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    Kernel Overview

      Kernel

      Overview of the eCos Kernel

The kernel is one of the key packages in all of eCos. It provides the

core functionality needed for developing multi-threaded applications:

The ability to create new threads in the system, either during startup

or when the system is already running.

Control over the various threads in the system, for example

manipulating their priorities.

A choice of schedulers, determining which thread should currently be

running.

A range of synchronization primitives, allowing threads to interact

and share data safely.

Integration with the system's support for interrupts and exceptions.

In some other operating systems the kernel provides additional

functionality. For example the kernel may also provide memory

allocation functionality, and device drivers may be part of the kernel

as well. This is not the case for eCos. Memory allocation is handled

by a separate package. Similary each device driver will typically be a

separate package. Various packages are combined and configured using

the eCos configuration technology to meet the requirements of the

application.

The eCos kernel package is optional. It is possible to write

single-threaded applications which do not use any kernel

functionality, for example RedBoot. Typically such applications are

based around a central polling loop, continually checking all devices

and taking appropriate action when I/O occurs. A small amount of

calculation is possible every iteration, at the cost of an increased

delay between an I/O event occurring and the polling loop detecting

the event. When the requirements are straightforward it may well be

easier to develop the application using a polling loop, avoiding the

complexities of multiple threads and synchronization between threads.

As requirements get more complicated a multi-threaded solution becomes

more appropriate, requiring the use of the kernel. In fact some of the

more advanced packages in eCos, for example the TCP/IP stack, use

multi-threading internally. Therefore if the application uses any of

those packages then the kernel becomes a required package, not an

optional one.

The kernel functionality can be used in one of two ways. The kernel

provides its own C API, with functions like

cyg_thread_create and

cyg_mutex_lock. These can be called directly from

application code or from other packages. Alternatively there are a

number of packages which provide compatibility with existing API's,

for example POSIX threads or µITRON. These allow application

code to call standard functions such as

pthread_create, and those functions are

implemented using the basic functionality provided by the eCos kernel.

Using compatibility packages in an eCos application can make it much

easier to reuse code developed in other environments, and to share

code.

Although the different compatibility packages have similar

requirements on the underlying kernel, for example the ability to

create a new thread, there are differences in the exact semantics. For

example, strict µITRON compliance requires that kernel

timeslicing is disabled. This is achieved largely through the

configuration technology. The kernel provides a number of

configuration options that control the exact semantics that are

provided, and the various compatibility packages require particular

settings for those options. This has two important consequences.

First, it is not usually possible to have two different compatibility

packages in one eCos configuration because they will have conflicting

requirements on the underlying kernel. Second, the semantics of the

kernel's own API are only loosely defined because of the many

configuration options. For example cyg_mutex_lock

will always attempt to lock a mutex, but various configuration options

determine the behaviour when the mutex is already locked and there is

a possibility of priority inversion.

The optional nature of the kernel package presents some complications

for other code, especially device drivers. Wherever possible a device

driver should work whether or not the kernel is present. However there

are some parts of the system, especially those related to interrupt

handling, which should be implemented differently in multi-threaded

environments containing the eCos kernel and in single-threaded

environments without the kernel. To cope with both scenarios the

common HAL package provides a driver API, with functions such as

cyg_drv_interrupt_attach. When the kernel package

is present these driver API functions map directly on to the

equivalent kernel functions such as

cyg_interrupt_attach, using macros to avoid any

overheads. When the kernel is absent the common HAL package implements

the driver API directly, but this implementation is simpler than the

one in the kernel because it can assume a single-threaded environment.

When a system involves multiple threads, a scheduler is needed to

determine which thread should currently be running. The eCos kernel

can be configured with one of two schedulers, the bitmap scheduler and

the multi-level queue (MLQ) scheduler. The bitmap scheduler is

somewhat more efficient, but has a number of limitations. Most systems

will instead use the MLQ scheduler. Other schedulers may be added in

the future, either as extensions to the kernel package or in separate

packages.

Both the bitmap and the MLQ scheduler use a simple numerical priority

to determine which thread should be running. The number of priority

levels is configurable via the option

CYGNUM_KERNEL_SCHED_PRIORITIES, but a typical

system will have up to 32 priority levels. Therefore thread priorities

will be in the range 0 to 31, with 0 being the highest priority and 31

the lowest. Usually only the system's idle thread will run at the

lowest priority. Thread priorities are absolute, so the kernel will

only run a lower-priority thread if all higher-priority threads are

currently blocked.

The bitmap scheduler only allows one thread per priority level, so if

the system is configured with 32 priority levels then it is limited to

only 32 threads — still enough for many applications. A simple

bitmap can be used to keep track of which threads are currently

runnable. Bitmaps can also be used to keep track of threads waiting on

a mutex or other synchronization primitive. Identifying the

highest-priority runnable or waiting thread involves a simple

operation on the bitmap, and an array index operation can then be used

to get hold of the thread data structure itself. This makes the

bitmap scheduler fast and totally deterministic.

The MLQ scheduler allows multiple threads to run at the same priority.

This means that there is no limit on the number of threads in the

system, other than the amount of memory available. However operations

such as finding the highest priority runnable thread are a little bit

more expensive than for the bitmap scheduler.

Optionally the MLQ scheduler supports timeslicing, where the scheduler

automatically switches from one runnable thread to another when some

number of clock ticks have occurred. Timeslicing only comes into play

when there are two runnable threads at the same priority and no higher

priority runnable threads. If timeslicing is disabled then a thread

will not be preempted by another thread of the same priority, and will

continue running until either it explicitly yields the processor or

until it blocks by, for example, waiting on a synchronization

primitive. The configuration options

CYGSEM_KERNEL_SCHED_TIMESLICE and

CYGNUM_KERNEL_SCHED_TIMESLICE_TICKS control

timeslicing. The bitmap scheduler does not provide timeslicing

support. It only allows one thread per priority level, so it is not

possible to preempt the current thread in favour of another one with

the same priority.

Another important configuration option that affects the MLQ scheduler

is CYGIMP_KERNEL_SCHED_SORTED_QUEUES. This

determines what happens when a thread blocks, for example by waiting

on a semaphore which has no pending events. The default behaviour of

the system is last-in-first-out queuing. For example if several

threads are waiting on a semaphore and an event is posted, the thread

that gets woken up is the last one that called

cyg_semaphore_wait. This allows for a simple and

fast implementation of both the queue and dequeue operations. However

if there are several queued threads with different priorities, it may

not be the highest priority one that gets woken up. In practice this

is rarely a problem: usually there will be at most one thread waiting

on a queue, or when there are several threads they will be of the same

priority. However if the application does require strict priority

queueing then the option

CYGIMP_KERNEL_SCHED_SORTED_QUEUES should be

enabled. There are disadvantages: more work is needed whenever a

thread is queued, and the scheduler needs to be locked for this

operation so the system's dispatch latency is worse. If the bitmap

scheduler is used then priority queueing is automatic and does not

involve any penalties.

Some kernel functionality is currently only supported with the MLQ

scheduler, not the bitmap scheduler. This includes support for SMP

systems, and protection against priority inversion using either mutex

priority ceilings or priority inheritance.

The eCos kernel provides a number of different synchronization

primitives: mutexes,

condition variables,

counting semaphores,

mail boxes and

event flags.

Mutexes serve a very different purpose from the other primitives. A

mutex allows multiple threads to share a resource safely: a thread

locks a mutex, manipulates the shared resource, and then unlocks the

mutex again. The other primitives are used to communicate information

between threads, or alternatively from a DSR associated with an

interrupt handler to a thread.

When a thread that has locked a mutex needs to wait for some condition

to become true, it should use a condition variable. A condition

variable is essentially just a place for a thread to wait, and which

another thread, or DSR, can use to wake it up. When a thread waits on

a condition variable it releases the mutex before waiting, and when it

wakes up it reacquires it before proceeding. These operations are

atomic so that synchronization race conditions cannot be introduced.

A counting semaphore is used to indicate that a particular event has

occurred. A consumer thread can wait for this event to occur, and a

producer thread or a DSR can post the event. There is a count

associated with the semaphore so if the event occurs multiple times in

quick succession this information is not lost, and the appropriate

number of semaphore wait operations will succeed.

Mail boxes are also used to indicate that a particular event has

occurred, and allows for one item of data to be exchanged per event.

Typically this item of data would be a pointer to some data structure.

Because of the need to store this extra data, mail boxes have a

finite capacity. If a producer thread generates mail box events

faster than they can be consumed then, to avoid overflow, it will be

blocked until space is again available in the mail box. This means

that mail boxes usually cannot be used by a DSR to wake up a

thread. Instead mail boxes are typically only used between threads.

Event flags can be used to wait on some number of different events,

and to signal that one or several of these events have occurred. This

is achieved by associating bits in a bit mask with the different

events. Unlike a counting semaphore no attempt is made to keep track

of the number of events that have occurred, only the fact that an

event has occurred at least once. Unlike a mail box it is not

possible to send additional data with the event, but this does mean

that there is no possibility of an overflow and hence event flags can

be used between a DSR and a thread as well as between threads.

The eCos common HAL package provides its own device driver API which

contains some of the above synchronization primitives. These allow

the DSR for an interrupt handler to signal events to higher-level

code. If the configuration includes the eCos kernel package then

the driver API routines map directly on to the equivalent kernel

routines, allowing interrupt handlers to interact with threads. If the

kernel package is not included and the application consists of just a

single thread running in polled mode then the driver API is

implemented entirely within the common HAL, and with no need to worry

about multiple threads the implementation can obviously be rather

simpler.

During normal operation the processor will be running one of the

threads in the system. This may be an application thread, a system

thread running inside say the TCP/IP stack, or the idle thread. From

time to time a hardware interrupt will occur, causing control to be

transferred briefly to an interrupt handler. When the interrupt has

been completed the system's scheduler will decide whether to return

control to the interrupted thread or to some other runnable thread.

Threads and interrupt handlers must be able to interact. If a thread

is waiting for some I/O operation to complete, the interrupt handler

associated with that I/O must be able to inform the thread that the

operation has completed. This can be achieved in a number of ways. One

very simple approach is for the interrupt handler to set a volatile

variable. A thread can then poll continuously until this flag is set,

possibly sleeping for a clock tick in between. Polling continuously

means that the cpu time is not available for other activities, which

may be acceptable for some but not all applications. Polling once

every clock tick imposes much less overhead, but means that the thread

may not detect that the I/O event has occurred until an entire clock

tick has elapsed. In typical systems this could be as long as 10

milliseconds. Such a delay might be acceptable for some applications,

but not all.

A better solution would be to use one of the synchronization

primitives. The interrupt handler could signal a condition variable,

post to a semaphore, or use one of the other primitives. The thread

would perform a wait operation on the same primitive. It would not

consume any cpu cycles until the I/O event had occurred, and when the

event does occur the thread can start running again immediately

(subject to any higher priority threads that might also be runnable).

Synchronization primitives constitute shared data, so care must be

taken to avoid problems with concurrent access. If the thread that was

interrupted was just performing some calculations then the interrupt

handler could manipulate the synchronization primitive quite safely.

However if the interrupted thread happened to be inside some kernel

call then there is a real possibility that some kernel data structure

will be corrupted.

One way of avoiding such problems would be for the kernel functions to

disable interrupts when executing any critical region. On most

architectures this would be simple to implement and very fast, but it

would mean that interrupts would be disabled often and for quite a

long time. For some applications that might not matter, but many

embedded applications require that the interrupt handler run as soon

as possible after the hardware interrupt has occurred. If the kernel

relied on disabling interrupts then it would not be able to support

such applications.

Instead the kernel uses a two-level approach to interrupt handling.

Associated with every interrupt vector is an Interrupt Service Routine

or ISR, which will run as quickly as possible so that it can service

the hardware. However an ISR can make only a small number of kernel

calls, mostly related to the interrupt subsystem, and it cannot make

any call that would cause a thread to wake up. If an ISR detects that

an I/O operation has completed and hence that a thread should be woken

up, it can cause the associated Deferred Service Routine or DSR to

run. A DSR is allowed to make more kernel calls, for example it can

signal a condition variable or post to a semaphore.

Disabling interrupts prevents ISRs from running, but very few parts of

the system disable interrupts and then only for short periods of time.

The main reason for a thread to disable interrupts is to manipulate

some state that is shared with an ISR. For example if a thread needs

to add another buffer to a linked list of free buffers and the ISR may

remove a buffer from this list at any time, the thread would need to

disable interrupts for the few instructions needed to manipulate the

list. If the hardware raises an interrupt at this time, it remains

pending until interrupts are reenabled.

Analogous to interrupts being disabled or enabled, the kernel has a

scheduler lock. The various kernel functions such as

cyg_mutex_lock and

cyg_semaphore_post will claim the scheduler lock,

manipulate the kernel data structures, and then release the scheduler

lock. If an interrupt results in a DSR being requested and the

scheduler is currently locked, the DSR remains pending. When the

scheduler lock is released any pending DSRs will run. These may post

events to synchronization primitives, causing other higher priority

threads to be woken up.

For an example, consider the following scenario. The system has a high

priority thread A, responsible for processing some data coming from an

external device. This device will raise an interrupt when data is

available. There are two other threads B and C which spend their time

performing calculations and occasionally writing results to a display

of some sort. This display is a shared resource so a mutex is used to

control access.

At a particular moment in time thread A is likely to be blocked,

waiting on a semaphore or another synchronization primitive until data

is available. Thread B might be running performing some calculations,

and thread C is runnable waiting for its next timeslice. Interrupts

are enabled, and the scheduler is unlocked because none of the threads

are in the middle of a kernel operation. At this point the device

raises an interrupt. The hardware transfers control to a low-level

interrupt handler provided by eCos which works out exactly which

interrupt occurs, and then the corresponding ISR is run. This ISR

manipulates the hardware as appropriate, determines that there is now

data available, and wants to wake up thread A by posting to the

semaphore. However ISR's are not allowed to call

cyg_semaphore_post directly, so instead the ISR

requests that its associated DSR be run and returns. There are no more

interrupts to be processed, so the kernel next checks for DSR's. One

DSR is pending and the scheduler is currently unlocked, so the DSR can

run immediately and post the semaphore. This will have the effect of

making thread A runnable again, so the scheduler's data structures are

adjusted accordingly. When the DSR returns thread B is no longer the

highest priority runnable thread so it will be suspended, and instead

thread A gains control over the cpu.

In the above example no kernel data structures were being manipulated

at the exact moment that the interrupt happened. However that cannot

be assumed. Suppose that thread B had finished its current set of

calculations and wanted to write the results to the display. It would

claim the appropriate mutex and manipulate the display. Now suppose

that thread B was timesliced in favour of thread C, and that thread C

also finished its calculations and wanted to write the results to the

display. It would call cyg_mutex_lock. This

kernel call locks the scheduler, examines the current state of the

mutex, discovers that the mutex is already owned by another thread,

suspends the current thread, and switches control to another runnable

thread. Another interrupt happens in the middle of this

cyg_mutex_lock call, causing the ISR to run

immediately. The ISR decides that thread A should be woken up so it

requests that its DSR be run and returns back to the kernel. At this

point there is a pending DSR, but the scheduler is still locked by the

call to cyg_mutex_lock so the DSR cannot run

immediately. Instead the call to cyg_mutex_lock

is allowed to continue, which at some point involves unlocking the

scheduler. The pending DSR can now run, safely post the semaphore, and

thus wake up thread A.

If the ISR had called cyg_semaphore_post directly

rather than leaving it to a DSR, it is likely that there would have

been some sort of corruption of a kernel data structure. For example

the kernel might have completely lost track of one of the threads, and

that thread would never have run again. The two-level approach to

interrupt handling, ISR's and DSR's, prevents such problems with no

need to disable interrupts.

eCos defines a number of contexts. Only certain calls are allowed from

inside each context, for example most operations on threads or

synchronization primitives are not allowed from ISR context. The

different contexts are initialization, thread, ISR and DSR.

When eCos starts up it goes through a number of phases, including

setting up the hardware and invoking C++ static constructors. During

this time interrupts are disabled and the scheduler is locked. When a

configuration includes the kernel package the final operation is a

call to 

linkend="kernel-schedcontrol">cyg_scheduler_start.

At this point interrupts are enabled, the scheduler is unlocked, and

control is transferred to the highest priority runnable thread. If the

configuration also includes the C library package then usually the C

library startup package will have created a thread which will call the

application's main entry point.

Some application code can also run before the scheduler is started,

and this code runs in initialization context. If the application is

written partly or completely in C++ then the constructors for any

static objects will be run. Alternatively application code can define

a function cyg_user_start which gets called after

any C++ static constructors. This allows applications to be written

entirely in C.

void

cyg_user_start(void)

{

    /* Perform application-specific initialization here */

}

It is not necessary for applications to provide a

cyg_user_start function since the system will

provide a default implementation which does nothing.

Typical operations that are performed from inside static constructors

or cyg_user_start include creating threads,

synchronization primitives, setting up alarms, and registering

application-specific interrupt handlers. In fact for many applications

all such creation operations happen at this time, using statically

allocated data, avoiding any need for dynamic memory allocation or

other overheads.

Code running in initialization context runs with interrupts disabled

and the scheduler locked. It is not permitted to reenable interrupts

or unlock the scheduler because the system is not guaranteed to be in

a totally consistent state at this point. A consequence is that

initialization code cannot use synchronization primitives such as

cyg_semaphore_wait to wait for an external event.

It is permitted to lock and unlock a mutex: there are no other threads

running so it is guaranteed that the mutex is not yet locked, and

therefore the lock operation will never block; this is useful when

making library calls that may use a mutex internally.

At the end of the startup sequence the system will call

cyg_scheduler_start and the various threads will

start running. In thread context nearly all of the kernel functions

are available. There may be some restrictions on interrupt-related

operations, depending on the target hardware. For example the hardware

may require that interrupts be acknowledged in the ISR or DSR before

control returns to thread context, in which case

cyg_interrupt_acknowledge should not be called

by a thread.

At any time the processor may receive an external interrupt, causing

control to be transferred from the current thread. Typically a VSR

provided by eCos will run and determine exactly which interrupt

occurred. Then the VSR will switch to the appropriate ISR, which can

be provided by a HAL package, a device driver, or by the application.

During this time the system is running at ISR context, and most of the

kernel function calls are disallowed. This includes the various

synchronization primitives, so for example an ISR is not allowed to

post to a semaphore to indicate that an event has happened. Usually

the only operations that should be performed from inside an ISR are

ones related to the interrupt subsystem itself, for example masking an

interrupt or acknowledging that an interrupt has been processed. On

SMP systems it is also possible to use spinlocks from ISR context.

When an ISR returns it can request that the corresponding DSR be run

as soon as it is safe to do so, and that will run in DSR context. This

context is also used for running alarm functions, and threads can

switch temporarily to DSR context by locking the scheduler. Only

certain kernel functions can be called from DSR context, although more

than in ISR context. In particular it is possible to use any

synchronization primitives which cannot block.  These include

cyg_semaphore_post,

cyg_cond_signal,

cyg_cond_broadcast,

cyg_flag_setbits, and

cyg_mbox_tryput. It is not possible to use any

primitives that may block such as

cyg_semaphore_wait,

cyg_mutex_lock, or

cyg_mbox_put. Calling such functions from inside

a DSR may cause the system to hang.

The specific documentation for the various kernel functions gives more

details about valid contexts.

In many APIs each function is expected to perform some validation of

its parameters and possibly of the current state of the system. This

is supposed to ensure that each function is used correctly, and that

application code is not attempting to perform a semaphore operation on

a mutex or anything like that. If an error is detected then a suitable

error code is returned, for example the POSIX function

pthread_mutex_lock can return various error codes

including EINVAL and EDEADLK.

There are a number of problems with this approach, especially in the

context of deeply embedded systems:

Performing these checks inside the mutex lock and all the other

functions requires extra cpu cycles and adds significantly to the code

size. Even if the application is written correctly and only makes

system function calls with sensible arguments and under the right

conditions, these overheads still exist.

Returning an error code is only useful if the calling code detects

these error codes and takes appropriate action. In practice the

calling code will often ignore any errors because the programmer

“knows” that the function is being

used correctly. If the programmer is mistaken then an error condition

may be detected and reported, but the application continues running

anyway and is likely to fail some time later in mysterious ways.

If the calling code does always check for error codes, that adds yet

more cpu cycles and code size overhead.

Usually there will be no way to recover from certain errors, so if the

application code detected an error such as EINVAL

then all it could do is abort the application somehow.

The approach taken within the eCos kernel is different. Functions such

as cyg_mutex_lock will not return an error code.

Instead they contain various assertions, which can be enabled or

disabled. During the development process assertions are normally left

enabled, and the various kernel functions will perform parameter

checks and other system consistency checks. If a problem is detected

then an assertion failure will be reported and the application will be

terminated. In a typical debug session a suitable breakpoint will have

been installed and the developer can now examine the state of the

system and work out exactly what is going on. Towards the end of the

development cycle assertions will be disabled by manipulating

configuration options within the eCos infrastructure package, and all

assertions will be eliminated at compile-time. The assumption is that

by this time the application code has been mostly debugged: the

initial version of the code might have tried to perform a semaphore

operation on a mutex, but any problems like that will have been fixed

some time ago. This approach has a number of advantages:

In the final application there will be no overheads for checking

parameters and other conditions. All that code will have been

eliminated at compile-time.

Because the final application will not suffer any overheads, it is

reasonable for the system to do more work during the development

process. In particular the various assertions can test for more error

conditions and more complicated errors. When an error is detected

it is possible to give a text message describing the error rather than

just return an error code.

There is no need for application programmers to handle error codes

returned by various kernel function calls. This simplifies the

application code.

If an error is detected then an assertion failure will be reported

immediately and the application will be halted. There is no

possibility of an error condition being ignored because application

code did not check for an error code.

Although none of the kernel functions return an error code, many of

them do return a status condition. For example the function

cyg_semaphore_timed_wait waits until either an

event has been posted to a semaphore, or until a certain number of

clock ticks have occurred. Usually the calling code will need to know

whether the wait operation succeeded or whether a timeout occurred.

cyg_semaphore_timed_wait returns a boolean: a

return value of zero or false indicates a timeout, a non-zero return

value indicates that the wait succeeded.

In conventional APIs one common error conditions is lack of memory.

For example the POSIX function pthread_create

usually has to allocate some memory dynamically for the thread stack

and other per-thread data. If the target hardware does not have enough

memory to meet all demands, or more commonly if the application

contains a memory leak, then there may not be enough memory available

and the function call would fail. The eCos kernel avoids such problems

by never performing any dynamic memory allocation. Instead it is the

responsibility of the application code to provide all the memory

required for kernel data structures and other needs. In the case of

cyg_thread_create this means a

cyg_thread data structure to hold the thread

details, and a char array for the thread stack.

In many applications this approach results in all data structures

being allocated statically rather than dynamically. This has several

advantages. If the application is in fact too large for the target

hardware's memory then there will be an error at link-time rather than

at run-time, making the problem much easier to diagnose. Static

allocation does not involve any of the usual overheads associated with

dynamic allocation, for example there is no need to keep track of the

various free blocks in the system, and it may be possible to eliminate

malloc from the system completely. Problems such

as fragmentation and memory leaks cannot occur if all data is

allocated statically. However, some applications are sufficiently

complicated that dynamic memory allocation is required, and the

various kernel functions do not distinguish between statically and

dynamically allocated memory. It still remains the responsibility of

the calling code to ensure that sufficient memory is available, and

passing null pointers to the kernel will result in assertions or

system failure.

    SMP Support

      SMP

      Support Symmetric Multiprocessing Systems

eCos contains support for limited Symmetric Multi-Processing (SMP).

This is only available on selected architectures and platforms.

The implementation has a number of restrictions on the kind of

hardware supported. These are described in .

The following sections describe the changes that have been made to the

eCos kernel to support SMP operation.

The system startup sequence needs to be somewhat different on an SMP

system, although this is largely transparent to application code. The

main startup takes place on only one CPU, called the primary CPU. All

other CPUs, the secondary CPUs, are either placed in suspended state

at reset, or are captured by the HAL and put into a spin as they start

up. The primary CPU is responsible for copying the DATA segment and

zeroing the BSS (if required), calling HAL variant and platform

initialization routines and invoking constructors. It then calls

cyg_start to enter the application. The

application may then create extra threads and other objects.

It is only when the application calls

cyg_scheduler_start that the secondary CPUs are

initialized. This routine scans the list of available secondary CPUs

and invokes HAL_SMP_CPU_START to start each

CPU. Finally it calls an internal function

Cyg_Scheduler::start_cpu to enter the scheduler

for the primary CPU.

Each secondary CPU starts in the HAL, where it completes any per-CPU

initialization before calling into the kernel at

cyg_kernel_cpu_startup. Here it claims the

scheduler lock and calls

Cyg_Scheduler::start_cpu.

Cyg_Scheduler::start_cpu is common to both the

primary and secondary CPUs. The first thing this code does is to

install an interrupt object for this CPU's inter-CPU interrupt. From

this point on the code is the same as for the single CPU case: an

initial thread is chosen and entered.

From this point on the CPUs are all equal, eCos makes no further

distinction between the primary and secondary CPUs. However, the

hardware may still distinguish between them as far as interrupt

delivery is concerned.

To function correctly an operating system kernel must protect its

vital data structures, such as the run queues, from concurrent

access. In a single CPU system the only concurrent activities to worry

about are asynchronous interrupts. The kernel can easily guard its

data structures against these by disabling interrupts. However, in a

multi-CPU system, this is inadequate since it does not block access by

other CPUs.

The eCos kernel protects its vital data structures using the scheduler

lock. In single CPU systems this is a simple counter that is

atomically incremented to acquire the lock and decremented to release

it. If the lock is decremented to zero then the scheduler may be

invoked to choose a different thread to run. Because interrupts may

continue to be serviced while the scheduler lock is claimed, ISRs are

not allowed to access kernel data structures, or call kernel routines

that can. Instead all such operations are deferred to an associated

DSR routine that is run during the lock release operation, when the

data structures are in a consistent state.

By choosing a kernel locking mechanism that does not rely on interrupt

manipulation to protect data structures, it is easier to convert eCos

to SMP than would otherwise be the case. The principal change needed to

make eCos SMP-safe is to convert the scheduler lock into a nestable

spin lock. This is done by adding a spinlock and a CPU id to the

original counter.

The algorithm for acquiring the scheduler lock is very simple. If the

scheduler lock's CPU id matches the current CPU then it can just increment

the counter and continue. If it does not match, the CPU must spin on

the spinlock, after which it may increment the counter and store its

own identity in the CPU id.

To release the lock, the counter is decremented. If it goes to zero

the CPU id value must be set to NONE and the spinlock cleared.

To protect these sequences against interrupts, they must be performed

with interrupts disabled. However, since these are very short code

sequences, they will not have an adverse effect on the interrupt

latency.

Beyond converting the scheduler lock, further preparing the kernel for

SMP is a relatively minor matter. The main changes are to convert

various scalar housekeeping variables into arrays indexed by CPU

id. These include the current thread pointer, the need_reschedule

flag and the timeslice counter.

At present only the Multi-Level Queue (MLQ) scheduler is capable of

supporting SMP configurations. The main change made to this scheduler

is to cope with having several threads in execution at the same

time. Running threads are marked with the CPU that they are executing on.

When scheduling a thread, the scheduler skips past any running threads

until it finds a thread that is pending. While not a constant-time

algorithm, as in the single CPU case, this is still deterministic,

since the worst case time is bounded by the number of CPUs in the

system.

A second change to the scheduler is in the code used to decide when

the scheduler should be called to choose a new thread. The scheduler

attempts to keep the n CPUs running the

n highest priority threads. Since an event or

interrupt on one CPU may require a reschedule on another CPU, there

must be a mechanism for deciding this. The algorithm currently

implemented is very simple. Given a thread that has just been awakened

(or had its priority changed), the scheduler scans the CPUs, starting

with the one it is currently running on, for a current thread that is

of lower priority than the new one. If one is found then a reschedule

interrupt is sent to that CPU and the scan continues, but now using

the current thread of the rescheduled CPU as the candidate thread. In

this way the new thread gets to run as quickly as possible, hopefully

on the current CPU, and the remaining CPUs will pick up the remaining

highest priority threads as a consequence of processing the reschedule

interrupt.

The final change to the scheduler is in the handling of

timeslicing. Only one CPU receives timer interrupts, although all CPUs

must handle timeslicing. To make this work, the CPU that receives the

timer interrupt decrements the timeslice counter for all CPUs, not

just its own. If the counter for a CPU reaches zero, then it sends a

timeslice interrupt to that CPU. On receiving the interrupt the

destination CPU enters the scheduler and looks for another thread at