user level upcall dispatcher
notices that the num signals eld is nonzero, and begins to process the signals. After each
Chapter 3. Kernel level implementation
72
signal entry is dequeued by the dispatcher, the signal eld is used to execute the appropriate
handler. The signal handler can then perform the desired operation using the supplied data
eld. The user level IPC and semaphore libraries contain handlers for each of the above signal
types.
3.9.13 Task timer service
The timer service allows tasks to register for a timed event. User level schedulers can use this
service to implement a sleep facility. A list of tasks, timer q, sorted by their wakeup time, is
maintained by the task timer.c module. This list is shared with the system clock interrupt
service routine, DTI service(), to ready tasks with expired wakeup values. The timer q lock
protects against concurrent accesses to the list. When a timed event expires, the associated
task is readied and delivered a TIMER UPCALL event.
QUEUE timer_q;
int
timer_q_lock;
/* list of tasks waiting for a timer event */
int task_timer_event( unsigned long wake_time );
is called by user level thread schedulers to register for a timed event.
The wake time eld species the future wakeup time. By using the current system clock time,
provided by kernel info->timer ticks, the future wakeup time can be calculated by adding
the desired number of clock ticks. The clock tick interrupt rate is determined at boot time, and
can be congured to any value within the hardware limits: 8.6 microseconds to 563 milliseconds.
Only one timed event is registered per task. Other timed events are queued by the user
level sleep service. Registering an earlier timed event will reset the previous setting to use the
nearer value.
The task descriptor eld timer link is used to link the task into the timer q. A linear
search through the timer q is done to locate the proper waketime slot.
task timer event()
Chapter 3. Kernel level implementation
73
3.10 Kernel management for global semaphores
The task signalling mechanism provides a way for tasks to send low level event messages to
each other. This service can be used to notify remote tasks that an event has occurred. Global
semaphores, on the other hand, provide a way for threads in a task to wait for a remote event
to occur. This service can be used as building block for higher level communication services.
Synpopsis
Interface availability
Description
kernel_sem_enqueue()
user kernel
Enqueues the caller task onto the semaphore.
kernel_sem_dequeue()
user kernel
Dequeues the next task from the semaphore.
kernel_sem_destroy()
user kernel
Dequeues all tasks from the semaphore.
Figure 3.14: Kernel level global semaphore support routines.
For example, a client thread wishing to send a message to a server may have to wait for
a free port buer to become available. The client thread performs a sem wait() on the port
queue, and when the server releases a buer, it issues a sem signal(). The client thread is
then unblocked to dequeue a free port buer and send its message.
While the user level global semaphore library implements the call sem wait() and the call
sem signal(), some kernel level assistance is required to preserve the ordering of sem wait()
calls. Threads must be unblocked by sem signal() in the same order that they were blocked
by sem wait(). Threads from multiple tasks may be blocked on the same semaphore. So
sem signal() must decide which task should be delivered the signal message. To do this, the
kernel implements a task queuing service, consisting of three system calls summarized in Figure
3.14.
These calls manage a set of task queues, one queue for each global semaphore. The queues
are stored in the task sem q[] table, declared in kernel sem.c:
QUEUE task_sem_q[NUM_GLOBAL_SEMS];
When a thread blocks on a global semaphore, the user semaphore library routine
calls kernel sem enqueue() to queue the threads's task descriptor onto the appropriate
Chapter 3. Kernel level implementation
74
entry. When the semaphore is signalled, kernel sem dequeue() is invoked,
which dequeues the task descriptor that contains the blocked thread. The task is then delivered a GLOBAL SEM SIGNAL SIGNAL message using task signal(). The task will then be
scheduled and its upcall dispatcher will unblock the thread.
In the event that a global semaphore holding blocked threads is destroyed, the
kernel sem destroy() call is invoked to traverse the appropriate task sem q[] list, and send
GLOBAL SEM DESTROY SIGNAL signals to each task. Each task will be scheduled and allowed to
unblock its threads.
Threads in a task can be blocked on multiple semaphores, so a task descriptor may be
queued on several task sem q[] queues at once. To allow this, each task descriptor contains
a list of global semaphore links, sem links[], in its shared region structure. A task is queued
on the same semaphore at most once, so the number of links required equals the number of
semaphores. The GLOBAL SEM LINK is dened as follows in shared region.h:
task sem q[]
typedef struct
{
signed int task_id:16;
signed int num_threads_waiting:16;
QUEUE_LINK link;
} GLOBAL_SEM_LINK;
/* this link's task identifier */
/* num threads in this task */
/* semaphore queue link */
eld identies the task descriptor that belongs to the link. When
kernel sem dequeue() removes a task from a semaphore queue, the task id eld is used
to identify which task contains the blocked thread.
The num threads waiting eld counts the number of threads in the task that are
blocked on the semaphore. This count is used by the user level semaphore library and the
kernel sem dequeue() routine to decide when a link should be enqueued or dequeued.
The
task id
Chapter 3. Kernel level implementation
75
3.11 Interrupt management
This section describes the kernel level interrupt handling and dispatching mechanism. Some
interrupts are handled directly by the kernel, such as the system clock tick. Other interrupts,
such as VMEbus interrupts, can be dynamically registered by tasks to call a user level routine.
A table of interrupt handler routines is maintained to keep track of user level and kernel level
handlers.
The interrupt.c module implements the interrupt dispatcher and user level system calls
use to register interrupt handlers. The table in Figure 3.15 summarizes the system call interface
exported to the user level by this module.
Synpopsis
Interface availability
Description
intr_register_user()
user kernel
Registers the task and enables the interrupt.
intr_deregister_user()
user kernel
Disables the interrupt bit and removes handler.
Figure 3.15: Interrupt handler registration system calls.
All interrupts on the 88100 are funnelled through one single exception vector, INT. From
there, the low level interrupt handler properly saves the execution context and branches to the
interrupt dispatcher. The interrupt dispatcher nds the recipient of the interrupt, and either
upcalls into user space to handle it, or calls a local kernel function.
3.11.1 User level preemption
Interrupts preempt user level threads and cause scheduling events to occur. For example, if
a timer interrupt occurs, a TICK UPCALL event is sent to the user level to indicate that it's
time to schedule the next ready thread. In a multiprocessor environment, special care must be
taken to ensure that the rescheduling of threads in the presence of spin-locks does not adversely
aect performance. A naive approach would allow interrupts to occur at any point during
user level execution. This can result in very poor performance if threads are using spin-locks
for concurrency protection. If a thread is preempted while holding a spin lock, then all other
Chapter 3. Kernel level implementation
76
threads that try to access the lock must wait until the original thread is rescheduled and releases
its lock. The original thread may not be rescheduled for some time, causing all other threads to
waste CPU time, uselessly spinning. Since spin locks are used throughout the user level kernel
to protect data structure accesses, this problem must be solved.
The solution implemented in the Raven kernel involves close participation between the
user level and kernel. Whenever the user level acquires a spin lock, a lock count variable
is incremented at the user level. Whenever an interrupt occurs that would cause an upcall
event into user space, the interrupt handler checks the lock count variable. A non-zero value
indicates that a critical section is currently being executed, and control must be returned to
the user. But before the interrupt handler returns control to the critical section, it sets the
upcall pending variable, to indicate that an interrupt occurred. When the user level regains
control and nishes its critical section, the upcall pending variable is checked, and if set, the
thread will save its context and branch to the upcall dispatcher. The upcall dispatcher will
perform the deferred event(s).
As implemented, the two variables, lock count and upcall pending are not stored as
conventional variables at all. Rather, each of them share the processor's r28 register, as shown
in Figure 3.16. This is done to ensure that access to the variables is atomic. For example,
to increment lock count, a single addu r28, r28, 1 instruction is performed. If lock count
was a conventional global variable, then incrementing it would require the use of a spin lock {
which cannot be done, since lock count itself is used within the locking routines.
upcall_
pending
16
lock_count
31
17
15
0
in_upcall
Figure 3.16: Register r28 usage.
Chapter 3. Kernel level implementation
77
A third variable is contained within the r28 register: in upcall, bit 16. This bit is set to
prevent preemption while the user level is processing an upcall event. The interrupt dispatcher
checks this bit whenever an upcall is attempted. If the bit is set, then an upcall is already is
progress. Upcalls cannot be nested, so the user's context is restored, and control returns back to
the point where the interrupt occurred, and upcall processing continues. Any upcall status[]
bits that were set as a result of the interrupt will be properly dispatched.
3.11.2 Handling interrupts
Due to the asynchronous nature of instruction execution on the 88100, low level interrupt handling is a fairly complex operation. Certain situations arise where instructions in the data
unit and oating point pipeline are not properly completed. The interrupt handler must complete these instructions. It is possible that some of the instructions may cause an exception
fault to occur, so the interrupt handler must be able to detect this and handle the exceptions.
Faulted instructions in the data unit pipeline must be decoded and completed by simulating
the instructions in software.
When a device issues an interrupt, and the processor interrupt disable bit (IND) is not set,
the processor stops fetching further instructions and tries to empty its pipelines. The IND bit
is automatically set to disable interrupts while the initial interrupt handler executes. Interrupts
remain disabled while execution continues throughout the kernel.
After the processor nishes cleaning up its internal state, execution resumes at the INT
vector handler in the exception vector table, excp vects. The exception vector table contains
1024 entries, one for each of the possible 88100 exception vectors. The INT vector saves the
user's stack pointer in a temporary scratch register, and branches to the kernel interrupt handler
INT handler() in intr handler.s:
exc1:
br.n
stcr
_INT_handler
r31, cr20
; hardware device interrupt
; cr20 <- user's stack pointer r31
The rst thing the interrupt handler does is check if the interrupt belongs to g88. g88 uses
Chapter 3. Kernel level implementation
78
the ABRT and SWI7 interrupt for its functioning, so any of those interrupts are immediately
redirected to the g88mon handler. The ABRT interrupt is set whenever the user presses controlC on the console. The SWI7 interrupt is used to propagate g88mon interrupts to all processors.
All other interrupts are processed by the kernel.
Interrupts are normally disabled while execution runs within the kernel. The only exception
to this rule is when the kernel is executing the idle loop. If an interrupt occurs during the idle
loop, no register state needs to be saved because the idle loop does not do anything useful. The
interrupt handler checks the processor EPSR3 register mode bit to see if the processor was in
supervisor mode at the time of the interrupt. If so, then the idle loop was running, and the
handler can jump directly to the interrupt dispatcher intr dispatch s() without saving any
state.
If execution was interrupted at the user level, the interrupt handler must save the user's
processor context. The user's context is comprised of 34 user level registers: r1 to r31, fpcr
and fpsr, and the execution address. This context is saved in one of two possible places:
Temporary storage on the kernel stack. If user level preemption has been deferred by
user lock management (i.e., if lock count and in upcall in register r28 are non-zero),
then the register context is temporarily saved onto the kernel stack. These registers are
restored when the interrupt handler completes and returns control to the user.
The user level thread context saved area. If user level preemption is not deferred (i.e.,
if lock count and in upcall in register r28 is zero), then the register context is saved
into a buer maintained at the user level. A pointer to this buer is retained in processor
register r29. When the user level thread scheduler runs a thread, it sets r29 to point to
the threads context save area. The interrupt handler saves the context directly into the
user supplied buer.
The EPSR register is the 88100 exception time shadow register for the PSR register. The EPSR reects the
value of the PSR before the exception occurred.
3
Chapter 3. Kernel level implementation
79
3.11.3 Dispatching interrupts
Once the processor context is saved, INT handler() calls the interrupt dispatcher in
interrupt.c to process the interrupt. There are two interrupt dispatchers:
void intr_dispatch_s( int cpu, int intr_vec, int intr_status );
This dispatch routine handles all interrupts while the processor was executing the idle
loop (i.e., when the processor is in supervisor mode). This dispatcher is fairly simple,
because there are no requirements to clean up a currently executing task.
TASK *intr_dispatch(int cpu, int intr_vec, int *context, TASK *curr_task);
This dispatch routine handles all interrupts occurring while the processor executes at the
user level. The routine has additional duties because the currently executing task must
be properly managed.
The interrupt dispatch module maintains a table, intr table[], which stores the handlers
for each of the 32 possible interrupts. Both of the interrupt dispatch routines use this table to
locate the appropriate interrupt service routine to handle the interrupt. The elements of this
table are dened by the INTR HANDLER structure:
typedef struct
{
TASK *user_task;
void (*routine)(int, int, TASK *);
int lock;
} INTR_HANDLER;
INTR_HANDLER intr_table[NUM_INTERRUPTS];
The user task eld designates the task that should be invoked to handle the interrupt.
This eld is set to NULL if the interrupt handler is local to the kernel. If the handler is local to
the kernel, routine contains the address of the interrupt service routine.
Chapter 3. Kernel level implementation
80
The routines intr register() and intr deregister() manage the intr table[] entries
to allow a user level task or local kernel function to be the recipient of any interrupt.
To handle a kernel level interrupt service routine, the dispatcher simply makes a function
call to the service routine. But for user level handlers, the procedure is more complicated. An
address space switch may be required to activate the proper interrupt handler task.
The dispatcher checks the appropriate interrupt entry, and if the currently executing task
is registered to handle that interrupt, then an INTR UPCALL event is given to the task. The
particular interrupt vector bit is recorded in the shared region intr status[] eld. The user
level interrupt dispatcher examines this eld to determine the proper interrupt handler for the
event. This action is demonstrated by the following code from intr dispatch s():
task->sr->intr_status[cpu] |= 1 << intr_vec;
task_upcall(cpu, task, 1 << INTR_UPCALL); /* no return */
is a non-returning call in the task scheduler that activates the specied task
and issues an upcall into its address space.
If the currently executing task does not handle the interrupt, but another task does, then
the current task must be switched out and the interrupt handling task must be switched in.
But before the current task is placed back onto the task ready queue, the thread which was
interrupted must be cleaned up so that it can be rescheduled. While the register context for
the thread has been properly saved by INT handler(), the user level thread kernel must be
notied that one of its threads has been preempted, so the thread can be placed back on the
user level thread ready queue. To do this, the current task is issued an RELINQUISH UPCALL.
The user level upcall handler will then take the currently executing thread and place it back
onto the thread ready queue. The upcall handler then immediately returns to the kernel via
task intr relinquish(). At this point, the current task is returned to the ready queue, and
the interrupt handler task is activated and issued an INTR UPCALL to handle the interrupt.
The Figure 3.17 owchart summarizes the interrupt dispatching process.
task upcall()
Chapter 3. Kernel level implementation
81
Begin
Did interrupt
occur in the
idle loop?
Yes
Use task_upcall() to
activate and upcall to
interrupt handler task.
Yes
Set upcall_pending ,
INTR_UPCALL flag and
return to user.
Yes
Upcall INTR_UPCALL to
current task.
No
Save context and call
intr_dispatch()
User level
upcalls
deferred?
No
Can current
task handle
interrupt?
No
Upcall RELINQUISH_UPCALL to
user level. Clean up thread and
return to kernel.
Use task_upcall() to activate
and upcall to interrupt handler
task.
Figure 3.17: Kernel interrupt dispatching process.
3.11.4 Kernel managed interrupts
Several interrupt sources are managed directly by kernel level service routines. These interrupt
vectors are the system clock tick timer (DTI), and software interrupt (SWI).
Chapter 3. Kernel level implementation
82
System clock tick
The system clock tick is the heartbeat generator for the kernel. The tick hardware used is the
timer component of the onboard MC68681 DUART chip. The Hypermodule connects the timer
output to the DTI interrupt vector. The MC68681 is initialized at boot time to a default or
user specied interrupt rate.
The clock interrupt service routine, DTI service(), checks the task timer q list, and readies
the tasks that have expired times. If an idle processor is available, the processor is delivered a
TASK RUN SWI interrupt to run the task.
After all such tasks have been readied, a TICK UPCALL event is issued to the currently
executing user level on the local processor. This upcall event is used by the user level as
a thread preemption timer to timeslice threads. The user level kernel counts the number of
TICK UPCALLS it receives, and relinquishes its processor when the value reaches a quantum.
There is a certain amount of trust placed in the user level to properly relinquish control when
its quantum has expired.
Clock ticks are not issued to each processor in the system at once. Only one processor at
a time has its clock tick interrupt vector enabled. When the tick occurs on a processor, the
DTI service() routine advances the tick interrupt to the next processor. Thus, tick interrupts
propagate around all processors evenly in a round-robin fashion. This helps reduce the lock
contention that would occur if all processors tried to access the task data structures that the
same time.
Remote processor interrupts
The Hypermodule contains a software interrupt register (SWI), which allows any processor
in the system to interrupt any other processor. A remote processor is issued an interrupt
when its associated bit is written in the SWI register. An external data structure, SWI MSG in
shared region.h, managed by the kernel software, allows event messages to be recorded about
the interrupt.
Chapter 3. Kernel level implementation
typedef struct
{
int msg;
int task_id;
int data;
int more_data;
int lock;
} SWI_MSG;
/*
/*
/*
/*
/*
83
SWI event message type */
event task id */
event type data */
more event type data */
spin-lock to protect this entry */
SWI_MSG swi_msgs[NUM_CPUS];
Each processor in the system maintains its own swi msg entry in a global swi msgs[]
table. To deliver an interrupt to a remote processor, a message and data is recorded into the
processor's swi msg entry. A lock must rst be acquired on the swi msg entry before access
to it is permitted. The local processor then sets the appropriate bit in the SWI register to
interrupt the remote processor.
The remote processor will receive the interrupt, and use the SWI service() routine to
handle it. SWI service() examines the swi msg entry and performs the appropriate action
based on the message and data.
The swi msgs[] table is allocated on a single physical page which is shared read/write
between all address spaces in the system. Also, the SWI register is mapped into all address
spaces. This allows any address space to eciently deliver a remote interrupt. System calls
from the user level are not required.
The following four interrupt messages can be issued using the swi msg entry:
TASK SUSPEND SWI
{ issued by task suspend() to suspend the execution of a task on a
processor.
TASK DESTROY SWI
{ issued by task destroy() to suspend execution of a task on a pro-
cessor.
TASK RUN SWI { issued by thread and task schedulers
or task.
to initiate execution of a new thread
Chapter 3. Kernel level implementation
TASK SIGNAL SWI
84
{ issued by task signal() to send a signal event to a task.
The interrupt module provides two routines to manage the delivery of remote interrupts.
The caller species a char array list of processors to interrupt. A non-zero entry in the list
indicates that the associated processor should be delivered an interrupt.
void intr_remote_cpus( char *cpus, int msg, int task_id );
This routine is used to send an interrupt to a list of processors, specied by the list cpus.
The swi msg entry for each processor in the list is acquired, the message is written, and
an interrupt is delivered.
int intr_remote_cpu( char *cpus, int msg, int task_id );
This routine is used to send an interrupt to one of the specied processors in the cpus
list. If the swi msg lock cannot be acquired (i.e., if the swi msg is currently being used
to deliver an interrupt by another processor), then the processor is skipped and the next
one in the list is tried. The routine returns OK after the rst interrupt is delivered, or
FAILED if no swi msg entry could be acquired.
These two routines are not exported to the user level. Since the swi msgs[] table and
the SWI register are mapped into the user address space, similar routines can be implemented
there, avoiding system calls to the kernel.
3.12 User/Kernel Shared memory regions
The Raven kernel relies on shared memory regions to help reduce communication costs between
the user and kernel levels. These regions contain high-use information that is frequently accessed
when making scheduling decisions at the kernel and user levels. Instead of invoking system calls
to pass this information between the user and kernel, read and write operations to the shared
regions are used to perform the same eect. Simple read and write operations are much cheaper
than system calls.
Chapter 3. Kernel level implementation
85
However, the use shared memory regions at the user level opens up system security holes.
Errant program behaviour and rogue processes can adversely modify the information contained
in the shared regions, causing any number of execution problems ranging from improper scheduling of tasks to fatal system crashes. This section summarizes the shared regions and describes
techniques used to help avoid their abuse.
3.12.1 The shared regions
There are three shared memory regions between the user level and kernel. The table in Figure
3.18 summarizes these regions and their protection status.
Region name
User Protection
Description
KERNEL_INFO
read-only
General purpose kernel state.
SHARED_REGION
read/write
Scheduling information between user/kernel.
SWI_MSGS
read/write
For passing SWI interrupt messages.
Figure 3.18: User/Kernel shared memory regions.
The KERNEL INFO structure is a read-only page that is mapped into all user level address
spaces. It is mapped read/write into the kernel address space.
The SHARED REGION structure is a read/write page that is pair-wise mapped between the
user/kernel address space. Each task maintains its own SHARED REGION structure for scheduling
purposes.
The SWI MSGS structure is a read/write page that is mapped into all address spaces. It is
used to pass software interrupt event information between processors. All address spaces require
access to this page so that software interrupts can be invoked without kernel intervention.
3.12.2 Abuse Prevention
The current technique used by the Raven kernel to reduce the tampering of shared memory
regions is to map the regions at non-obvious locations in the virtual address space. If regions are
allocated in sparse areas of the address space, chances are that erroneous or malicious program
Chapter 3. Kernel level implementation
86
behaviour will result in memory access exceptions before the shared regions are discovered.
However, this technique is certainly not failsafe.
In order to fully protect shared memory regions at the user level from invalid access, the
processor must have the ability to mark sections of memory in the user space as privileged.
Accessing these sections would require the executing code to have the same level of privilege.
The user level kernel could use this feature to enable its code and shared regions with a certain
level of privilege, higher than the level given to user application code. However, most modern
microprocessors, including the 88100, do not allow this kind of exibility, so alternatives must
be considered.
One alternative to mimic multiple levels of privilege within the same address space is to dynamically change the address mapping tables as the privileged code executes. When a function
in the user kernel requires information from a shared region, it could map in the appropriate
page, access the data, and unmap the page. Special purpose address mapping system calls
could be coded very eciently in assembler for this purpose. However, additional costs such as
TLB misses and ushing could make this approach impractical. If the translation tables were
accessible to user programs, then these system calls could be avoided, but the security situation
would be even worse due to the exposure of translation tables.
Other techniques to improve security in this area are being investigated.
Chapter 4
User level kernel implementation
Each user level task running in the system requires a user level kernel library. This library
contains all of the operating system services that are not implemented in the supervisor kernel,
such as: thread management, synchronization primitives, and interprocess communication.
This chapter discusses the implementation of the user level kernel, and how it interacts with
the supervisor kernel and user programs.
User level application code
Thread management
Synchronization
threads.c
task_signal.c
Interprocess
communication
ports.c
rpc_port.c
ctxsw.s
semaph.c
grpc_port.c
ready_queue.c
lock.s
global_port.c
System call
dispatching
upcall.c
exception.c
interrupt.c
syscalls.c
syscall.s
Upcall and interrupt dispatching
Figure 4.19: User level kernel source code organization.
Figure 4.19 shows the modular breakdown of the user level kernel, and the source code les
involved. This section begins by discussing the user level upcalling dispatching mechanism. We
87
Chapter 4. User level kernel implementation
88
then then continue by describing the threading environment and support modules.
4.1 Upcall handling
When a task is loaded into the system, task create() allocates a memory space, as in Figure
3.8, and preallocates an initial stack for the task to execute in. This stack is known as the
upcall stack, and is used by the processor to handle upcalls events into the task. Since upcall
events to the same task can occur in parallel on multiple processors, a separate upcall stack for
each processor is required. The upcall stacks are located at well known location at the end of
heap space.
User level task
run_
handler()
DACC_
handler()
ether_
handler()
DI_
handler()
tick_
handler()
relinquish_
handler()
timer_
handler()
kill_
handler()
run_
handler()
excp_
dispatch()
intr_
dispatch()
signal_
handler()
tick_
handler()
timer_
handler()
relinquish_
handler()
kill_
handler()
upcall_
dispatch()
Kernel level
Figure 4.20: User level upcall dispatching routines.
When the kernel upcalls into user space, it places the upcall event ags into the task's
shared region upcall status[] entry1 . All upcalls into the user space are funnelled through
a single dispatch routine, upcall dispatch() in upcall.c. upcall dispatch() scans the
1
The this cpu() function returns the local processor number.
Chapter 4. User level kernel implementation
89
entry for event bits and handles each of the agged events by calling the
appropriate upcall handler, clearing the event bits as it goes. Figure 4.20 shows a typical set of
upcall handlers, including an exception handler routine and two interrupt device driver routines.
Preemption is deferred at all times on the local processor during upcall handling. This
prevents nested upcalls to occur. For example, if an interrupt occurs during upcall dispatching,
the interrupt event is recorded, but control returns directly to the user level to proceed with
upcall dispatching. The upcall dispatcher will notice the occurrence of any event by examining
its upcall status[] entry, and handle it appropriately. Preemption is deferred by setting the
in upcall ag in the processor register r28. The kernel level upcall dispatcher examines this
ag before issuing any upcall, and if set, does not issue an upcall.
The upcall mechanism maintains a table of upcall handlers, upcall table[]. This table
is comprised of a static list of upcall handler functions, one for each upcall event type. The
kernel task management module denes 8 possible upcall events that must be handled by the
dispatcher. The following Figure 4.21 describes the actions performed by each of the upcall
event handlers.
A second upcall dispatch routine upcall resume() is provided to invoke dispatching by the
user level spin lock library. When a spin-lock is held, upcall preemption is deferred until the
lock is released. The lock release code checks to see if an upcall is pending, and if so, branches
to upcall resume() to dispatch the pending upcalls.
While upcall dispatch() is invoked by the kernel in the upcall context, upcall resume()
is invoked within the calling thread's context. So before proceeding with upcall dispatching,
upcall resume() must rst save the thread's context and put the thread back on the ready
queue.
upcall status[]
4.1.1 User level interrupt and exception handlers
All of the entries in the upcall table[] are dened at compile time to point to well known
handler routines in the user level kernel. However, two of these upcall handlers, excp handler()
Chapter 4. User level kernel implementation
90
Upcall Event
Handler function
RUN_UPCALL
run_handler()
Description
TICK_UPCALL
tick_handler()
RELINQUISH_UPCALL
relinquish_handler()
TIMER_UPCALL
timer_handler()
SIGNAL_UPCALL
signal_handler()
At least one message is waiting in the shared region
signal queue. Dequeue each signal message and call
their signal handlers.
KILL_UPCALL
kill_handler()
The task scheduler has destroyed this task, so the user
level state must be cleaned up, and control returned to
the kernel.
EXCP_UPCALL
excp_handler()
A processor exception has occurred. The handler checks
the excp_status[] entry to determine the exception
code and branches to the appropriate exception handler.
INTR_UPCALL
intr_handler()
A device interrupt occurred that is registered for this
task. The intr_status[] entry is used to determine
the interrupt vector and user interrupt handler.
The task has been given a processor, and can begin
scheduling threads.
A system clock tick occurred, schedule another thread. A
quantum count is incremented, and if expired, the
processor is relinquished to the kernel.
The task scheduler has requested that this task
relinquish the processor.
The task timer service has indicated that a timed event in
this task has expired. Check the list of sleeping threads
and ready any threads that have expired waketimes.
Figure 4.21: Summary of upcall events and the user level upcall handlers.
and intr handler(), furnish a level of indirection to user provided handlers.
The user provided handlers are invoked from within the context of the upcall dispatching
mechanism. Preemption is deferred, and execution runs on the upcall stack. Handlers that execute within this context must not invoke thread management routines that may cause a thread
context switch to occur. Thus, handler routines must be careful not to block on semaphores or
such. Spin locks are allowed because they do not involve context switching.
Interrupt handers
The interrupt.c module maintains a table of user provided interrupt handlers, intr table[].
Each entry in the table contains a pointer to a handler routine:
Chapter 4. User level kernel implementation
91
typedef struct
{
void (*routine)(int);
int lock;
} INTR_HANDLER;
INTR_HANDLER intr_table[NUM_INTERRUPTS];
The following two routines are provided by the interrupt.c module to manage the entries
in intr table[]:
int intr_register( int intr_vec, void *handler );
This routine registers a user supplied handler routine to be called whenever the hardware
interrupt vector intr vec is signalled. The list of hardware interrupt vectors are listed
in the kernel le registers.h. The kernel system call intr register user() is called
to register the interrupt with the kernel and set the interrupt enable bit for the vector.
Returns OK if successful, or FAILED if an error occurred.
int intr_deregister( int intr_vec );
This routine disables the interrupt vector intr vec and removes the user level handler
routine from the intr table[]. The kernel system call intr deregister user() is used
to disable the interrupt enable bit for the vector. Returns OK if successful, or FAILED if
an error occurred.
Exception handers
The exception.c module is basically a mirror of the interrupt.c module. It provides the
same functionality, except that its attention is directed towards processor exceptions rather
than interrupt exceptions.
One use for the user level handling of exceptions is to trap the data access exception DACC.
This exception occurs whenever a memory load or store operation happens outside of the user
level address space. This could be used to implement a user level paging mechanism. Or,
Chapter 4. User level kernel implementation
92
a handler could be constructed that would detect thread stack overows and automatically
allocate more room. Handlers for other exceptions, such as code violations and division by
zero, could be written to destroy the oending thread context, rather than destroying the
whole task.
exception.c maintains a table of user provided exception handlers, excp table[]. Each
entry in the table contains a pointer to a handler routine:
typedef struct
{
void (*routine)();
int lock;
} EXCP_HANDLER;
EXCP_HANDLER excp_table[NUM_EXCEPTIONS];
Similar to the
excp table[]:
interrupt.c
module, two routines are provided to manage the entries in
int excp_register( int excp_vec, void *handler );
This routine registers a user supplied handler routine to be called whenever the exception
vector excp vec is signalled. The list of hardware exception vectors are listed in the kernel
le registers.h. The kernel system call excp register user() is called to register the
exception with the kernel. Returns OK if successful, or FAILED if an error occurred.
int excp_deregister( int intr_vec );
This routine removes the user level handler routine from the excp table[], and calls the
kernel system call excp deregister user() to deregister the exception from the kernel.
Returns OK if successful, or FAILED if an error occurred.
4.2 User level spin-locks
The user level spin locking code, lock.s, provides the same interface as the kernel version, but
it is implemented quite dierently at the user level. The user level spin-lock mechanism needs
Chapter 4. User level kernel implementation
93
to prevent thread preemption while a lock is held. If a thread is preempted while holding a lock,
other threads in the system will busy-wait trying to acquire the lock until the thread is rescheduled and the lock is released. To solve this problem, a special locking protocol is implemented
between lock wait()/lock free() and the kernel level upcall dispatcher mechanism.
void lock_wait(int *lock);
The lock wait() call tries to acquire a lock by spin-waiting on a cached copy of the lock,
just like the supervisor kernel version. However, when it does acquire the lock, the lock count
variable in register r28 is incremented to notify the kernel level upcall dispatching mechanism
that a lock is held. If an interrupt or other task scheduling event occurs, the kernel upcall
dispatcher checks the locks held counter and defers the upcall event if it is non-zero. The
upcall dispatcher will not preempt the user level when the lock count is non-zero.
void lock_free(int *lock);
writes a 0 value to the lock variable, and decrements the lock count variable.
If lock count reaches zero, and an upcall is deferred (bit 16 of r28 is set), then control is
passed immediately to the upcall resume() call. upcall resume() invokes the user level
upcall dispatcher, where the deferred upcall is properly handled.
lock free()
4.3 Thread management
The thread management library manages the creation, destruction, and scheduling of lightweight
threads of control. Threads are preemptively timesliced, and and freely migrate from processor
to processor, in an eort to balance workload. A central 32 level round robin priority queue
manages the scheduling ordering of threads.
4.3.1 Thread descriptor structure
The thread descriptor control block structure,
that makes up a thread:
TCB
dened in
kernel.h,
comprises the data
Chapter 4. User level kernel implementation
typedef struct
{
int id;
int state;
int priority;
char name[THREAD_NAME_SIZE+1];
/*
/*
/*
/*
94
thread identifier */
scheduling state */
priority level, 0 to 31 */
string name for thread */
int stack_region;
int *context;
/* VM region of thread stack */
/* thread register context area */
int sem_wait_id;
unsigned int wake_time;
/* sem this thread is waiting on */
/* time to wake this sleeping thread */
QUEUE_LINK link;
int lock;
} TCB;
/* queue link in system queues */
/* spin-lock for this thread */
TCB thread_table[THREAD_TABLE_SIZE];
TCB *thread_free_q;
int thread_free_q_lock;
All thread descriptors are stored within the static thread table[] array. At initialization
time, free thread descriptors are linked into the thread free q, to make descriptor allocation
an easy dequeue operation.
Threads are referred to by their index into this table, the identier eld id. The state
eld maintains the threads scheduling state. priority contains the scheduling priority for the
thread, a value from 0 to 31. The name eld stores a string name for the thread (useful for
debugging purposes).
Each thread is allocated a page-aligned context save area and stack from the kernel memory
allocator, vm alloc(). The region descriptor belonging to the stack is saved in the stack region
eld. The address of the context save area is stored in the context eld. The thread's stack is
positioned immediately below this context buer. Figure 4.22 shows this relationship.
The sem wait id eld is used by the local semaphore library to store the identier of the
semaphore that the thread is blocked on. If the thread is ever destroyed while blocked on a
semaphore, sem wait id is consulted to remove the thread from the appropriate semaphore
Chapter 4. User level kernel implementation
95
Context save area,
256 bytes
context
Allocated page
aligned from
vm_alloc()
Thread stack buffer
Stack pointer r31,
grows downward.
Figure 4.22: Thread context save area and stack buer.
entry.
The wake time eld is used by the thread sleep() facility to record the future wakeup
time of the thread.
The link eld is a queue link that links the thread descriptor into the various user level
thread management queues. There are several of these queues: the ready queue, semaphore
queues, and IPC queues.
Finally, the lock eld provides a spin lock to protect against concurrent accesses to the
thread descriptor data structure.
4.3.2 Thread context switching
The low level thread context switching routines provide the basic mechanism to load and save
the thread execution context. The thread context is a collection of 29 general purpose registers:
r1 { r25, the frame pointer r30, the stack pointer r31, and oating point registers fpcr and
fpsr. In addition to the processor registers, a thread instruction pointer is also maintained.
Two assembler routines are provided by the ctxsw.s module to assist in the saving and
loading of thread contexts: load context() and save context().
void load_context(int *context, int *thread_lock);
This routine loads the processor registers with the context buer provided by context. The
thread descriptor must be previously locked to prevent writes to the context area while it is being
loaded (if an interrupt occurs in the middle of loading a thread's context, the interrupt handler
Chapter 4. User level kernel implementation
96
must not overwrite this context or inconsistency would result). Processor register r29 is set to
point to the context buer. (At interrupt time, the kernel interrupt handler consults r29 to
nd the context save address.) When the context is nished being loaded, the thread descriptor
is unlocked (hence the reason for passing the thread lock parameter), and a jump instruction
branches to the address contained in the thread instruction pointer. load context() never
returns to the caller.
A thread's context is saved in one of two ways: by the kernel interrupt handler when
a interrupt occurs, or by the thread scheduler to switch out a running thread. The thread
scheduler uses save context() to save the calling thread's context registers.
int save_context(int *context, int *thread_lock);
save context() saves the
calling thread's register context into the supplied context buer.
Once saved, the thread descriptor lock is released. save context() returns a non-zero value
to the caller. When the thread context is loaded again sometime in the future, the thread
instruction pointer returns back to the same location where save context() was called from.
Except in this case, a zero value is returned. Therefore, the caller must check the return
value to see if execution is continuing, or if the thread has been restored. The following code
demonstrates this:
/* save the thread's context and drop through */
if ( save_context(running_thread->context, &running_thread->lock) )
/* When thread context is reloaded, control returns here. */
/* This return statement jumps back to the thread.
*/
return;
/* normal execution drops through to here */
4.3.3 Thread ready queue
The thread scheduler uses a 32 level round robin priority queue to order ready threads. Priority
level 0 is low, priority level 31 is high. A low priority thread will never be scheduled to run if
Chapter 4. User level kernel implementation
97
there is a higher priority thread waiting. However, this ordering does not span tasks. A low
priority thread in a remote task will be allowed to run even though there are higher priority
threads in the local task.
The ready queue.c module implements a priority queue structure and interface that is
similar to the kernel level task ready queue. The readyq table[] keeps track of 32 queues, one
queue for each priority level. The hint variable is used to cache the highest priority level with
a non-empty queue, to help speed up dequeue operations. A spin-lock ready q lock protects
against concurrent accesses to the ready queue structures.
static QUEUE readyq_table[NUM_READYQ_PRIORITIES];
static int hint;
static int ready_q_lock;
Three routines are provided which operate on the above structures:
TCB *dequeue_ready_thread();
Dequeues the next available ready thread from the ready queue, and decrements the
shared region num ready threads counter.
void enqueue_ready_thread(TCB *thread);
Puts the thread descriptor thread into the THREAD READY state, enqueues it onto the ready
queue, and increments the shared region num ready threads counter. If an idle remote
processor is available, an TASK RUN SWI interrupt will be delivered to that processor to
run the thread. Otherwise, if a processor has not been previously requested, the kernel
system call task request cpu(), drops down to the task scheduler to request another
processor for the task.
void remove_ready_thread(TCB *thread);
Removes the specied thread descriptor from the ready queue structure, and decrements
the shared region num ready threads counter. This is used by the thread destroy() and
thread suspend() routines to remove a thread from anywhere within the ready queue.
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98
4.3.4 Thread scheduling
The thread scheduler is the heart of the user level kernel. When the upcall dispatcher nishes
handling all of the upcall events, control is passed to the scheduler. Ready threads are dequeued
from the ready queue, and executed via load context(). When a thread blocks, the scheduler
saves the thread's context, and schedules a new thread.
There are two main thread scheduling routines: sched() and sched no save(). These
routines are used internally by the thread package, and are not intended for general purpose
use by user programs. A third interface, thread sched() is available to user programs for
voluntarily relinquishing control.
void sched(TCB *running_thread);
The sched() routine is responsible for saving the running thread context and dispatching
a new thread. The caller is assumed to already have placed the running thread descriptor on
the proper queue; this routine only saves the context, it does not do any queuing. After saving
the caller's context, sched() dequeues the next ready thread, sets its state to THREAD RUNNING,
and performs load context() to execute the thread. If there is no ready thread available, the
task relinquish() system call relinquishes control of the processor back to the kernel task
scheduler.
void sched_no_save();
is the same as sched(), except that the caller's context is not saved. This
is used primarily as the nal call by the upcall dispatcher, which doesn't run in a thread context,
to begin scheduling threads. sched no save() dequeues the next ready thread, sets its state
to THREAD RUNNING, and performs load context() to execute the thread. If there is no ready
thread available, the task relinquish() system call relinquishes control of the processor back
to the kernel task scheduler.
sched no save()
Chapter 4. User level kernel implementation
99
void thread_sched();
is a user-callable thread library routine. This call performs a voluntary
thread relinquishment. The caller thread is placed on the ready queue, and sched() is called
to save the thread context and schedule another thread.
thread sched()
4.3.5 Thread creation
All threads are created using the library call thread create(). thread create() allocates a
free thread descriptor from the thread free q, allocates a stack, and sets up the thread's initial
execution context. The execution context is built from the thread entry point, exit point, initial
stack address, and parameters. The exit point for a thread is called when the thread \runs o
the end" of its function, and is set to the thread destroy() call.
int thread_create( int *thread_id, void (func)(), char *name,
int priority, int stack_size, int ready, int num_args, ...);
species the entry point for the thread. name is a null-delimited string which identies
the thread (useful for debugging purposes). priority species the ready queue scheduling
priority.
stack size species the size of the buer to allocate for the thread's stack. Currently, all
stack buers are allocated using the kernel memory allocator, vm alloc(). Thus, stacks are
always allocated on page boundaries. The smallest stack size is therefore 4096 bytes.
The ready eld species the scheduling state that the thread should be placed in after it
is created. A non-zero value passed in ready signies that the thread be placed on the ready
queue and executed if a processor is available. If zero is passed in ready, the thread is placed
in the THREAD SUSPENDED state, to be later resumed by thread resume().
The num args eld species the number of parameter arguments to pass to the created
thread. The thread arguments are specied immediately after the num args eld as a variable
argument list.
func
Chapter 4. User level kernel implementation
100
If the thread creation was successful, the thread identier is returned in thread id, and OK
is returned. Otherwise, FAILED is returned if an error occurred.
4.3.6 Destroying a thread
The thread destroy() routine removes a thread from the local task. The thread's stack is
freed, and the thread descriptor is placed back onto the thread free q. In a multiprocessor
environment, however, the specied thread to destroy may be executing on a remote processor.
In this case, an interrupt must be delivered to the remote processor to suspend execution of
the thread.
If the currently executing thread is destroying itself, then additional work needs to be done
to properly free the threads stack buer. The currently executing thread cannot free its own
stack, or else execution will have no stack to continue with. Instead, the thread descriptor
is placed in the THREAD DYING state, and is queued onto the thread dying q. A special lowpriority idle cleanup() thread is created at initialization time, which waits for threads to be
placed on the thread dying q, and frees the thread stacks when they become available.
int thread_destroy( int id );
The thread to destroy is specied by id.
4.3.7 Suspending and resuming a thread
A thread can suspend execution of itself using the thread suspend() call. This routine places
the thread descriptor into the THREAD SUSPENDED state, and will not be executed again until it
is resumed via thread resume().
int thread_suspend();
This function suspends the caller thread indenitely.
int thread_resume( int id );
This function resumes normal scheduling priority of the specied thread.
Chapter 4. User level kernel implementation
101
4.3.8 Sleeping a thread
The thread sleeping facility is used to suspend the execution of threads for a predetermined
length of time. The sleep facility is built on top of the task timer service, which generates upcall
events at specied times.
The timer handler() upcall routine and thread sleep() routine share a sorted queue,
thread sleep q. Sleeping threads are placed on this queue, sorted in the order of wakeup
time. When timer handler() is called, expired threads on the thread sleep q are placed on
the ready queue and executed.
int thread_sleep( unsigned long sleep_time );
puts the caller thread to sleep for the specied number of clock ticks. The
number of clock ticks per second is a value that is interactively set at kernel boot time.
thread sleep()
4.4 Semaphore management
Semaphores provide a way to synchronize actions and communicate events between threads. A
common use for semaphores is to provide mutual exclusion around a piece of sensitive code.
Other uses include controlling producer/consumer type problems between threads. For example,
the interprocess communication system uses semaphores to synchronize message passing events
between threads.
The user level semaphore library, sem.c and global sem.c, provides two types of semaphores
through the same function call interface. Lightweight semaphores, local to an address space
can be created for exclusive use between threads in the same address space (used by the local
IPC implementation). Global semaphores can also be created, which allow threads in remote
address spaces to synchronize between each other (used by the global IPC implementation).
The global semaphore implementation requires special hooks into the kernel, and is therefore
slightly more costly in terms of performance. A parameter in the sem create() call allows the
caller to specify whether a local or global semaphore should be created.
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102
4.4.1 Local semaphores
Local semaphores are used only between threads in the same address space. They are specially
optimized for the local case, and no kernel involvement is required. The semaphore library
keeps track of each semaphore using a statically allocated table of descriptors:
typedef struct semaph_s
{
int id;
/* semaphore identifier */
int state;
/* SEM_FREE or SEM_USED */
int sequence;
/* for deletion protection */
int count;
/* semaphore count variable */
int lock;
/* spin-lock protecting this entry */
QUEUE wait_q;
/* queue of waiting threads */
struct semaph_s *next_sem;
} SEMAPH;
SEMAPH sem_table[SEM_TABLE_SIZE];
SEMAPH *sem_free_head;
int
sem_lock;
/* semaphore free list */
/* spin-lock protecting free list */
The total number of semaphores in a task is bounded at compile time by the SEM TABLE SIZE
constant in the sem.h le. Each semaphore descriptor maintains a count value and a queue for
storing blocked threads. The sequence eld allows the semaphore routines to check whether
a deletion has occurred. Each semaphore descriptor is linked into a free list, sem free head,
making semaphore allocation a simple operation.
4.4.2 Global semaphores
Global semaphores can be used to synchronize events between threads in separate address
spaces, as well as their local address space. The global semaphore routines are similar in
semantics to the local case. However, they dier completely in implementation, some kernel
support is necessary.
Each address space shares a global semaphore table, global sem table dened as:
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typedef struct
{
int lock;
int free_sem_q;
GLOBAL_SEM sems[NUM_GLOBAL_SEMS];
} GLOBAL_SEM_TABLE;
GLOBAL_SEM_TABLE *global_sem_table;
This structure is allocated by the rst task in the system, and is shared between all further
created tasks. The main element in this structure is the table of semaphore descriptors, sems.
The free sem q eld implements a list head pointer to start a linked list of free semaphore
descriptors.
The rst task that is loaded into the system uses vm alloc() to allocate a region for the
table. The region identier is then stored in the global nameserver. Future tasks that are
loaded into the system query the nameserver for the region identier, and map the table in
using vm share().
The global semaphore descriptors stored in sems comprise the semaphore's main data structure unit:
typedef struct
{
int id;
int owner_task;
int sequence;
int count;
int lock;
int next_sem;
} GLOBAL_SEM;
/*
/*
/*
/*
/*
/*
semaphore identifier */
owner task_id of this semaphore */
destroy sequence counter */
semaphore count */
spin-lock protecting this entry */
semaphore free list next pointer */
Other data structures involved in global semaphore operation reside in the local shared
region structure, and in the kernel. These data structures are used to support the queuing of
blocked threads in disjoint address spaces. Threads in dierent tasks can block on the same
global semaphore. The sem signal() signal operation will unblock one of the threads, but
which one? A fair protocol would unblock the thread that has been waiting the longest.
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The local semaphore implementation supports this fairness by using a FIFO queue to sort
the blocked thread descriptors. A queue is also used in the global semaphore case to provide
fairness. However, this queue stores task descriptors, not threads.
Each global semaphore in the system has a task queue associated with it. This queue links
together all the tasks in the system that contain threads which are blocked on the semaphore.
Since multiple threads in the same task can block on several dierent semaphores, each task
in the system could be linked onto several dierent global semaphore queues. Thus, each task
needs to maintain a table of queue links, one for each possible global semaphore.
The table of queue links for each task is located in the task's kernel/user shared region, as
sem links[]. When a thread in a task blocks on a global semaphore, the task is linked onto
the semaphore queue using its local link from sem links[]. When a thread in another task
invokes the sem signal() operation, the next task in the queue is dequeued and delivered a
GLOBAL SEM SIGNAL SIGNAL message using the task signal() mechanism. The signal handler
for this message then wakes up and runs the appropriate thread.
The semaphore task queues are maintained within the kernel. Whenever a user thread
blocks on a semaphore, a system call is required to place the local task on the semaphore
queue: kernel sem enqueue(). However, since the task is now queued on the semaphore,
further threads that block on the same semaphore do not invoke the kernel operation. Likewise,
the kernel sem dequeue() system call removes the next task in the queue and delivers it a
signal.
4.4.3 Waiting on a semaphore
The sem wait() call decrements the semaphore count variable. If the count remains zero and
above, then the call returns immediately. Otherwise, the calling thread is enqueued onto the
semaphore's queue, and the thread scheduler is called to run the next thread. The thread will
remain on the semaphore's queue until it is unblocked by sem signal() or sem destroy(), or
until the thread is destroyed.
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int sem_wait( int sem_id, int no_block );
The semaphore is specied by sem id. If no block is set to a non-zero value, then sem wait()
will always return to the caller, regardless of the value of the count variable. This function returns OK if successful, WOULD BLOCK when no block is set and the call would normally block,
DESTROYED if the semaphore was deleted, or FAILED if the specied sem id is invalid.
4.4.4 Signalling a semaphore
The sem signal() call increments the semaphore count variable, and readies the next waiting
thread from the semaphore's queue using enqueue ready thread(). The readied thread will
eventually resume execution when a processor is available.
int sem_signal(int sem_id);
The sem id parameter species the semaphore to signal.
4.4.5 Allocating a semaphore
The sem create() call allocates a free semaphore descriptor and initializes the descriptor entries. The next free semaphore descriptor is dequeued from the sem free head queue. The
initial value of the semaphore count can be specied by the caller. Either global or local
semaphores can be allocated by specifying the global parameter.
int sem_create( int *sem_id, int count, int global );
The initial semaphore count value is specied by the parameter count. Passing a non-zero
value as the global parameter will create a global semaphore, otherwise a local semaphore will
be created.
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4.4.6 Destroying a semaphore
When a program is nished with a semaphore, it should be returned to the system using the
sem destroy() call. Destroying a semaphore requires more work than enqueuing the descriptor
on the free list, however. Any blocked threads waiting on the semaphore must be released, or
they would remain blocked forever. The blocked threads are dequeued one at a time, and readied
with the enqueue ready thread() call. Before releasing the blocked threads, the semaphore
sequence eld is incremented. When a thread resumes execution in sem wait(), it checks the
sequence value against the previous value. If the values dier, then sem wait() can return a
DESTROYED value.
When destroying a global semaphore, however, any remote tasks that are queued behind the
semaphore must be dequeued and delivered a GLOBAL SEM DESTROY SIGNAL message. Since the
global semaphore queues are maintained within the kernel, the kernel sem destroy() system
call performs this function.
4.4.7 Kernel intervention
The global semaphore library requires some special purpose kernel support for its operation.
One of the main goals of the system is to reduce the overall number of these kernel interactions.
This subsection summarizes the situations where kernel system calls are necessary during the
global semaphore operations.
The following steps are executed by the semaphore wait operation. A kernel call is only
necessary if there are no blocked threads on the semaphore.
1. Return if semaphore count is greater than zero.
2. Block calling thread.
3. Call kernel sem enqueue() if this is the rst thread in the local task blocked on the
semaphore.
The semaphore signal routine makes a system call only if there is a waiting thread:
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1. If the semaphore count indicates a blocked thread, call kernel sem dequeue().
2. Return to caller.
4.4.8 Miscellaneous semaphore operations
The following operations can be useful in certain situations.
int sem_count( int sem_id );
This function returns the count value of the specied semaphore.
int sem_reset(int sem_id, int count);
This function allows the caller to change a semaphore count value.
4.4.9 Semaphore library initialization
The sem init() semaphore library initialization routine rst initializes the local semaphore
data structures. The sem table[] is initialized and a free list q is created.
Then, and the global semaphore initialization routine is invoked. This routine allocates
or maps in the global semaphore table, and initializes the local data structures. The global
nameserver is queried to see if the GLOBAL SEM REGION NAME string exists in the database. If
so, the region identier for the semaphore table is returned, and vm share() is used to map in
the memory. If the string is not registered, then a new table is allocated with vm alloc(), and
its region identier is stored in the nameserver database.
4.5 Interprocess communication
The user level kernel supports both port-based synchronous send/receive/reply and asynchronous send/receive communication models. These models are suciently dierent in implementation that each require its own library interface: the synchronous port library, rpc port.c
and grpc port.c, and asynchronous port libraries, ports.c and global ports.c.
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The port based approach to interprocess communication uses a port number as the mailbox
address for messages. In a client/server model, the server waits for messages to be received on
a port, the client sends messages to a port. When a server cannot receive messages as fast as
they are sent, the messages are buered by the port library. A port must be properly created
before any IPC can take place through the interface.
For performance reasons, the port user level libraries distinguish between local ports and
global ports. Local ports are used only for communication within a single address space. The
implementation of these ports are based on local semaphores and the local thread scheduler.
Local ports are more ecient than global ports because all of their interaction is limited to the
local address space.
Global ports are used for communication across address spaces, as well as within. These
ports make extensive use of shared memory between the client/server address spaces to help
reduce communication costs. In addition to reducing data copying, the shared memory also
helps reduce the number of kernel interventions required to invoke the port communication
protocols.
Local port messages are always passed by reference to reduce data copying. However, since
pointers don't make sense in remote address spaces, global port messages cannot use pointers.
Instead, to emulate pass-by-reference, a virtual memory region descriptor of the data buer can
be passed, and the vm share() and vm move() system calls can be used to map the region into
the local address space. This technique can eliminate data copying when moving data between
address spaces. For small pieces of data, however, the overhead of memory mapping outweighs
the data copy, and therefore this technique should be reserved for larger chunks of data.
4.5.1 The synchronous port library
The user level library supports the synchronous style send/receive/reply protocol. Using a
transaction identier returned by the receive operation, server threads can defer the reply stage
until a later time. The transaction identier is used by the reply call to identify the proper
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client message to reply.
The following two subsections describe the implementation of the local and global port
libraries. Then, the user interface is described.
Local synchronous port implementation
The local synchronous port library maintains a table of port descriptors rpc port msgs[]. This
descriptor is used to keep track of threads blocked on sends, receives, and replies.
typedef
{
int
int
int
struct port_s
id;
state;
sequence;
int send_sem;
QUEUE send_msg_q;
QUEUE reply_wait_q;
struct port_s *next_port;
int lock;
} RPC_PORT;
/* port identifier */
/* USED or FREE status */
/* incremented at each port_destroy */
/* semaphore to block receiver */
/* queue of sender messages */
/* queue of threads waiting for a reply */
/* next port descriptor link */
/* spin lock protecting this port */
The semaphore send sem is used to block server threads on the receive operation. This
semaphore counts the number sender threads currently blocked on a send. When a client thread
sends a message, the message is queued into the send msg q, and the send sem is signalled to
wake the server thread. The client thread is placed on the reply wait q and blocks waiting for
a reply. The server thread dequeues the client from this queue and unblocks it when it performs
the reply operation.
Each thread in a task is allocated its own synchronous message descriptor. All local messages
are passed by reference. This message descriptor maintains pointers to the passed data buers,
and a queue link for attaching to a port:
Chapter 4. User level kernel implementation
typedef struct
{
int id;
char *send_data;
int
send_len;
char *reply_data;
int *reply_len;
QUEUE_LINK link;
} RPC_MSG;
/*
/*
/*
/*
/*
/*
110
port id this message is queued on */
pointer to send data */
length of send_data */
pointer to reply data */
pointer to reply data length */
link for port msg queue */
Global synchronous port implementation
Each task in the system shares a region of shared memory that contains a table of global port
descriptors. When a global port is created, a free descriptor is allocated from the free list. A
region buer for storing messages is allocated by the owner task, and is shared to all clients
that want to interact on the port.
This port descriptor maintains head/tail pointers to the shared port message buers. Each
port descriptor contains three queues: the msg free q links the free message descriptors together; the msg send q head links the send messages together; and msg reply q head which
links the reply messages together.
A global semaphore in each port descriptor is used to control access to the port's msg free q.
When a client thread sends a message, a message buer is dequeued from the shared buer. If
there are no free buers, then the thread blocks on the msg free sem. When a server releases a
message buer back to the free list, it performs a sem signal() operation to unblock a waiting
thread.
The task signalling facility is used to communicate low level send/receive/reply events to
the client and server address spaces. These events are not sent on every interaction, but only in
the worst-case moments discussed below. The task signalling facility, task signal(), is used
to communicate these events to remote address spaces. The following three signal messages are
dened for this use:
the
GRPC PORT SEND SIGNAL
message is sent by a client thread to a server task with a
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blocked server thread to wake the server thread and tell it that a message awaits. If a
send message is already present in the queue, then the library assumes that the server
has already been delivered a signal message, so sending another is not necessary.
the GRPC PORT REPLY SIGNAL message is sent by a server thread to a client task to indicate
to the client task that a server is replying to a client's message. When the client task
receives this signal, it can wake the client thread, and deliver the reply message to it.
the GRPC PORT DESTROY SIGNAL message is sent to all client tasks that are blocked on a
port when the port is destroyed. The client threads can then be unblocked and told that
the port was destroyed.
The task signal() mechanism can deliver these messages without kernel intervention on
the local processor. If a remote processor is executing the destination task, then that processor
is interrupted by the user level intr remote cpu() service. Otherwise, if a remote processor is
idle, then that processor is interrupted to handle the signal message. If all processors are busy,
then a system call is required to deliver the signal.
Sending a synchronous message
int rpc_port_send( int port_id, char *send_data, int send_len,
int *reply_data, int *reply_len);
This function queues a message of length send len bytes on port id and blocks waiting
for a reply. For a global port, the send data buer is copied into the ports shared buer
region. For a local port, a pointer to the data is passed. reply data is assumed to point to
pre-allocated buer space for the reply message, whose length is specied in reply len. Upon
successful return, the global port reply data is copied in reply data buer and the length in
reply len. The call returns OK if successful, DESTROYED if the port was destroyed while waiting
for a reply, and FAILED if the port does not exist.
The following client thread demonstrates this call:
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void client(port)
{
char reply_msg[20];
char *send_msg = "send message";
int reply_len;
rpc_port_send(port, send_msg, strlen(send_msg)+1, &reply_msg, &reply_len);
printf("reply message is:%s length:%d\n", reply_msg, reply_len);
}
Before the client thread can queue a message into the shared port buer, the buer must be
mapped into the client's address space. rpc port send() will automatically do this mapping on
the rst message send. However, when a client is nished communicating across a global port,
the port buer must be unmapped explicitly. The rpc port dereference() call, described
below, does this.
Receiving a synchronous message
int rpc_port_recv( int port_id, char **recv_data, int *recv_len,
char **reply_data);
This function blocks the calling thread and waits for a message to arrive on the specied port.
When a message arrives, recv data and recv len is returned to contain a pointer and length
of the received data, and the reply data eld points to the reply buer. This buer is used
by the server to copy the reply message into. When successful, the function returns a msg id,
which identies the synchronous transaction. This msg id is supplied to the rpc port reply()
function to issue a reply on the transaction. The following code demonstrates this interaction:
void server(int port)
{
char *recv_msg;
int recv_len;
char *reply_data;
char *reply_msg = "reply message";
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int msg_id;
msg_id = rpc_port_recv(port, &recv_msg, &recv_len, &reply_data);
printf("received message:%s length:%d\n", recv_msg, recv_len);
/* copy reply message into supplied reply buffer */
strcpy(reply_data, reply_msg);
rpc_port_reply(port, msg_id, strlen(reply_msg));
}
Replying a synchronous message
int rpc_port_reply( int port_id, int msg_id, int reply_len);
This routine is called by the server thread to reply to an rpc port send() call. The msg id
species the transaction number obtained from rpc port recv(). reply len species the
length of data copied into the reply data buer from rpc port recv.
Creating a synchronous port
int rpc_port_create( int *port_id );
This is the creation routine for local synchronous ports (global ports are suciently dierent
to require a separate routine). The created port is returned in port id.
int grpc_port_create( int *port_id, int max_data_len, int num_msg_bufs);
This is the creation routine for global synchronous ports. The maximum port message
size, max data len, and queue size, num msg bufs, are specied so that a queue buer can be
allocated. The created port is returned in port id. Returns OK if successful, FAILED if there
are no free ports.
Destroying a synchronous port
int rpc_port_destroy( int port_id );
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This function destroys the specied synchronous port. Only the port creator's address space
can destroy the port. All clients blocked on an rpc port send() are released.
int rpc_port_dereference( int port_id );
When a client address space is nished sending messages to a global port, it can \dereference" the port. This causes the shared memory region between the client and server to be
unmapped from the client address space.
Kernel intervention
The interprocess communication library is designed to reduce the number of user/kernel interactions required to interact between client and server address spaces. Pure kernel implementations
require at least three kernel calls per send/receive/reply interaction: one call at each of the send,
receive, and reply stages.
The local port communication library does not require any kernel support at all. The global
port library does however require kernel system calls in certain cases. Global interprocess
communication is based on the task-to-task asynchronous signalling mechanism. The amount
of kernel intervention required is directly based on the amount of kernel intervention in the
signalling mechanism. An invocation of the signal mechanism breaks down into the following
steps:
1. If the destination task is running on a remote processor, deliver it an interrupt and return.
2. If there is an idle processor, deliver it an interrupt and return.
3. Otherwise, make a system call to perform the signal.
The signalling mechanism uses software interrupts to deliver messages to remote processors,
avoiding system calls on the local processor. A system call is required only if the third step is
reached.
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The synchronous IPC mechanism tries to reduce the number of kernel calls by invoking
task signals only when necessary. The following lists summarize the stages of the synchronous
operations and where system calls and task signals potentially occur.
The rpc port send() operation requires at most one system call per invocation, or possibly
a task signal invocation:
1.
for a free send buer, causing a system call only for the rst blocked thread
(described in 4.4.7).
sem wait()
2. Copy message into buer.
3. If the send queue is empty, deliver a signal to the destination task.
4. Block sending thread.
5. Wake up and copy reply message to local buer.
6.
sem signal()
free buer queue.
The steps followed by the rpc port recv() operation does not require any kernel support
at all:
1. If a message awaits, return a pointer to the message buer.
2. If the port is empty, block the thread.
3. Wake up and return a pointer to the message buer.
rpc port reply()
requires a task signal invocation only when there are no reply messages
queued for the client:
1. Enqueue the reply message.
2. If there are existing reply messages in the queue, simply return.
3. Otherwise, deliver a task signal to the client task.
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4.5.2 The asynchronous port library
Asynchronous send/receive communication can be used between threads when no reply message
is required. This method can more ecient than synchronous interactions because there is
potentially less context switching involved between the client and server thread. Client send
messages are copied into a buer, to be picked up by the server. Client sending threads do not
block unless the port queue becomes full.
Threads communicate their message data through a FIFO message buer. In the local port
case, this buer is dynamically allocate into the local memory space. In the global port case, a
shared region is allocated between client and servers, similar to the global synchronous ports.
The local port implementation uses local semaphores to synchronize client/server access to
the port message buer queues. The sender thread waits for free message buer to become
available. The free message buer is dequeued, and the send message is copied into it. The
message is queued onto the port send queue, and the send semaphore is signalled to wake the
server thread.
The global port implementation relies on a shared region of port descriptors to manage the
port queue. One global semaphore is used to synchronize access to the port free message queue.
Client threads wait on the free message queue semaphore for buers to become available. The
server threads block waiting for sent messages.
When a client thread sends a message, a GLOBAL PORT SEND SIGNAL message is delivered to
the server task to notify server threads that a message is waiting. This signal is sent only when
server threads are blocked, and when no messages are queued in the send buer. That is, the
signal is sent only when the send queue is empty.
Sending an asynchronous message
int port_send( int port_id, char *msg, int msg_size, int no_block );
Sends a message pointed to by msg to the specied port. The message buer of length
msg size is copied into the port's message queue. If the message queue is full, this routine will
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return immediately without queuing the message if no block is set to NO BLOCK. If no block
is BLOCK, the calling thread will be blocked until room is made available in the message queue.
Returns OK if the message was queued successfully, WOULD BLOCK if the queue was full, DESTROYED
if the port was destroyed while blocked, or FAILED if the port does not exist.
The following client thread demonstrates this call:
void client(port)
{
char *send_msg = "send message";
port_send(port, send_msg, strlen(send_msg)+1, BLOCK);
}
Before a client thread in a remote address space can send messages on a global port, the port
message queue region must be mapped into the client's address space. The port send() routine
will automatically map in the port message queue on the rst invocation. When communication
across the port is later discontinued by the client, the port queue must be explicitly unmapped.
The port dereference() call, described below, performs this operation.
Receiving an asynchronous message
int port_recv( int port_id, char *msg, int *msg_size, int no_block );
Receives a message from the specied port. If the port queue is empty, this will return
immediately if no block is set to NO BLOCK. Otherwise, the caller thread will be blocked until
a message arrives. msg must be a preallocated buer to hold at least msg size bytes. The
message is automatically copied into this buer when it is dequeued from the port. Returns
OK if a message was successfully received, DESTROYED if the port was destroyed while blocked,
WOULD BLOCK if the caller thread would be blocked, or FAILED if the port does not exist.
The following server thread demonstrates this call:
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void server(port)
{
char recv_msg[20];
int recv_size;
port_recv(port, recv_msg, recv_size, BLOCK);
printf("message received:%s length:%d\n", recv_msg, recv_size);
}
Creating an asynchronous port
int port_create( int *port_id, int max_msg_size, int queue_size, int global );
This is the asynchronous port creation routine. The max msg size parameter species
the maximum length in bytes of the messages passed on this port. queue size species the
maximum number of messages that can be queued up on the port. If global is PORT GLOBAL,
then a global port will be created. Otherwise, if global is PORT LOCAL, then a local port will
be created. Returns OK if successful, FAILED if there was a parameter error, or if there are no
more port descriptors.
Destroying an asynchronous port
int port_destroy( int port_id );
Destroys the specied local or global port. Remote and local threads blocked on an operation
to the port will be woken with a DESTROYED failure result. Returns OK if successful, FAILED if
the port does not exist, or if the port did not originate in the caller's task.
int port_dereference(int port_id);
When client threads in an address space are nished accessing a port, this routine will clean
up the shared memory region associated with the port. Returns OK if successful, FAILED if the
port does not exist.
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Kernel intervention
Similar to the synchronous port library, the asynchronous library only requires kernel support
for supporting global communication. The task signalling mechanism is used in the same
manner to help reduce kernel intervention.
The port send() call may require a system call if the sem wait() call blocks, or a signal
may be delivered:
1.
sem wait()
for a free message buer.
2. Copy message into buer.
3. Deliver a signal only if the send queue is empty and a server thread is blocked.
The port receive() call may require a system call by sem signal() to wake any blocked
sender threads:
1. Block if there are no messages.
2. Wake up, dequeue message and copy it into the caller's buer.
3. Issue sem signal() on the port free buer queue.
4.6 The global nameserver
The user level library maintains a simple nameserver data structure that is shared read/write
by all address spaces in the system. The nameserver database stores (string,integer) pairs,
using the string as a search key. User programs can register strings, with them associated an
integer value. The integer value can be used to pass handles for various objects, or for other
miscellaneous purposes. For example, global ports can be registered, so that remote address
spaces can query for server port identiers.
The following structure is used for each entry in the nameserver table[]:
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typedef struct
{
char string[NAMESERVER_STRING_SIZE];
int data;
} NAMESERVER_ENTRY;
The complete nameserver interface is provided through the following three function calls:
nameserver register(), nameserver deregister(), and nameserver find().
4.6.1 Registering with the nameserver
Any null-character delimited string, of up to NAMESERVER STRING SIZE characters, can be stored
in the nameserver database. The register routine simply performs a linear search through the
nameserver table[], and nds the rst available slot.
int nameserver_register( char *str, int data );
The null-terminated string str is stored in the nameserver table[], with its associated
data value, data. If the table is full, FAILED is returned. The string and data value are copied
into the table entry. The string is truncated if not enough space is available. Returns OK if
successful.
4.6.2 Deregistering a string
The deregister routine removes a string and its data value from the nameserver table. A linear
search nd the table entry, which is then invalidated.
int nameserver_deregister( char *str );
The null-terminated string str species the string to invalidate. Returns OK if successful,
or FAILED otherwise.
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4.6.3 Nameserver initialization
The task initialization upcall must initialize the nameserver data structure. The initialization
code checks to see if a nameserver table already exists in the system. The nameserver region
identier is passed to a task through the TASK ARGS structure when calling task create().
The rst task in the system allocates the table using vm alloc(). All child tasks are passed
the nameserver table region identier through TASK ARGS. Child tasks, when initializing, uses
the region identier to map in the table with vm share().
4.7 User/User Shared memory regions
Just as the supervisor kernel implements shared regions between the user and kernel spaces, the
user kernel library also makes use of shared regions between user/user address spaces. These
regions are used by the interprocess communication library and global nameserver to facilitate
data sharing between tasks. The supervisor kernel has no knowledge of these regions.
These regions are susceptible to the same problems that the user/kernel shared regions face,
although the aects of such problems are slightly less critical. Damaging a user/user shared
region may prevent interprocess communication across a port, but it won't fatally crash the
system as damaging the user/kernel shared region would.
4.7.1 Globally shared regions
The user level kernel library shares three regions between each task in the system. The rst
task that is loaded into the system and executed is responsible for allocating each region. Other
tasks that are subsequently loaded share these regions. Each region is shared with read/write
protection:
The global nameserver region, which contains a table of key/value pairs for passing handles
and other information.
Chapter 4. User level kernel implementation
122
The asynchronous port descriptor region, which contains a table of asynchronous port
descriptors.
The synchronous port descriptor region, which contains a table of descriptors for the
synchronous port library.
4.7.2 Client/Server shared regions
The synchronous and asynchronous port creation routines allocate a buer in the server task
to queue messages between the client and server. Clients that wish to communicate across
the port rst map in the buer region to their address space. So these regions are only made
available to those address spaces that ask for it.
Other shared memory regions can be easily created by user programs for application specic
purposes.
Chapter 5
Performance Evaluation
This chapter presents some performance benchmarks for various aspects of the kernel operation.
We begin by describing the tools used to obtain performance data. Then, several benchmarks
are presented and analyzed.
5.1 Benchmark tools
There are two tools available in the system which can provide performance measurements.
The Hypermodule's on-board Z8536 Counter/Timer can be congured as a 32-bit timer, with
microsecond resolution. The following routines are available within the kernel and at the user
level to control the timer:
unsigned long timer_start();
unsigned long timer_stop(unsigned long start);
The timer hardware is initialized at boot time to continually cycle through a 32-bit counter
register. The counting frequency is 2MHz, giving the counter a cycle time of about 35.7 minutes
with 1/2 microsecond accuracy. The timer start() function takes a snapshot of the counter
register, returning the counter value. This value is then passed to timer stop(), where another
snapshot is taken, and the time between snapshots is returned. This value should then be
divided by 2 to convert to microseconds.
The second tool is only available while running under the simulator. The simulator provides
a special device used to support an instruction tracing feature. This feature allows a processor's
execution stream to be saved for later analysis, such as counting the number of instructions
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124
executed. A simulator device allows the tracing to be turned on and o, at program control, and
categorized into slots. The following macros can be used by any code to control the instruction
tracing device:
/* 'x' is the category slot */
#define SIMTRACE_ON(x) (*(int *) SIMTRACE_CTRL = x)
#define SIMTRACE_OFF(x) (*(int *) (SIMTRACE_CTRL+4) = x)
5.2 Function calls vs. System calls
To help put the performance results in perspective, here we consider the cost of a null system
call compared to the cost of a null procedure call. A null procedure with four parameters
requires at least 6 instructions on the 88100: four instructions to build the argument list, one
instruction to call the routine, and one instruction to return.
/* func() is the null workhorse */
void func(int a, int b, int c, int d)
{
return;
}
The kernel system call interface funnels all system calls through a single trap vector. An
integer passed across the trap is used to determine the correct system call. The initial system
call handler has the job of preserving user level state and setting up the kernel execution
context. This involves some register saving and processor conguration. In all, 56 instructions
are required to invoke a null system call with 4 parameters.
The basic setup cost and administration for system calls is an order of magnitude greater
than a local function call. Table 5.2 summarizes this comparison.
5.3 Thread management performance
This section details the costs of thread creation and scheduling. Thread management operations
are primitives that closely aect the performance of tightly coupled parallel applications. In
Chapter 5. Performance Evaluation
125
Instructions
local function call 6
kernel system call 56
Table 5.2: 4 parameter user level function call vs. kernel system call.
thread create()
dispatch and destroy
thread sched()
1 CPU
38.1 usec
22.3 usec
12.8 usec
2 CPU
35.7 usec
16.2 usec
8.2 usec
3 CPU
34.3 usec
13.5 usec
6.5 usec
4 CPU
34.1 usec
11.4 usec
5.1 usec
Table 5.3: Thread management performance.
these applications, threads are used extensively to perform jobs in parallel, and schedule events
from one to another. Often, the raw performance of the underlying thread scheduler can become
a bottleneck.
5.3.1 Thread creation performance
This benchmark measures the overall performance of the thread create() routine. A simple
program was written which creates 100 threads, but does not execute them. Execution times
were collected for four dierent processor congurations: one, two, three, and four processor
versions. In each case, the same number of worker threads is used to create the null thread as
there are processors. For example, the four processor measurement uses four server threads to
create 25 null threads each, totalling to 100 threads.
Table 5.3 summarizes the results. The basic uniprocessor thread create call completes every
38.1 microseconds. This creation time could be signicantly improved by modifying the thread
stack allocator. Currently, thread create() performs the system call vm alloc() to allocate
its thread stack. An algorithm that allocated a large number of thread stacks at once would
reduce the expensive vm alloc() calls.
When more processors are added to the system, performance improves because there of the
Chapter 5. Performance Evaluation
126
parallel creation, but only slightly, until levelling o after about 4 processors. This is probably
caused by a spin-lock contention bottleneck, seen in three places during thread creation: locking
the free thread descriptor queue to allocate a descriptor; locking the task descriptor to perform
the vm alloc() system call; and locking the task's address map descriptor to perform the
vm alloc() system call. In addition to reducing the amount of code executed, removing the
vm alloc() call for stack allocation may also reduce lock contention.
5.3.2 Thread resumption and destruction performance
Using the 100 threads created for the previous benchmark test, the threads are resumed using
thread resume(), executed, and nally destroy themselves using thread destroy(). The last
thread to destroy itself prints out the execution time for the complete run.
Table 5.3 summarizes the execution times for one, two, three, and four processors. A thread
is executed and removed in 19.3 microseconds. This execution time includes putting the thread
on the ready queue, invoking the scheduler to dispatch it, and invoking the thread destroy()
routine to kill the thread. Adding more processors has a more signicant eect on the execution
time, compared to the thread creation benchmark. While there is still lock contention for the
ready queue list, this contention is not nearly as long as found in the vm alloc() system call.
Greater parallelism is achieved in this case.
5.3.3 Thread context switching
This benchmark measures the basic context switching performance of the thread sched()
library call. 10 threads are created, and each of them performs the following tight loop:
void worker( iterations )
{
while ( iterations-- )
thread_sched();
}
Chapter 5. Performance Evaluation
127
100,000 iterations were performed by each thread, and the overall execution time was
recorded. This number is divided by 100,000 to determine the approximate performance of
the thread sched() call. The calling thread's context is saved, and its descriptor is placed on
the ready queue. The next available thread is dequeued and its context is restored.
Table 5.3 shows the average performance of thread sched(). A basic context switch from
one thread to another on a uniprocessor occurs in 12.8 microseconds. The ready queue oers
some lock contention for when additional processors are added. The remainder of the processor
time is spent saving and loading the 29 thread context registers1 .
5.4 Interrupt handling performance
The performance of interrupt handling is a critical concern for high speed device drivers and
kernel scheduling performance. Interrupt handling must be as lightweight as possible to ensure
low latency dispatch times to device drivers. Interrupt handling and dispatching in a monolithic
kernel is fairly straightforward: trap the interrupt, save context, and call the interrupt service
routine. In the Raven kernel, since device drivers are implemented at the user level, device
interrupts must take the journey up into the user level for processing. However, once at the
user level, execution can continue with application processing.
An experiment was constructed to measure the execution latency time to dispatch an interrupt to a service routine. The kernel interrupt handler was modied to take a timestamp at
the earliest convenience. This timestamp is then compared with the timestamp acquired at the
beginning of the interrupt handler. Three dierent interrupt handler scenarios were measured:
1. A kernel level interrupt handler. Invoking this handler is a local kernel call.
2. A user level interrupt handler in a task that is activated on the interrupted processor.
Invoking this handler requires an upcall into the user space.
A small optimization can be made during the register save operation because the register save is occuring
during a well known point in the thread scheduler. This allows less registers to be saved than the whole processor
context.
1
Chapter 5. Performance Evaluation
Time (usec)
kernel invoke
7.21
user invoke
14.0
user switch/invoke 30.6
128
Instructions
86
194
421
Table 5.4: Interrupt service routine invocation latencies.
3. A user level interrupt handler in a task that is not currently activated on the interrupted
processor. Invoking this handler requires that the current task be switched out and the
interrupt handler task be switched in. Performing this operation requires two upcalls into
user space and a system call.
Table 5.4 summarizes the average times, in microseconds, to invoke each service routine.
Also, the number of instructions per invocation is shown. The cheapest invocation time of 7.21
microseconds is naturally inside the kernel. No special setup is required to upcall into user
space. Also, the user level register context can be saved in a cheaper fashion, since it will be
directly restored by the kernel at the end of the interrupt.
Invoking a user space handler is about twice as expensive. The user level register context
must be properly saved into the user level context save area, and an upcall must be performed to
the user level. Once at the user level, the upcall dispatcher must place the previously executing
thread on the ready queue, and nally call the service routine.
Switching address spaces before calling the service routine is the most expensive invocation
operation. The old address space must be upcalled to handle any cleanup and placed on the
task ready queue before the new address space can be invoked.
At rst glance, this benchmark appears to show that user level device drivers are much more
expensive than kernel device drivers because of the interrupt dispatching overhead. However,
one must also consider that even a kernel device driver needs to communicate with the user level
at some point. User level code must eventually be executed to operate on the data provided
by the device driver. Depending on the device, this may involve an extra data copy operation
Chapter 5. Performance Evaluation
129
to move the data between the user application and kernel device driver. Moreover, there is
the additional costs of scheduling and activating the user application when the device driver is
ready for more. All of these costs are automatically taken care of by the interrupt dispatcher
and upcall mechanism.
5.5 Task signalling performance
The asynchronous task signalling facility is used throughout the kernel to provide synchronization and event passing between tasks. The performance of this facility is an important factor
for global thread synchronization and interprocess communication.
The signalling mechanism relies on interprocessor interrupts or a system call to deliver signal
messages. The sequence of steps performed by a signal invocation is summarized as follows:
1. If the destination task is running on a remote processor, deliver it an interrupt and return.
2. If there is an idle processor, deliver it an interrupt and return.
3. Otherwise, make a system call to perform the signal.
The remote interrupt mechanism allows the local processor to ooad all of the queuing and
task invocation work to the destination processor. The initiating processor can continue with
its own work while the signal is processed. If signal invocation makes a kernel call to perform
its work, the local processor handles the signal message queuing and readying, and dispatching
of the destination task.
A benchmark was constructed to measure the dierence between a software interrupted
signal message and a kernel mediated signal message. Two tasks were created. One task
contains a signal handler that will be invoked by the other task. Three test cases were measured:
a kernel mediated signal, a software interrupt with the remote task activated on the interrupted
processor, and a software interrupt with the remote task not activated on the interrupted
processor. The results for a single invocation are shown in Table 5.5.
Chapter 5. Performance Evaluation
130
Time (usec)
kernel signal
32.4
user active signal 13.2
user inactive signal 15.6
Table 5.5: Task signalling invocation latencies.
The kernel mediated signal is more than twice as expensive as the software interrupt versions.
This time is mostly consumed by the task switch that must occur on the local processor. The
initiating task must be switched out, and the destination task must be switched in a delivered
the signal. Sending a signal to a remote processor that is running the task is the least expensive.
No task switching is required, but there is the cost of handling the interrupt and saving the
interrupt thread's context. Interrupting an idle processor is only slightly more expensive. The
destination task must be activated, but there is no thread state to save.
5.6 Interprocess communication performance
The performance of the interprocess communication primitives is a vital factor for highthroughput client/server applications. In these types applications, IPC performance is the
most common bottleneck.
The user level IPC libraries make extensive use of shared memory and scheduling primitives
that do not require kernel intervention. Two experiment programs were constructed to measure
the performance of the IPC libraries under various conditions. The rst set of experiments test
the local interprocess communication primitives. Then a second set of experiments test the
global IPC primitives. These libraries are suciently dierent in implementation that it makes
sense to test and analyze them separately.
Chapter 5. Performance Evaluation
131
5.6.1 Local communication
Both synchronous and asynchronous port based communication was tested. The performance
of these libraries weighs heavily on the performance of the low level thread scheduling modules.
The operations performed by the local IPC primitives are mostly thread context switching and
enqueue/dequeue operations.
The test program creates a port and some threads to communicate across the port. The
number of client and server threads was varied, as well as the number of physical processors. A
4 byte message is passed several thousand times each case, and the individual results averaged.
The rst set of tests measures the asynchronous port performance. A port with a 20 element
queue was created. The results of these tests are shown in Table 5.6. The rst test creates 1
sender thread and 1 receiver thread (denoted 1-send/1-recv). While the average send/receive
time for this case is 13.2 microseconds, the actual latency between a single send/receive pair is
much higher. The 13.2 microsecond time is achieved because the sending thread can ll up the
port queue at full speed, and then transfer control to the receiver thread which can spend all of
its time draining the queue. Thus the overall time is greatly reduced due to the amortization
of message buering in the queue.
The addition of more processors to this case does not help much. In theory, the sender should
be able to supply data fast enough to keep the receiver occupied. However, by examining the
port status descriptors at various intervals during the benchmark, it appeared that the port
remained full during most of the computation. This caused senders to block, thus forcing
context switches between the senders and receivers. A more balanced workload on the sender
side would have helped reduce this problem.
The synchronous message passing case requires much more work per transaction, because
the sender always blocks waiting for a reply. Therefore the 1-send/1-recv/reply uniprocessor
case is heavily bounded by the thread scheduling performance. Adding more processors causes
additional work to be done. This appears to be due to scheduling overhead. When a client
thread places a message on a port queue, the client will deliver a remote interrupt to the next
Chapter 5. Performance Evaluation
1 CPU
1-send/1-recv
13.2 usec
10-send/10-recv
14.3
1-send/1-recv/reply
32.6
10-send/10-recv/reply 18.1
132
2 CPU
12.6 usec
12.0
32.9
17.6
3 CPU
12.7 usec
10.3
33.0
17.0
4 CPU
12.7 usec
10.0
33.0
16.8
Table 5.6: Performance of asynchronous and synchronous local ports, 4 byte data message.
available idle processor to wake the server thread. Since there is only one server thread and
one client thread, the cost of managing the context switch on the local processor turns out to
be less expensive than delivering an interrupt to a remote processor. Interrupting a remote
processor to run a thread causes that processor to examine the ready queue, increasing lock
contention for other processors in the system.
If the client and server sides have more threads to work with, as in the 10-client thread
10-server thread case, performance is greatly improved because the message queue can be
maintained at a non-empty state, and servers can read messages as fast as clients can place
them.
However, none of these local port cases show much improvement when more processors are
added. Most of the time spent seems to be wasted on lock contention. There are two hot spots:
the port spin-lock or the thread scheduler ready queue lock. The workload that the client and
server threads perform are basically null operations, so most of the execution time is spent
chasing after locks within the thread scheduler.
5.6.2 Global interprocess communication
This section measures the performance throughput of the global interprocess communication
service. This test combines many of the primitive system services: remote interrupt dispatching,
task signal dispatching, global semaphores, and task and thread scheduling.
The global IPC test cases create two address spaces: a server task, and a client task. The
server task allocates a port descriptor containing 20 message buers and advertises it on the
Chapter 5. Performance Evaluation
1 CPU
1-send/1-recv
43.2 usec
2-send/2-recv
44.5
10-send/10-recv
44.3
1-send/1-recv/reply
145
2-send/2-recv/reply
108
10-send/10-recv/reply 89.3
133
2 CPU
33.9 usec
32.2
33.9
124
95.3
92.5
3 CPU
32.1 usec
31.8
32.0
124.1
90.3
94.9
4 CPU
32.0 usec
31.0
32.5
123.8
89.6
95.6
Table 5.7: IPC performance for asynchronous and synchronous global ports, 4 byte data message.
nameserver. A number of server threads are created to listen for messages on the port. The
client task queries the nameserver for the port descriptor and creates a number of client threads
to bombard the server with messages.
Table 5.7 contains performance results for various combinations of processors and threads.
The simple case of 1-send thread and 1-receive thread synchronous send/receive/reply demonstrates the worst case performance of 121 microseconds per interaction. Each iteration requires
two processor relinquishments and two upcalls. This gure is improved slightly in the 2 processor case, because the client and servers reside on separate processors most of the time. Invoking
the remote task to signal a message is done via a software interrupt, and an address space switch
is not required.
However, synchronous performance does not increase by the expected amount when more
threads and processors are added. In fact, performance is reduced when more processors are
added to the 10-sender 10-receiver case. In a uniprocessor system, the all the sender threads
block, then all the receivers return replies. However, when processors are added, the thread
and task schedulers seems to constantly jump around in a form of hysteresis, causing more
scheduling events to occur than optimal.
The asynchronous message transfers are much faster overall because of the reduced context
switching requirements between the client and server. The client threads have no problem
keeping the port queue full of data for the server threads. Increasing the number of threads
Chapter 5. Performance Evaluation
Async IPC
total IPC calls 200,000
kernel calls
16,000
user interrupts 68,000
134
Sync IPC
300,000
56,000
108,000
Table 5.8: IPC primitive breakdown.
results in an apparent slight performance hit. This is possibly due to the increased number of
thread descriptors being managed throughout the system. Both the single thread and multiple
thread case are able to keep the port queue full, so have multiple threads on a single processor
does not help.
As seen in the local case, performance sharply declines as more processors are added to the
system. The reason for this again is lock contention. The workload performed by the client
and server threads is null, so all their eort is spent trying to access the port queue. Much of
the overall processor time is spent waiting on the port queue.
Kernel Intervention
The global interprocess communication library attempts to reduce the number of kernel interactions by using a task signalling mechanism that can send event messages without kernel
intervention. To illustrate this, the number of low level primitive invocations was measured for
the 2-thread server, 2-thread client, benchmarks shown above for synchronous and asynchronous
IPC.
These measurements are broken into the following three categories: the total number of IPC
primitive calls, the number of kernel mediated calls, and the number of software interrupt mediated calls. Table 5.8 summarizes the results for asynchronous and synchronous benchmarks.
These results demonstrate the reduced dependency on kernel interaction of the user level
IPC library. Better results could be achieved using an improved task and thread scheduler.
Such a scheduling system could properly schedule threads that are communicating so as to
reduce the number of context switches and event messages.
Chapter 5. Performance Evaluation
Time (usec)
vm alloc() 4KB
38.5
vm alloc() 64KB 149
vm alloc() 1MB
2080
vm free() 4KB
23.3
vm free() 64KB
145
vm free() 1MB
2140
vm share() 4KB
30.1
vm share() 64KB 116
vm share() 1MB
1650
vm move() 4KB
34.0
vm move() 64KB
132
vm move() 1MB
1720
135
Instructions
389
1516
20637
303
1575
21745
303
1126
14682
324
1264
15353
Table 5.9: Virtual memory operation execution times.
5.7 Memory management performance
This section performs some experiments to measure the execution time of various virtual memory system calls. The system calls tested are: vm alloc(), vm free(), vm share(), and
vm move(). Each call is benchmarked by repeatedly calling the routines using region sizes
of 4KB, 64KB, and 1MB.
The calls were all performed within a single thread, so no parallel performance numbers
were measured. However, one can deduce that the parallel performance of these operations will
be relatively poor due to high lock contention. Approximately 90% of the work done by these
operations occur within a critical section, protected by the address map lock, or by the task
descriptor lock. This lock contention is on a per-address space basis, so parallel invocations will
happily coexist if allocations occur amongst disjoint address spaces.
Each call was executed a number of times in a tight loop. Table 5.9 shows the average
execution time and instruction counts for each call, for varying region sizes.
The time to execute each call on a single page size of 4KB is relatively high compared to
the time for larger page sizes. This is due to the high setup costs for each call. Using multiple
Chapter 5. Performance Evaluation
Frame size (bytes)
128
512
1024
1500
136
Time (sec)
18.6
9.03
8.71
8.57
eciency
45.1%
92.9%
96.3%
97.9%
Table 5.10: Time to transfer 10MB of data over Ethernet.
page sizes allows the per-page cost to be amortized over a large number of pages, thus reducing
the relative execution time drastically.
5.8 Ethernet driver performance
A simple test program was constructed to test the performance of the Ethernet device driver.
The test program runs on two machines connected by Ethernet. One machine continuously
sends data to the receiver machine. 10 megabytes of data is transmitted between the machines
using a range of Ethernet frame sizes. Table 5.10 summarizes the results of the test, showing
the percentage eciency compared to the 10Mbps Ethernet speed.
Chapter 6
Related Work
This chapter examines recent work in the eld of multiprocessor operating systems, and how
it relates to the design of the Raven kernel. The operating systems topic is rather broad. The
research emphasis in recent years has turned away from implementing feature rich environments,
to nding more ecient and streamlined ways of doing things. For example, rather than building
an operating system that contains everything that anyone would ever need, recent research in
the eld identies the basic operating system components and improves upon them. The Raven
kernel was designed and implemented in the same spirit.
Amongst these basic operating system components that are particularly relevant to multiprocessor systems are critical section management, thread management and scheduling, and
interprocess communication. Much attention to these areas has been spent in recent years to
improve the performance and their characteristics.
6.1 Low-level mutual exclusion
There is a large body of research work related to the implementation and analysis of mutual
exclusion synchronization primitives on shared-memory multiprocessors. Early work in this area
detailed software algorithms where the only atomic operations provided by the hardware are
memory read and write, [Dij65] [Knu66]. The main disadvantage of these software dominated
approaches is their ineciency. Since then, however, more powerful atomic operations supplied
by hardware has made mutual exclusion more ecient. Before looking at these operations,
consider where mutual exclusion is used.
Operating systems rely on low-level mutual exclusion algorithms to protect against parallel
137
Chapter 6. Related Work
138
access to system data structures and hardware devices. In a uniprocessor system, a common use
for mutual exclusion are to protect data structures that are shared between interrupt handlers
and device drivers. Disabling preemption by masking interrupts around sensitive code is an
eective way to provide this capability. However, the addition of multiple processors in a
system complicates the situation. Disabling interrupts alone does not stop other processors in
the system from accessing the protected resource.
The technique of spin-locking has long been an elementary operation that can provide
mutual exclusion between separate processors. The algorithm is to spin-wait for a shared lock
variable to become available, and then mark it unavailable, thus claiming the lock. When the
critical section is over, the lock variable is marked available again. In many systems, the lock
\acquire" stage requires an atomic operation, such as memory exchange or test-and-set.
While lock contention can be reduced by carefully designing critical sections to minimize
their overlap, spin-locking can be wasteful of available computing resources because of the busy
wait nature of the algorithm. In addition to completely consuming local processor cycles, the
spinning read/write cycle of test-and-set can generate a constant barrage of memory transactions. In a shared memory multiprocessor environment where main memory accesses share a
common bus, this activity can degrade the performance of all processors in the system. Experiments in [And89] show that this algorithm is worthwhile for systems with less than six
processors. However, this experiment only shows the impact of memory bus contention against
spinning processors, and not processors doing other work.
A simple optimization to the spin lock involves the use of memory caching to reduce global
memory bus contention. This technique relies on cache coherency to maintain proper copies of
lock variables. Rather than accessing the lock variable directly in memory, the lock variable is
read into the local processor cache. All spinning occurs out of the cache. When a lock value
changes, the system's cache coherency algorithm propagates the new value to the appropriate
caches. On some systems however, the cost of maintaining cache coherency can become a bottleneck. The Raven kernel implements the above method because the Hypermodule hardware
Chapter 6. Related Work
139
and the 88100 provide the necessary cache coherency protocols.
In the absence of cache coherency, spinning memory transactions can be reduced by using
a backo algorithm. If acquiring a lock fails, then delay for a period of time and try again.
This algorithm is similar to the Ethernet's exponential backo [MB76]. However in this case,
performance can still be poor for a small number of spinning processors because the lock acquire
stage will continue to backo even when the lock is released. The waiting processor remains
consumed by the backo delay. This algorithm is not appropriate in the current implementation
of the Raven kernel because of the small number of processors in the system. Also, the overhead
required to implement backo timing would consume a high proportion of lock acquire stage.
The technique of queuelocks has been shown to reduce memory bus contention even in the
presence of many processors, [And89], [GT90], [MCS91a]. The idea behind queuelocks is to
make each thread spin only for one other thread to release a lock. If one thread waits for a
lock holder, another thread will wait for the rst waiting thread. This relationship allows each
thread to spin on a dierent memory location. However, the advantages of this method are
oset by the additional overhead costs in the bookkeeping of lock queue data structures. This
overhead is not justied in the current implementation of the Raven kernel because of the small
number of processors.
Experiments have shown that spin-locking on global locks does not scale well beyond eight
processors [And89] [KLMO91]. While memory bus contention is signicantly reduced using
caching, performance eventually becomes bounded by spinning processor cycles. This bottleneck appears to become a factor in systems with more than eight processors. Also, the cost of
cache coherency in some systems can impose other bottlenecks to the system.
An alternative to spin-waiting involves techniques based on wait-free synchronization [Her91]
[Her90] [MCS91b] and data structures known as lock-free objects [Ber91] [MP91]. The idea
here is optimistic: allow concurrent data accesses without blocking. After a modication to
a data structure is made, the algorithm checks to see if structures are consistent, and if not,
the operation is rolled-back. However, these algorithms require additional hardware support
Chapter 6. Related Work
140
beyond simple test-and-set or compare-and-swap to operate eciently. The Hypermodule and
88100 instruction set does not directly support these algorithms, but they can be constructed
using more primitive features.
6.2 Threads
Operating systems have long provided lightweight threads of control to support a general purpose concurrent programming model for address spaces. Threads are used in uniprocessor
systems as a structuring aid and to help overlap input/output with computation. In multiprocessor systems, threads are also used to exploit true parallelism. Several techniques for thread
management and their associated performance characteristics are measured in [ALL89].
Thread management is usually implemented as either a kernel level service, or in user
address spaces as a threading library linked with executables. Kernel level threads benet from
better integration with the other kernel supported services, such as priority scheduling and
input/output. The kernel maintains control over all scheduling decisions, so thread priorities can
be obeyed across address spaces. Threads performing input/output system calls or interprocess
communication can be properly blocked and rescheduled as their operations complete.
Traditional microkernel architectures such as Mach [TR87] and the V-Kernel [Che88] demonstrate the use of kernel threads. However, the performance of these systems inherently suer
due to the costs of crossing user/kernel boundaries to perform thread management functions.
Every single thread context switch and library call requires a kernel call. In addition, since
the kernel level interface is usually intended to be used by all varieties of user programs, the
threading interface must be general purpose and cannot take advantage of any local special
purpose optimizations.
The Raven kernel implements threads at the user level to avoid the above performance
problems and provide more convenient interfaces to the user.
Pure user level threading implementations can perform thread management operations at
least an order of magnitude faster than their kernel level counterparts. The cost of invoking
Chapter 6. Related Work
141
thread operations is at most the cost of a local procedure call, which in some cases can be
optimized to inline macro routines. Many such user level threading packages exist for the Unix
environment [Gol86] [Doe87] [SM90]. These packages multiplex a number of user dened threads
on top of a single kernel implemented process. While these packages avoid kernel invocation
for most thread services, they introduce problems of their own:
Blocking system calls stop all threads in the address space. While select() can be used to
alleviate this problem for routines such as open(), close(), read(), and write(), other
potentially blocking calls such as mkdir(), rename(), ioctl(), stat() and asynchronous
events such as page faults are more dicult to deal with.
Poor performance resulting from improper scheduling decisions imposed by the kernel
during low-level thread mutual exclusion. Spin locks are commonly used between threads
in the same address to provide lightweight mutual exclusion (blocking semaphore management is too heavyweight for some operations). If a spin lock is acquired and held by a
thread which is subsequently switched out, other threads in the system trying to acquire
the lock will hopelessly busy wait until the holder is allowed to complete its critical section. A thread can be switched out for a number of reasons, such as the expiry of a time
quantum, or the arrival of other external interrupting conditions.
One technique which tries alleviate parts of the above problems allows lightweight user
level threads to be executed on top of kernel supported middle-weight threads of control. This
technique is used by Mach's C-Threads library [CD90] and SunOS's LWP [PKB+ 91] [SS92].
The user level threads reside as data structures in the user level address space. Kernel level
threads are used as virtual processors to execute the user level threads. User level threads can
be successfully scheduled around blocking system calls, but low-level synchronization problems
still exist.
To solve the scheduling problems that low-level synchronization code introduces requires
some special support by the operating system that can detect when it is inappropriate for
Chapter 6. Related Work
142
context switching to occur. In the Psyche operating system [SLM89], rst class user level
threads [MSLM91] share locking information between the lock management routines and the
kernel. Soon before preemption is required, the kernel provides the user level with a two-minute
warning ag. User level synchronization code can check this ag prior to acquiring a lock, and
voluntarily relinquish control to the kernel if it deems necessary. While this technique does
not completely remove inappropriate scheduling decisions, the number of them is signicantly
reduced. The solution implemented in the Raven kernel eliminates this problem.
The scheduler activations technique [ABLL92] provides a more sure-re way of preventing
lock synchronization problems by allowing critical sections to complete before preempting the
processor. This is the same idea used by the Raven kernel, but scheduler activations implements
it quite dierently.
When an event occurs that would normally cause preemption during a critical section, an
upcall into the user space occurs. The user level kernel recognizes that a critical section is in
progress by checking the interrupted address, and jumps directly to the code that will complete
the critical section. This code is in fact a copy of the original, except that the tail end contains
a relinquishment call to the scheduler.
Instead of upcalling to the user level during a critical section, the Raven kernel defers the
preemption by setting an \upcall deferred" bit and returns to the user execution. At the end
of the critical section, the user checks the bit and relinquishes if it indicates so.
The scheduler activation technique allows lock operations to be as ecient as possible, because they do not require to manage any preemption status variables. However, additional
overhead is required by the upcall handler to dispatch control to the appropriate copy of the
critical section. The instruction pointer at the time of preemption must be examined and compared with a list of critical section handlers. This makes nested locking and critical sections with
multiple return points dicult to manage. Special compiler support is required to automate
the code copying. The upcall dispatcher in the Raven kernel avoids this diculty altogether.
Chapter 6. Related Work
143
6.3 Interprocess communication
In traditional operating systems, the kernel has mediated the interprocess communication mechanism. User programs wishing to communicate with remote services were required to invoke
kernel operations to perform the communication protocol. This process is now seen as being inecient due to the increased relative costs of crossing user/kernel boundaries compared
to simple procedure calls. Interprocess communication systems are concentrating on reducing
data copying costs and latencies by using memory mapping techniques and software supported
scheduling mechanisms.
Recent versions of the Mach 3.0 kernel have improved on typical kernel mediated IPC implementations by introducing the continuation [DBRD91]. Continuations facilitate the passage
of execution control through the kernel scheduling primitives by allowing execution context to
be handed o to another thread. This can eliminate scheduling overhead and queuing within
the kernel. In the fast path best case, the sender thread executes within the context of the
receiver thread.
The Lightweight Remote Procedure Call (LRPC) [BALL89] mechanism also involves kernel
intervention to pass messages between client and server, but takes advantage of architectural
details of the DEC SRC Firey. Execution progresses through the kernel and into the remote
address space using a special purpose stack structure that is used by both the sender and
receiver. Frequently used parameters are cached in processor registers.
As with the thread scheduling implementations discussed above, recent interprocess communication design have been removed from the kernel and implemented at the user level. This
allows users to directly invoke IPC primitives without the added costs of crossing user/kernel
boundaries. A level of indirection is removed because instead of invoking the kernel, the user
now directly communicates with the remote process.
The URPC technique [BALL90] relies on pair-wise shared memory between the client and
server processes to pass message data. The message delivery system is controlled by low priority
threads that poll the message queues looking for work. The threads only poll while the system
Chapter 6. Related Work
144
is idle. While this polling mechanism can produce low latency message transfers, this best case
scenario only occurs when there is no other work for the system. Therefore, this model is not
appropriate for systems with constant workloads. The Raven kernel is intended to be used in
applications where good IPC performance is required under load.
The split level scheduling technique and memory mapped streams implemented in the continuous media system [GA91] describes one way of using shared memory and scheduling techniques to reduce communication bottlenecks. Split level scheduling allows scheduling decisions
to be made at both the kernel and user level. A shared data structure between the user/kernel
level facilitates the communication of thread scheduling information. This sharing of information is similar to the Raven kernel, but in Raven, the amount of information shared is much less.
In order to properly honour thread priorities and real-time events in remote address spaces,
much more scheduling detail must be shared between the user and kernel.
The address-valued signal mechanism introduced in [CK93] describes a hardware assisted
low level signalling mechanism. This is a hardware solution to the Raven kernel's task signalling
facility. The hardware maintains a FIFO queue of signal interrupts, making it especially easy
and ecient for one address space to send a low level synchronization message to another
address space. Virtual addresses are used to direct the signal to any particular address space,
and the hardware handles the rest. The remote processor is interrupted and presented the next
signal on the hardware FIFO queue. Higher level communication protocols, such as RPC, can
be built with this low level mechanism.
Chapter 7
Conclusion
7.1 Summary
This report presented a new multiprocessor operating system, known as the Raven kernel. The
Raven kernel provides a multitasking, time sliced, environment for user level programs to execute in. The system provides the notion of tasks, virtual memory, threads, and interprocess
communication. However, unlike traditional microkernel architectures, the Raven kernel implements many of these services completely in user space. The motivation behind this design was
to improve system performance by reducing the number of user/kernel boundary changes.
An overview of the runtime environment used for the Raven kernel was provided. The
hardware platform used is the Motorola 88100 four processor Hypermodule, with 32MB of
shared memory. A special kernel debugger based on gdb allows kernel code to be interactively
debugged and tested.
The implementation of the kernel services was then described. The kernel consists of several
modules that provide three main services to user level programs:
Task management (allocation and scheduling).
Virtual memory management (memory allocation and mapping).
Low level upcall dispatching.
The description of the user level kernel followed. The following operating system services
were implemented completely at the user space:
Preemptive thread scheduling. Threads migrate from processor to processor in an eort
to balance the load.
145
Chapter 7. Conclusion
146
Device interrupt handlers. Hardware interrupts are eciently funnelled up from the kernel
to user level registered handlers.
Semaphores, used to synchronize events between threads in remote address spaces.
Interprocess communication. A synchronous and asynchronous port based messaging
scheme was implemented using shared memory queues and low level synchronization routines.
A nameserver database for fast lookup of global names.
A set of primitive low level event signalling routines made interprocess communication and
scheduling possible without kernel intervention:
The intr remote cpu() processor interruption facility delivers hardware interrupts to
remote processors.
The task signal() signalling facility sends asynchronous event message to remote tasks.
The performance results show that reduced kernel intervention and improved performance
is possible using these techniques. However, the trade-o between performance is stability. To
reduce communication bottlenecks between address spaces, extensive use of shared memory
regions is employed. These shared memory regions are left exposed to malicious processes or
errant program behaviour. Therefore, the system is suited towards dedicated environments
where programs are trusted.
7.2 Future Work
The Raven kernel is intended to provide the basis for an ecient and lightweight parallel
programming environment for high-speed parallel applications. Work will be continued in this
area to improve performance and add functionality.
The performance results showed that scheduling and lock contention overhead contributed
to most of the interprocess communication bottleneck. The current task and thread scheduler
Chapter 7. Conclusion
147
makes scheduling decisions based solely on the round-robin fairness scheme. A better task and
thread scheduler could be designed which would identify communicating threads, and try to
schedule them together to reduce context switching bottlenecks.
Appendix A
Kernel system call interface
This appendix summarizes the supervisor level system call interface that is available for general
purpose user programs. The rst section presents the system calls intended by use for user level
kernels. The second section presents the interface intended for user application programs.
A.1 System calls provided to the user level kernel
This section summarizes the system calls provided to user level kernels. User application programs should not call these routines directly.
A.1.1 Task management system calls
int
int
int
int
int
task_timer_event( int wakeup_time );
task_request_cpu();
task_relinquish_cpu();
task_intr_relinquish_cpu();
task_cleanup( int task_id );
A.1.2 Interrupt management system calls
These system calls register and enable interrupts to the user level.
int intr_register_user( int intr_vec );
int intr_deregister_user( int intr_vec );
A.1.3 Exception management system calls
These system calls register and enable exceptions to the user level.
148
Appendix A. Kernel system call interface
149
int excp_register_user( int intr_vec );
int excp_deregister_user( int intr_vec );
A.1.4 Global semaphore management
int kernel_sem_enqueue(int sem_id);
int kernel_sem_dequeue(int sem_id);
A.2 System calls provided for application programs
This section summarizes the system calls provided by the supervisor kernel for application
programs.
A.2.1 Task management system calls
int
int
int
int
task_suspend( int task_id );
task_resume( int task_id );
task_signal( int task_id, int signal, int user_data );
task_create( int *task_id, int priority, TASK_EXEC_HDR *hdr,
int code_region, int data_region, TASK_ARGS *args );
int task_destroy( int task_id );
int task_info( int task_id );
A.2.2 Virtual memory management system calls
int
int
int
int
vm_alloc(int *region_id, int task_id, void **addr, int size, int attrb );
vm_free(int region_id);
vm_move(int region_id, int dest_task_id, void **dest_addr, int dest_attrb);
vm_share( int src_region_id, int *dest_region_id, int dest_task_id,
void **dest_addr, int dest_attrb );
int vm_map_device( int *region_id, void *phys_addr, void **addr, int size );
int vm_unmap_device( int region_id );
A.2.3 Console input/output
void kprint(char *str);
Appendix A. Kernel system call interface
int kgetstr(char *str, int len);
A.2.4 Program loader
int read_exec_hdr(char *filespec, TASK_EXEC_HDR *hdr);
int read_exec( int code_region, int data_region );
150
Appendix B
User kernel library call interface
This appendix summarizes the user level kernel library call interface that is available for general
purpose user programs. All of these calls are prototyped in the <user/threads.h> header le.
B.1 Thread management
int thread_me();
int thread_sleep( unsigned long sleep_time );
int thread_suspend();
int thread_resume( int id );
void thread_sched();
int thread_create( int *thread_id, void (func)(), char *name, int priority,
int stacksize, int ready, int num_args, ...);
int thread_destroy( int id );
B.2 Synchronization primitives
/* Spin lock routines */
void lock_wait(int *lock);
void lock_free(int *lock);
/* semaphore routines */
int sem_wait( int sem_id, int no_block );
int sem_signal( int sem_id );
int sem_reset( int sem_id, int count );
int sem_count( int sem_id );
int sem_create( int *sem_id, int count, int global );
int sem_destroy( int sem_id );
B.3 Asynchronous Send/Receive port IPC
int port_send( int port_id, char *msg, int msg_size, int no_block );
int port_recv( int port_id, char *msg, int *msg_size, int no_block );
151
Appendix B. User kernel library call interface
152
int port_create( int *port_id, int max_msg_size, int queue_size, int global );
int port_destroy( int port_id );
/* for global ports only -- these do memory mappings of port queues */
int port_reference(int port_id);
int port_dereference(int port_id);
B.4 Synchronous Send/Receive/Reply port IPC
int rpc_port_send( int port_id, char
char *reply_data,
int rpc_port_recv( int port_id, char
char **reply_data
int rpc_port_reply( int port_id, int
int rpc_port_create(int *port_id);
int rpc_port_destroy(int port_id);
*send_data, int send_len,
int *reply_len );
**recv_data, int *recv_len,
);
msg_id, int reply_len );
/* special calls for global rpc_ports */
int grpc_port_create(int *port_id, int max_data_len, int num_msg_bufs);
int rpc_port_dereference(int port_id);
B.5 Nameserver
int nameserver_find(char *str, int *data);
int nameserver_register(char *str, int data);
int nameserver_deregister(char *str);
B.6 User level memory management
/* Zone memory allocator routines */
void *zone_alloc( int zone_id );
int zone_free( void *buf );
int zone_create( int *zone_id, int size, int alloc_size );
int zone_destroy( int zone_id );
void *malloc(int size);
#define free(buf) (zone_free(buf))
B.7 Interrupt and exception management
int intr_register( int intr_vec, void *handler );
int intr_deregister( int intr_vec )
Appendix B. User kernel library call interface
int excp_register( int excp_vec, void *routine, int global );
int excp_deregister( int excp_vec );
153
Appendix C
Unix version
A Unix version of the user level interface was implemented to aid in the development and
testing of user level programs. The Unix version implements a non-preemptive thread scheduler,
properly integrated with Unix lesystem I/O using a select() wrapper.
The following function prototypes document the Unix version interface:
int thread_me();
int thread_sleep( long sleep_time );
int thread_suspend( int id );
int thread_resume( int id );
void thread_sched();
int thread_create( int *thread_id, void (func)(), char *name, int priority,
int stacksize, int num_args, ...);
int thread_destroy( int id );
int sem_wait( int sem_id, int no_block );
int sem_signal( int sem_id );
int sem_reset( int sem_id, int count );
int sem_count( int sem_id );
int sem_create( int *sem_id, int count, int global );
int sem_destroy( int sem_id );
int port_send( int port_id, int *msg, int msg_size );
154
Appendix C. Unix version
int port_recv( int port_id, void **msg, int *msg_size );
int port_create( int *port_id, int msg_size, int queue_size, int attrib );
int port_destroy( int port_id );
/* for Unix I/O */
int Read( int fd, char *buf, int nbytes );
int ReadN( int fd, char *buf, int nbytes );
int Write( int fd, char *buf, int nbytes );
int WriteN( int fd, char *buf, int nbytes );
int Open( char *filespec, int flags, int mode );
int Close(int fd);
int Socket( int domain, int type, int protocol );
int Accept(int fd, struct sockaddr *addr, int *addrlen);
int Connect(int fd, struct sockaddr *name, int namelen);
155
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