26. Symmetric Multiprocessing Services¶

26.2.1. Uniprocessor versus SMP Parallelism¶

Uniprocessor systems have long been used in embedded systems. In this hardware model, there are some system execution characteristics which have long been taken for granted:

• one task executes at a time
• hardware events result in interrupts

There is no true parallelism. Even when interrupts appear to occur at the same time, they are processed in largely a serial fashion. This is true even when the interupt service routines are allowed to nest. From a tasking viewpoint, it is the responsibility of the real-time operatimg system to simulate parallelism by switching between tasks. These task switches occur in response to hardware interrupt events and explicit application events such as blocking for a resource or delaying.

With symmetric multiprocessing, the presence of multiple processors allows for true concurrency and provides for cost-effective performance improvements. Uniprocessors tend to increase performance by increasing clock speed and complexity. This tends to lead to hot, power hungry microprocessors which are poorly suited for many embedded applications.

The true concurrency is in sharp contrast to the single task and interrupt model of uniprocessor systems. This results in a fundamental change to uniprocessor system characteristics listed above. Developers are faced with a different set of characteristics which, in turn, break some existing assumptions and result in new challenges. In an SMP system with N processors, these are the new execution characteristics.

• N tasks execute in parallel
• hardware events result in interrupts

There is true parallelism with a task executing on each processor and the possibility of interrupts occurring on each processor. Thus in contrast to their being one task and one interrupt to consider on a uniprocessor, there are N tasks and potentially N simultaneous interrupts to consider on an SMP system.

This increase in hardware complexity and presence of true parallelism results in the application developer needing to be even more cautious about mutual exclusion and shared data access than in a uniprocessor embedded system. Race conditions that never or rarely happened when an application executed on a uniprocessor system, become much more likely due to multiple threads executing in parallel. On a uniprocessor system, these race conditions would only happen when a task switch occurred at just the wrong moment. Now there are N-1 tasks executing in parallel all the time and this results in many more opportunities for small windows in critical sections to be hit.

26.2.2. Task Affinity¶

RTEMS provides services to manipulate the affinity of a task. Affinity is used to specify the subset of processors in an SMP system on which a particular task can execute.

By default, tasks have an affinity which allows them to execute on any available processor.

Task affinity is a possible feature to be supported by SMP-aware schedulers. However, only a subset of the available schedulers support affinity. Although the behavior is scheduler specific, if the scheduler does not support affinity, it is likely to ignore all attempts to set affinity.

The scheduler with support for arbitary processor affinities uses a proof of concept implementation. See https://devel.rtems.org/ticket/2510.

26.2.3. Task Migration¶

With more than one processor in the system tasks can migrate from one processor to another. There are three reasons why tasks migrate in RTEMS.

• The scheduler changes explicitly via rtems_task_set_scheduler() or similar directives.
• The task resumes execution after a blocking operation. On a priority based scheduler it will evict the lowest priority task currently assigned to a processor in the processor set managed by the scheduler instance.
• The task moves temporarily to another scheduler instance due to locking protocols like Migratory Priority Inheritance or the Multiprocessor Resource Sharing Protocol.

Task migration should be avoided so that the working set of a task can stay on the most local cache level.

The current implementation of task migration in RTEMS has some implications with respect to the interrupt latency. It is crucial to preserve the system invariant that a task can execute on at most one processor in the system at a time. This is accomplished with a boolean indicator in the task context. The processor architecture specific low-level task context switch code will mark that a task context is no longer executing and waits that the heir context stopped execution before it restores the heir context and resumes execution of the heir task. So there is one point in time in which a processor is without a task. This is essential to avoid cyclic dependencies in case multiple tasks migrate at once. Otherwise some supervising entity is necessary to prevent life-locks. Such a global supervisor would lead to scalability problems so this approach is not used. Currently the thread dispatch is performed with interrupts disabled. So in case the heir task is currently executing on another processor then this prolongs the time of disabled interrupts since one processor has to wait for another processor to make progress.

It is difficult to avoid this issue with the interrupt latency since interrupts normally store the context of the interrupted task on its stack. In case a task is marked as not executing we must not use its task stack to store such an interrupt context. We cannot use the heir stack before it stopped execution on another processor. So if we enable interrupts during this transition we have to provide an alternative task independent stack for this time frame. This issue needs further investigation.

26.2.4. Clustered Scheduling¶

We have clustered scheduling in case the set of processors of a system is partitioned into non-empty pairwise-disjoint subsets. These subsets are called clusters. Clusters with a cardinality of one are partitions. Each cluster is owned by exactly one scheduler instance.

Clustered scheduling helps to control the worst-case latencies in multi-processor systems, see Brandenburg, Bjorn B.: Scheduling and Locking in Multiprocessor Real-Time Operating Systems. PhD thesis, 2011.http://www.cs.unc.edu/~bbb/diss/brandenburg-diss.pdf. The goal is to reduce the amount of shared state in the system and thus prevention of lock contention. Modern multi-processor systems tend to have several layers of data and instruction caches. With clustered scheduling it is possible to honour the cache topology of a system and thus avoid expensive cache synchronization traffic. It is easy to implement. The problem is to provide synchronization primitives for inter-cluster synchronization (more than one cluster is involved in the synchronization process). In RTEMS there are currently four means available

• events,
• message queues,
• semaphores using the Chapter 12 Section 2.3 - Priority Inheritance protocol (priority boosting), and
• semaphores using the Chapter 12 Section 2.5 - Multiprocessor Resource Sharing Protocol (MrsP).

The clustered scheduling approach enables separation of functions with real-time requirements and functions that profit from fairness and high throughput provided the scheduler instances are fully decoupled and adequate inter-cluster synchronization primitives are used. This is work in progress.

For the configuration of clustered schedulers see Chapter 24 Section 21.9 - Configuring Clustered Schedulers.

To set the scheduler of a task see SCHEDULER_IDENT - Get ID of a scheduler and Chapter 26 Section 4.8 - TASK_SET_SCHEDULER - Set scheduler of a task.

26.2.5. Task Priority Queues¶

Due to the support for clustered scheduling the task priority queues need special attention. It makes no sense to compare the priority values of two different scheduler instances. Thus, it is not possible to simply use one plain priority queue for tasks of different scheduler instances.

One solution to this problem is to use two levels of queues. The top level queue provides FIFO ordering and contains priority queues. Each priority queue is associated with a scheduler instance and contains only tasks of this scheduler instance. Tasks are enqueued in the priority queue corresponding to their scheduler instance. In case this priority queue was empty, then it is appended to the FIFO. To dequeue a task the highest priority task of the first priority queue in the FIFO is selected. Then the first priority queue is removed from the FIFO. In case the previously first priority queue is not empty, then it is appended to the FIFO. So there is FIFO fairness with respect to the highest priority task of each scheduler instances. See also Brandenburg, Bjorn B.: A fully preemptive multiprocessor semaphore protocol for latency-sensitive real-time applications. In Proceedings of the 25th Euromicro Conference on Real-Time Systems (ECRTS 2013), pages 292-302, 2013.http://www.mpi-sws.org/~bbb/papers/pdf/ecrts13b.pdf.

Such a two level queue may need a considerable amount of memory if fast enqueue and dequeue operations are desired (depends on the scheduler instance count). To mitigate this problem an approch of the FreeBSD kernel was implemented in RTEMS. We have the invariant that a task can be enqueued on at most one task queue. Thus, we need only as many queues as we have tasks. Each task is equipped with spare task queue which it can give to an object on demand. The task queue uses a dedicated memory space independent of the other memory used for the task itself. In case a task needs to block, then there are two options

• the object already has task queue, then the task enqueues itself to this already present queue and the spare task queue of the task is added to a list of free queues for this object, or
• otherwise, then the queue of the task is given to the object and the task enqueues itself to this queue.

In case the task is dequeued, then there are two options

• the task is the last task on the queue, then it removes this queue from the object and reclaims it for its own purpose, or
• otherwise, then the task removes one queue from the free list of the object and reclaims it for its own purpose.

Since there are usually more objects than tasks, this actually reduces the memory demands. In addition the objects contain only a pointer to the task queue structure. This helps to hide implementation details and makes it possible to use self-contained synchronization objects in Newlib and GCC (C++ and OpenMP run-time support).

26.2.6. Scheduler Helping Protocol¶

The scheduler provides a helping protocol to support locking protocols like Migratory Priority Inheritance or the Multiprocessor Resource Sharing Protocol. Each ready task can use at least one scheduler node at a time to gain access to a processor. Each scheduler node has an owner, a user and an optional idle task. The owner of a scheduler node is determined a task creation and never changes during the life time of a scheduler node. The user of a scheduler node may change due to the scheduler helping protocol. A scheduler node is in one of the four scheduler help states:

help yourself

This scheduler node is solely used by the owner task. This task owns no resources using a helping protocol and thus does not take part in the scheduler helping protocol. No help will be provided for other tasks.

help active owner

This scheduler node is owned by a task actively owning a resource and can be used to help out tasks. In case this scheduler node changes its state from ready to scheduled and the task executes using another node, then an idle task will be provided as a user of this node to temporarily execute on behalf of the owner task. Thus lower priority tasks are denied access to the processors of this scheduler instance. In case a task actively owning a resource performs a blocking operation, then an idle task will be used also in case this node is in the scheduled state.

help active rival

This scheduler node is owned by a task actively obtaining a resource currently owned by another task and can be used to help out tasks. The task owning this node is ready and will give away its processor in case the task owning the resource asks for help.

help passive

This scheduler node is owned by a task obtaining a resource currently owned by another task and can be used to help out tasks. The task owning this node is blocked.

The following scheduler operations return a task in need for help

• unblock,
• change priority,
• yield, and
• ask for help.

A task in need for help is a task that encounters a scheduler state change from scheduled to ready (this is a pre-emption by a higher priority task) or a task that cannot be scheduled in an unblock operation. Such a task can ask tasks which depend on resources owned by this task for help.

In case it is not possible to schedule a task in need for help, then the scheduler nodes available for the task will be placed into the set of ready scheduler nodes of the corresponding scheduler instances. Once a state change from ready to scheduled happens for one of scheduler nodes it will be used to schedule the task in need for help.

The ask for help scheduler operation is used to help tasks in need for help returned by the operations mentioned above. This operation is also used in case the root of a resource sub-tree owned by a task changes.

The run-time of the ask for help procedures depend on the size of the resource tree of the task needing help and other resource trees in case tasks in need for help are produced during this operation. Thus the worst-case latency in the system depends on the maximum resource tree size of the application.

26.2.7. Critical Section Techniques and SMP¶

As discussed earlier, SMP systems have opportunities for true parallelism which was not possible on uniprocessor systems. Consequently, multiple techniques that provided adequate critical sections on uniprocessor systems are unsafe on SMP systems. In this section, some of these unsafe techniques will be discussed.

In general, applications must use proper operating system provided mutual exclusion mechanisms to ensure correct behavior. This primarily means the use of binary semaphores or mutexes to implement critical sections.

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void legacy_code_with_interrupt_disable_enable( void )
{
    rtems_interrupt_level level;
    rtems_interrupt_disable( level );
    /* Some critical stuff */
    rtems_interrupt_enable( level );
}

RTEMS_INTERRUPT_LOCK_DEFINE( static, lock, "Name" );

void smp_ready_code_with_interrupt_lock( void )
{
    rtems_interrupt_lock_context lock_context;
    rtems_interrupt_lock_acquire( &lock, &lock_context );
    /* Some critical stuff */
    rtems_interrupt_lock_release( &lock, &lock_context );
}

26.2.8. Task Unique Data and SMP¶

Per task variables are a service commonly provided by real-time operating systems for application use. They work by allowing the application to specify a location in memory (typically a void *) which is logically added to the context of a task. On each task switch, the location in memory is stored and each task can have a unique value in the same memory location. This memory location is directly accessed as a variable in a program.

This works well in a uniprocessor environment because there is one task executing and one memory location containing a task-specific value. But it is fundamentally broken on an SMP system because there are always N tasks executing. With only one location in memory, N-1 tasks will not have the correct value.

This paradigm for providing task unique data values is fundamentally broken on SMP systems.

26.2.9. OpenMP¶

OpenMP support for RTEMS is available via the GCC provided libgomp. There is libgomp support for RTEMS in the POSIX configuration of libgomp since GCC 4.9 (requires a Newlib snapshot after 2015-03-12). In GCC 6.1 or later (requires a Newlib snapshot after 2015-07-30 for <sys/lock.h> provided self-contained synchronization objects) there is a specialized libgomp configuration for RTEMS which offers a significantly better performance compared to the POSIX configuration of libgomp. In addition application configurable thread pools for each scheduler instance are available in GCC 6.1 or later.

The run-time configuration of libgomp is done via environment variables documented in the libgomp manual. The environment variables are evaluated in a constructor function which executes in the context of the first initialization task before the actual initialization task function is called (just like a global C++ constructor). To set application specific values, a higher priority constructor function must be used to set up the environment variables.

#include <stdlib.h>
void __attribute__((constructor(1000))) config_libgomp( void )
{
    setenv( "OMP_DISPLAY_ENV", "VERBOSE", 1 );
    setenv( "GOMP_SPINCOUNT", "30000", 1 );
    setenv( "GOMP_RTEMS_THREAD_POOLS", "1$2@SCHD", 1 );
}

The environment variable GOMP_RTEMS_THREAD_POOLS is RTEMS-specific. It determines the thread pools for each scheduler instance. The format for GOMP_RTEMS_THREAD_POOLS is a list of optional <thread-pool-count>[$<priority>]@<scheduler-name> configurations separated by : where:

• <thread-pool-count> is the thread pool count for this scheduler instance.
• $<priority> is an optional priority for the worker threads of a thread pool according to pthread_setschedparam. In case a priority value is omitted, then a worker thread will inherit the priority of the OpenMP master thread that created it. The priority of the worker thread is not changed by libgomp after creation, even if a new OpenMP master thread using the worker has a different priority.
• @<scheduler-name> is the scheduler instance name according to the RTEMS application configuration.

In case no thread pool configuration is specified for a scheduler instance, then each OpenMP master thread of this scheduler instance will use its own dynamically allocated thread pool. To limit the worker thread count of the thread pools, each OpenMP master thread must call omp_set_num_threads.

Lets suppose we have three scheduler instances IO, WRK0, and WRK1 with GOMP_RTEMS_THREAD_POOLS set to "1@WRK0:3$4@WRK1". Then there are no thread pool restrictions for scheduler instance IO. In the scheduler instance WRK0 there is one thread pool available. Since no priority is specified for this scheduler instance, the worker thread inherits the priority of the OpenMP master thread that created it. In the scheduler instance WRK1 there are three thread pools available and their worker threads run at priority four.

26.2.10. Thread Dispatch Details¶

This section gives background information to developers interested in the interrupt latencies introduced by thread dispatching. A thread dispatch consists of all work which must be done to stop the currently executing thread on a processor and hand over this processor to an heir thread.

On SMP systems, scheduling decisions on one processor must be propagated to other processors through inter-processor interrupts. So, a thread dispatch which must be carried out on another processor happens not instantaneous. Thus several thread dispatch requests might be in the air and it is possible that some of them may be out of date before the corresponding processor has time to deal with them. The thread dispatch mechanism uses three per-processor variables,

• the executing thread,
• the heir thread, and
• an boolean flag indicating if a thread dispatch is necessary or not.

Updates of the heir thread and the thread dispatch necessary indicator are synchronized via explicit memory barriers without the use of locks. A thread can be an heir thread on at most one processor in the system. The thread context is protected by a TTAS lock embedded in the context to ensure that it is used on at most one processor at a time. The thread post-switch actions use a per-processor lock. This implementation turned out to be quite efficient and no lock contention was observed in the test suite.

The current implementation of thread dispatching has some implications with respect to the interrupt latency. It is crucial to preserve the system invariant that a thread can execute on at most one processor in the system at a time. This is accomplished with a boolean indicator in the thread context. The processor architecture specific context switch code will mark that a thread context is no longer executing and waits that the heir context stopped execution before it restores the heir context and resumes execution of the heir thread (the boolean indicator is basically a TTAS lock). So, there is one point in time in which a processor is without a thread. This is essential to avoid cyclic dependencies in case multiple threads migrate at once. Otherwise some supervising entity is necessary to prevent deadlocks. Such a global supervisor would lead to scalability problems so this approach is not used. Currently the context switch is performed with interrupts disabled. Thus in case the heir thread is currently executing on another processor, the time of disabled interrupts is prolonged since one processor has to wait for another processor to make progress.

It is difficult to avoid this issue with the interrupt latency since interrupts normally store the context of the interrupted thread on its stack. In case a thread is marked as not executing, we must not use its thread stack to store such an interrupt context. We cannot use the heir stack before it stopped execution on another processor. If we enable interrupts during this transition, then we have to provide an alternative thread independent stack for interrupts in this time frame. This issue needs further investigation.

The problematic situation occurs in case we have a thread which executes with thread dispatching disabled and should execute on another processor (e.g. it is an heir thread on another processor). In this case the interrupts on this other processor are disabled until the thread enables thread dispatching and starts the thread dispatch sequence. The scheduler (an exception is the scheduler with thread processor affinity support) tries to avoid such a situation and checks if a new scheduled thread already executes on a processor. In case the assigned processor differs from the processor on which the thread already executes and this processor is a member of the processor set managed by this scheduler instance, it will reassign the processors to keep the already executing thread in place. Therefore normal scheduler requests will not lead to such a situation. Explicit thread migration requests, however, can lead to this situation. Explicit thread migrations may occur due to the scheduler helping protocol or explicit scheduler instance changes. The situation can also be provoked by interrupts which suspend and resume threads multiple times and produce stale asynchronous thread dispatch requests in the system.

26.3.1. Setting Affinity to a Single Processor¶

On some embedded applications targeting SMP systems, it may be beneficial to lock individual tasks to specific processors. In this way, one can designate a processor for I/O tasks, another for computation, etc.. The following illustrates the code sequence necessary to assign a task an affinity for processor with index processor_index.

#include <rtems.h>
#include <assert.h>

void pin_to_processor(rtems_id task_id, int processor_index)
{
    rtems_status_code sc;
    cpu_set_t         cpuset;
    CPU_ZERO(&cpuset);
    CPU_SET(processor_index, &cpuset);
    sc = rtems_task_set_affinity(task_id, sizeof(cpuset), &cpuset);
    assert(sc == RTEMS_SUCCESSFUL);
}

It is important to note that the cpuset is not validated until the rtems_task_set_affinity call is made. At that point, it is validated against the current system configuration.

26.4.1. GET_PROCESSOR_COUNT - Get processor count¶

CALLING SEQUENCE:

uint32_t rtems_get_processor_count(void);

DIRECTIVE STATUS CODES:

The count of processors in the system.

DESCRIPTION:

On uni-processor configurations a value of one will be returned.

On SMP configurations this returns the value of a global variable set during system initialization to indicate the count of utilized processors. The processor count depends on the physically or virtually available processors and application configuration. The value will always be less than or equal to the maximum count of application configured processors.

NOTES:

None.

26.4.2. GET_CURRENT_PROCESSOR - Get current processor index¶

CALLING SEQUENCE:

uint32_t rtems_get_current_processor(void);

DIRECTIVE STATUS CODES:

The index of the current processor.

DESCRIPTION:

On uni-processor configurations a value of zero will be returned.

On SMP configurations an architecture specific method is used to obtain the index of the current processor in the system. The set of processor indices is the range of integers starting with zero up to the processor count minus one.

Outside of sections with disabled thread dispatching the current processor index may change after every instruction since the thread may migrate from one processor to another. Sections with disabled interrupts are sections with thread dispatching disabled.

NOTES:

None.

26.4.3. SCHEDULER_IDENT - Get ID of a scheduler¶

CALLING SEQUENCE:

rtems_status_code rtems_scheduler_ident(
    rtems_name  name,
    rtems_id   *id
);

DIRECTIVE STATUS CODES:

RTEMS_SUCCESSFUL	Successful operation.
RTEMS_INVALID_ADDRESS	The id parameter is NULL.
RTEMS_INVALID_NAME	Invalid scheduler name.

DESCRIPTION:

Identifies a scheduler by its name. The scheduler name is determined by the scheduler configuration. See Chapter 24 Section 21.9 - Configuring Clustered Schedulers and Chapter 24 Section 21.7 - Configuring a Scheduler Name.

NOTES:

None.

26.4.4. SCHEDULER_GET_PROCESSOR_SET - Get processor set of a scheduler¶

CALLING SEQUENCE:

rtems_status_code rtems_scheduler_get_processor_set(
    rtems_id   scheduler_id,
    size_t     cpusetsize,
    cpu_set_t *cpuset
);

DIRECTIVE STATUS CODES:

RTEMS_SUCCESSFUL	Successful operation.
RTEMS_INVALID_ID	Invalid scheduler instance identifier.
RTEMS_INVALID_ADDRESS	The cpuset parameter is NULL.
RTEMS_INVALID_NUMBER	The processor set buffer is too small for the set of processors owned by the scheduler instance.

DESCRIPTION:

Returns the processor set owned by the scheduler instance in cpuset. A set bit in the processor set means that this processor is owned by the scheduler instance and a cleared bit means the opposite.

NOTES:

None.

26.4.5. SCHEDULER_ADD_PROCESSOR - Add processor to a scheduler¶

CALLING SEQUENCE:

rtems_status_code rtems_scheduler_add_processor(
    rtems_id scheduler_id,
    uint32_t cpu_index
);

DIRECTIVE STATUS CODES:

RTEMS_SUCCESSFUL	Successful operation.
RTEMS_INVALID_ID	Invalid scheduler instance identifier.
RTEMS_NOT_CONFIGURED	The processor is not configured to be used by the application.
RTEMS_INCORRECT_STATE	The processor is configured to be used by the application, however, it is not online.
RTEMS_RESOURCE_IN_USE	The processor is already assigned to a scheduler instance.

DESCRIPTION:

Adds a processor to the set of processors owned by the specified scheduler instance.

NOTES:

Must be called from task context. This operation obtains and releases the objects allocator lock.

26.4.6. SCHEDULER_REMOVE_PROCESSOR - Remove processor from a scheduler¶

CALLING SEQUENCE:

rtems_status_code rtems_scheduler_remove_processor(
    rtems_id scheduler_id,
    uint32_t cpu_index
);

DIRECTIVE STATUS CODES:

RTEMS_SUCCESSFUL	Successful operation.
RTEMS_INVALID_ID	Invalid scheduler instance identifier.
RTEMS_INVALID_NUMBER	The processor is not owned by the specified scheduler instance.
RTEMS_RESOURCE_IN_USE	The set of processors owned by the specified scheduler instance would be empty after the processor removal and there exists a non-idle task that uses this scheduler instance as its home scheduler instance.

DESCRIPTION:

Removes a processor from set of processors owned by the specified scheduler instance.

NOTES:

Must be called from task context. This operation obtains and releases the objects allocator lock. Removing a processor from a scheduler is a complex operation that involves all tasks of the system.

26.4.7. TASK_GET_SCHEDULER - Get scheduler of a task¶

CALLING SEQUENCE:

rtems_status_code rtems_task_get_scheduler(
    rtems_id  task_id,
    rtems_id *scheduler_id
);

DIRECTIVE STATUS CODES:

RTEMS_SUCCESSFUL	successful operation
RTEMS_INVALID_ADDRESS	scheduler_id is NULL
RTEMS_INVALID_ID	invalid task id

DESCRIPTION:

Returns the scheduler identifier of a task identified by task_id in scheduler_id.

NOTES:

None.

26.4.8. TASK_SET_SCHEDULER - Set scheduler of a task¶

CALLING SEQUENCE:

rtems_status_code rtems_task_set_scheduler(
    rtems_id task_id,
    rtems_id scheduler_id
);

DIRECTIVE STATUS CODES:

RTEMS_SUCCESSFUL	successful operation
RTEMS_INVALID_ID	invalid task or scheduler id
RTEMS_INCORRECT_STATE	the task is in the wrong state to perform a scheduler change

DESCRIPTION:

Sets the scheduler of a task identified by task_id to the scheduler identified by scheduler_id. The scheduler of a task is initialized to the scheduler of the task that created it.

NOTES:

None.

EXAMPLE:

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#include <rtems.h> #include <assert.h> void task(rtems_task_argument arg); void example(void) { rtems_status_code sc; rtems_id task_id; rtems_id scheduler_id; rtems_name scheduler_name; scheduler_name = rtems_build_name('W', 'O', 'R', 'K'); sc = rtems_scheduler_ident(scheduler_name, &scheduler_id); assert(sc == RTEMS_SUCCESSFUL); sc = rtems_task_create( rtems_build_name('T', 'A', 'S', 'K'), 1, RTEMS_MINIMUM_STACK_SIZE, RTEMS_DEFAULT_MODES, RTEMS_DEFAULT_ATTRIBUTES, &task_id ); assert(sc == RTEMS_SUCCESSFUL); sc = rtems_task_set_scheduler(task_id, scheduler_id); assert(sc == RTEMS_SUCCESSFUL); sc = rtems_task_start(task_id, task, 0); assert(sc == RTEMS_SUCCESSFUL); }

26.4.9. TASK_GET_AFFINITY - Get task processor affinity¶

CALLING SEQUENCE:

rtems_status_code rtems_task_get_affinity(
    rtems_id   id,
    size_t     cpusetsize,
    cpu_set_t *cpuset
);

DIRECTIVE STATUS CODES:

RTEMS_SUCCESSFUL	successful operation
RTEMS_INVALID_ADDRESS	cpuset is NULL
RTEMS_INVALID_ID	invalid task id
RTEMS_INVALID_NUMBER	the affinity set buffer is too small for the current processor affinity set of the task

DESCRIPTION:

Returns the current processor affinity set of the task in cpuset. A set bit in the affinity set means that the task can execute on this processor and a cleared bit means the opposite.

NOTES:

None.

26.4.10. TASK_SET_AFFINITY - Set task processor affinity¶

CALLING SEQUENCE:

rtems_status_code rtems_task_set_affinity(
    rtems_id         id,
    size_t           cpusetsize,
    const cpu_set_t *cpuset
);

DIRECTIVE STATUS CODES:

RTEMS_SUCCESSFUL	successful operation
RTEMS_INVALID_ADDRESS	cpuset is NULL
RTEMS_INVALID_ID	invalid task id
RTEMS_INVALID_NUMBER	invalid processor affinity set

DESCRIPTION:

Sets the processor affinity set for the task specified by cpuset. A set bit in the affinity set means that the task can execute on this processor and a cleared bit means the opposite.

NOTES:

This function will not change the scheduler of the task. The intersection of the processor affinity set and the set of processors owned by the scheduler of the task must be non-empty. It is not an error if the processor affinity set contains processors that are not part of the set of processors owned by the scheduler instance of the task. A task will simply not run under normal circumstances on these processors since the scheduler ignores them. Some locking protocols may temporarily use processors that are not included in the processor affinity set of the task. It is also not an error if the processor affinity set contains processors that are not part of the system.