From ba9dfcf92c8aeb832c6f2c7ea7c4150064960d63 Mon Sep 17 00:00:00 2001 From: Sebastian Huber Date: Thu, 20 Aug 2020 10:54:56 +0200 Subject: c-user: Split up scheduling concepts Introduce a background section. This makes it easier to automatically generate parts of the scheduling concepts documentation in the future. Update #3993. --- c-user/scheduling-concepts/background.rst | 327 ++++++++++++++++ c-user/scheduling-concepts/directives.rst | 424 +++++++++++++++++++++ c-user/scheduling-concepts/index.rst | 19 + c-user/scheduling-concepts/introduction.rst | 42 ++ c-user/scheduling-concepts/smp-schedulers.rst | 69 ++++ .../uniprocessor-schedulers.rst | 113 ++++++ 6 files changed, 994 insertions(+) create mode 100644 c-user/scheduling-concepts/background.rst create mode 100644 c-user/scheduling-concepts/directives.rst create mode 100644 c-user/scheduling-concepts/index.rst create mode 100644 c-user/scheduling-concepts/introduction.rst create mode 100644 c-user/scheduling-concepts/smp-schedulers.rst create mode 100644 c-user/scheduling-concepts/uniprocessor-schedulers.rst (limited to 'c-user/scheduling-concepts') diff --git a/c-user/scheduling-concepts/background.rst b/c-user/scheduling-concepts/background.rst new file mode 100644 index 0000000..7f8a63d --- /dev/null +++ b/c-user/scheduling-concepts/background.rst @@ -0,0 +1,327 @@ +.. SPDX-License-Identifier: CC-BY-SA-4.0 + +.. Copyright (C) 2011 Petr Benes +.. Copyright (C) 2010 Gedare Bloom +.. Copyright (C) 1988, 2008 On-Line Applications Research Corporation (OAR) + +Background +========== + +.. index:: scheduling algorithms + +Scheduling Algorithms +--------------------- + +RTEMS provides a plugin framework that allows it to support multiple +scheduling algorithms. RTEMS includes multiple scheduling algorithms, and the +user can select which of these they wish to use in their application at +link-time. In addition, the user can implement their own scheduling algorithm +and configure RTEMS to use it. + +Supporting multiple scheduling algorithms gives the end user the option to +select the algorithm which is most appropriate to their use case. Most +real-time operating systems schedule tasks using a priority based algorithm, +possibly with preemption control. The classic RTEMS scheduling algorithm which +was the only algorithm available in RTEMS 4.10 and earlier, is a fixed-priority +scheduling algorithm. This scheduling algorithm is suitable for uniprocessor +(e.g., non-SMP) systems and is known as the *Deterministic Priority +Scheduler*. Unless the user configures another scheduling algorithm, RTEMS +will use this on uniprocessor systems. + +.. index:: priority scheduling + +Priority Scheduling +------------------- + +When using priority based scheduling, RTEMS allocates the processor using a +priority-based, preemptive algorithm augmented to provide round-robin +characteristics within individual priority groups. The goal of this algorithm +is to guarantee that the task which is executing on the processor at any point +in time is the one with the highest priority among all tasks in the ready +state. + +When a task is added to the ready chain, it is placed behind all other tasks of +the same priority. This rule provides a round-robin within a priority group +scheduling characteristic. This means that in a group of equal priority tasks, +tasks will execute in the order they become ready or FIFO order. Even though +there are ways to manipulate and adjust task priorities, the most important +rule to remember is: + +.. note:: + + Priority based scheduling algorithms will always select the highest priority + task that is ready to run when allocating the processor to a task. + +Priority scheduling is the most commonly used scheduling algorithm. It should +be used by applications in which multiple tasks contend for CPU time or other +resources, and there is a need to ensure certain tasks are given priority over +other tasks. + +There are a few common methods of accomplishing the mechanics of this +algorithm. These ways involve a list or chain of tasks in the ready state. + +- The least efficient method is to randomly place tasks in the ready chain + forcing the scheduler to scan the entire chain to determine which task + receives the processor. + +- A more efficient method is to schedule the task by placing it in the proper + place on the ready chain based on the designated scheduling criteria at the + time it enters the ready state. Thus, when the processor is free, the first + task on the ready chain is allocated the processor. + +- Another mechanism is to maintain a list of FIFOs per priority. When a task + is readied, it is placed on the rear of the FIFO for its priority. This + method is often used with a bitmap to assist in locating which FIFOs have + ready tasks on them. This data structure has :math:`O(1)` insert, extract + and find highest ready run-time complexities. + +- A red-black tree may be used for the ready queue with the priority as the + key. This data structure has :math:`O(log(n))` insert, extract and find + highest ready run-time complexities while :math:`n` is the count of tasks in + the ready queue. + +RTEMS currently includes multiple priority based scheduling algorithms as well +as other algorithms that incorporate deadline. Each algorithm is discussed in +the following sections. + +.. index:: scheduling mechanisms + +Scheduling Modification Mechanisms +---------------------------------- + +RTEMS provides four mechanisms which allow the user to alter the task +scheduling decisions: + +- user-selectable task priority level + +- task preemption control + +- task timeslicing control + +- manual round-robin selection + +Each of these methods provides a powerful capability to customize sets of tasks +to satisfy the unique and particular requirements encountered in custom +real-time applications. Although each mechanism operates independently, there +is a precedence relationship which governs the effects of scheduling +modifications. The evaluation order for scheduling characteristics is always +priority, preemption mode, and timeslicing. When reading the descriptions of +timeslicing and manual round-robin it is important to keep in mind that +preemption (if enabled) of a task by higher priority tasks will occur as +required, overriding the other factors presented in the description. + +.. index:: task priority + +Task Priority and Scheduling +^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +The most significant task scheduling modification mechanism is the ability for +the user to assign a priority level to each individual task when it is created +and to alter a task's priority at run-time. The maximum priority level depends +on the configured scheduler. A lower priority level means higher priority +(higher importance). The maximum priority level of the default uniprocessor +scheduler is 255. + +.. index:: preemption + +Preemption +^^^^^^^^^^ + +Another way the user can alter the basic scheduling algorithm is by +manipulating the preemption mode flag (``RTEMS_PREEMPT_MASK``) of individual +tasks. If preemption is disabled for a task (``RTEMS_NO_PREEMPT``), then the +task will not relinquish control of the processor until it terminates, blocks, +or re-enables preemption. Even tasks which become ready to run and possess +higher priority levels will not be allowed to execute. Note that the +preemption setting has no effect on the manner in which a task is scheduled. +It only applies once a task has control of the processor. + +.. index:: timeslicing +.. index:: round robin scheduling + +Timeslicing +^^^^^^^^^^^ + +Timeslicing or round-robin scheduling is an additional method which can be used +to alter the basic scheduling algorithm. Like preemption, timeslicing is +specified on a task by task basis using the timeslicing mode flag +(``RTEMS_TIMESLICE_MASK``). If timeslicing is enabled for a task +(``RTEMS_TIMESLICE``), then RTEMS will limit the amount of time the task can +execute before the processor is allocated to another task. Each tick of the +real-time clock reduces the currently running task's timeslice. When the +execution time equals the timeslice, RTEMS will dispatch another task of the +same priority to execute. If there are no other tasks of the same priority +ready to execute, then the current task is allocated an additional timeslice +and continues to run. Remember that a higher priority task will preempt the +task (unless preemption is disabled) as soon as it is ready to run, even if the +task has not used up its entire timeslice. + +.. index:: manual round robin + +Manual Round-Robin +^^^^^^^^^^^^^^^^^^ + +The final mechanism for altering the RTEMS scheduling algorithm is called +manual round-robin. Manual round-robin is invoked by using +the ``rtems_task_wake_after`` directive with a time interval of +``RTEMS_YIELD_PROCESSOR``. This allows a task to give up the processor and be +immediately returned to the ready chain at the end of its priority group. If +no other tasks of the same priority are ready to run, then the task does not +lose control of the processor. + +.. index:: dispatching + +Dispatching Tasks +----------------- + +The dispatcher is the RTEMS component responsible for allocating the processor +to a ready task. In order to allocate the processor to one task, it must be +deallocated or retrieved from the task currently using it. This involves a +concept called a context switch. To perform a context switch, the dispatcher +saves the context of the current task and restores the context of the task +which has been allocated to the processor. Saving and restoring a task's +context is the storing/loading of all the essential information about a task to +enable it to continue execution without any effects of the interruption. For +example, the contents of a task's register set must be the same when it is +given the processor as they were when it was taken away. All of the +information that must be saved or restored for a context switch is located +either in the TCB or on the task's stacks. + +Tasks that utilize a numeric coprocessor and are created with the +``RTEMS_FLOATING_POINT`` attribute require additional operations during a +context switch. These additional operations are necessary to save and restore +the floating point context of ``RTEMS_FLOATING_POINT`` tasks. To avoid +unnecessary save and restore operations, the state of the numeric coprocessor +is only saved when a ``RTEMS_FLOATING_POINT`` task is dispatched and that task +was not the last task to utilize the coprocessor. + +.. index:: task state transitions + +Task State Transitions +---------------------- + +Tasks in an RTEMS system must always be in one of the five allowable task +states. These states are: executing, ready, blocked, dormant, and +non-existent. + +A task occupies the non-existent state before a ``rtems_task_create`` has been +issued on its behalf. A task enters the non-existent state from any other +state in the system when it is deleted with the ``rtems_task_delete`` +directive. While a task occupies this state it does not have a TCB or a task +ID assigned to it; therefore, no other tasks in the system may reference this +task. + +When a task is created via the ``rtems_task_create`` directive, it enters the +dormant state. This state is not entered through any other means. Although +the task exists in the system, it cannot actively compete for system resources. +It will remain in the dormant state until it is started via the +``rtems_task_start`` directive, at which time it enters the ready state. The +task is now permitted to be scheduled for the processor and to compete for +other system resources. + +.. figure:: ../../images/c_user/states.png + :width: 70% + :align: center + :alt: Task State Transitions + +A task occupies the blocked state whenever it is unable to be scheduled to run. +A running task may block itself or be blocked by other tasks in the system. +The running task blocks itself through voluntary operations that cause the task +to wait. The only way a task can block a task other than itself is with the +``rtems_task_suspend`` directive. A task enters the blocked state due to any +of the following conditions: + +- A task issues a ``rtems_task_suspend`` directive which blocks either itself + or another task in the system. + +- The running task issues a ``rtems_barrier_wait`` directive. + +- The running task issues a ``rtems_message_queue_receive`` directive with the + wait option, and the message queue is empty. + +- The running task issues a ``rtems_event_receive`` directive with the wait + option, and the currently pending events do not satisfy the request. + +- The running task issues a ``rtems_semaphore_obtain`` directive with the wait + option and the requested semaphore is unavailable. + +- The running task issues a ``rtems_task_wake_after`` directive which blocks + the task for the given time interval. If the time interval specified is + zero, the task yields the processor and remains in the ready state. + +- The running task issues a ``rtems_task_wake_when`` directive which blocks the + task until the requested date and time arrives. + +- The running task issues a ``rtems_rate_monotonic_period`` directive and must + wait for the specified rate monotonic period to conclude. + +- The running task issues a ``rtems_region_get_segment`` directive with the + wait option and there is not an available segment large enough to satisfy the + task's request. + +A blocked task may also be suspended. Therefore, both the suspension and the +blocking condition must be removed before the task becomes ready to run again. + +A task occupies the ready state when it is able to be scheduled to run, but +currently does not have control of the processor. Tasks of the same or higher +priority will yield the processor by either becoming blocked, completing their +timeslice, or being deleted. All tasks with the same priority will execute in +FIFO order. A task enters the ready state due to any of the following +conditions: + +- A running task issues a ``rtems_task_resume`` directive for a task that is + suspended and the task is not blocked waiting on any resource. + +- A running task issues a ``rtems_message_queue_send``, + ``rtems_message_queue_broadcast``, or a ``rtems_message_queue_urgent`` + directive which posts a message to the queue on which the blocked task is + waiting. + +- A running task issues an ``rtems_event_send`` directive which sends an event + condition to a task that is blocked waiting on that event condition. + +- A running task issues a ``rtems_semaphore_release`` directive which releases + the semaphore on which the blocked task is waiting. + +- A timeout interval expires for a task which was blocked by a call to the + ``rtems_task_wake_after`` directive. + +- A timeout period expires for a task which blocked by a call to the + ``rtems_task_wake_when`` directive. + +- A running task issues a ``rtems_region_return_segment`` directive which + releases a segment to the region on which the blocked task is waiting and a + resulting segment is large enough to satisfy the task's request. + +- A rate monotonic period expires for a task which blocked by a call to the + ``rtems_rate_monotonic_period`` directive. + +- A timeout interval expires for a task which was blocked waiting on a message, + event, semaphore, or segment with a timeout specified. + +- A running task issues a directive which deletes a message queue, a semaphore, + or a region on which the blocked task is waiting. + +- A running task issues a ``rtems_task_restart`` directive for the blocked + task. + +- The running task, with its preemption mode enabled, may be made ready by + issuing any of the directives that may unblock a task with a higher priority. + This directive may be issued from the running task itself or from an ISR. A + ready task occupies the executing state when it has control of the CPU. A + task enters the executing state due to any of the following conditions: + +- The task is the highest priority ready task in the system. + +- The running task blocks and the task is next in the scheduling queue. The + task may be of equal priority as in round-robin scheduling or the task may + possess the highest priority of the remaining ready tasks. + +- The running task may reenable its preemption mode and a task exists in the + ready queue that has a higher priority than the running task. + +- The running task lowers its own priority and another task is of higher + priority as a result. + +- The running task raises the priority of a task above its own and the running + task is in preemption mode. diff --git a/c-user/scheduling-concepts/directives.rst b/c-user/scheduling-concepts/directives.rst new file mode 100644 index 0000000..5b1246f --- /dev/null +++ b/c-user/scheduling-concepts/directives.rst @@ -0,0 +1,424 @@ +.. SPDX-License-Identifier: CC-BY-SA-4.0 + +.. Copyright (C) 1988, 2008 On-Line Applications Research Corporation (OAR) + +Directives +========== + +This section details the scheduler manager. A subsection is dedicated to each +of these services and describes the calling sequence, related constants, usage, +and status codes. + +.. raw:: latex + + \clearpage + +.. _rtems_scheduler_ident: + +SCHEDULER_IDENT - Get ID of a scheduler +--------------------------------------- + +CALLING SEQUENCE: + .. code-block:: c + + rtems_status_code rtems_scheduler_ident( + rtems_name name, + rtems_id *id + ); + +DIRECTIVE STATUS CODES: + .. list-table:: + :class: rtems-table + + * - ``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 :ref:`ConfigurationSchedulerTable` + and :ref:`CONFIGURE_SCHEDULER_NAME`. + +NOTES: + None. + +.. raw:: latex + + \clearpage + +.. _rtems_scheduler_ident_by_processor: + +SCHEDULER_IDENT_BY_PROCESSOR - Get ID of a scheduler by processor +----------------------------------------------------------------- + +CALLING SEQUENCE: + .. code-block:: c + + rtems_status_code rtems_scheduler_ident_by_processor( + uint32_t cpu_index, + rtems_id *id + ); + +DIRECTIVE STATUS CODES: + .. list-table:: + :class: rtems-table + + * - ``RTEMS_SUCCESSFUL`` + - Successful operation. + * - ``RTEMS_INVALID_ADDRESS`` + - The ``id`` parameter is ``NULL``. + * - ``RTEMS_INVALID_NAME`` + - Invalid processor index. + * - ``RTEMS_INCORRECT_STATE`` + - The processor index is valid, however, this processor is not owned by + a scheduler. + +DESCRIPTION: + Identifies a scheduler by a processor. + +NOTES: + None. + +.. raw:: latex + + \clearpage + +.. _rtems_scheduler_ident_by_processor_set: + +SCHEDULER_IDENT_BY_PROCESSOR_SET - Get ID of a scheduler by processor set +------------------------------------------------------------------------- + +CALLING SEQUENCE: + .. code-block:: c + + rtems_status_code rtems_scheduler_ident_by_processor_set( + size_t cpusetsize, + const cpu_set_t *cpuset, + rtems_id *id + ); + +DIRECTIVE STATUS CODES: + .. list-table:: + :class: rtems-table + + * - ``RTEMS_SUCCESSFUL`` + - Successful operation. + * - ``RTEMS_INVALID_ADDRESS`` + - The ``id`` parameter is ``NULL``. + * - ``RTEMS_INVALID_SIZE`` + - Invalid processor set size. + * - ``RTEMS_INVALID_NAME`` + - The processor set contains no online processor. + * - ``RTEMS_INCORRECT_STATE`` + - The processor set is valid, however, the highest numbered online + processor in the specified processor set is not owned by a scheduler. + +DESCRIPTION: + Identifies a scheduler by a processor set. The scheduler is selected + according to the highest numbered online processor in the specified + processor set. + +NOTES: + None. + +.. raw:: latex + + \clearpage + +.. _rtems_scheduler_get_maximum_priority: + +SCHEDULER_GET_MAXIMUM_PRIORITY - Get maximum task priority of a scheduler +------------------------------------------------------------------------- + +CALLING SEQUENCE: + .. code-block:: c + + rtems_status_code rtems_scheduler_get_maximum_priority( + rtems_id scheduler_id, + rtems_task_priority *priority + ); + +DIRECTIVE STATUS CODES: + .. list-table:: + :class: rtems-table + + * - ``RTEMS_SUCCESSFUL`` + - Successful operation. + * - ``RTEMS_INVALID_ID`` + - Invalid scheduler instance identifier. + * - ``RTEMS_INVALID_ADDRESS`` + - The ``priority`` parameter is ``NULL``. + +DESCRIPTION: + Returns the maximum task priority of the specified scheduler instance in + ``priority``. + +NOTES: + None. + +.. raw:: latex + + \clearpage + +.. _rtems_scheduler_map_priority_to_posix: + +SCHEDULER_MAP_PRIORITY_TO_POSIX - Map task priority to POSIX thread prority +--------------------------------------------------------------------------- + +CALLING SEQUENCE: + .. code-block:: c + + rtems_status_code rtems_scheduler_map_priority_to_posix( + rtems_id scheduler_id, + rtems_task_priority priority, + int *posix_priority + ); + +DIRECTIVE STATUS CODES: + .. list-table:: + :class: rtems-table + + * - ``RTEMS_SUCCESSFUL`` + - Successful operation. + * - ``RTEMS_INVALID_ADDRESS`` + - The ``posix_priority`` parameter is ``NULL``. + * - ``RTEMS_INVALID_ID`` + - Invalid scheduler instance identifier. + * - ``RTEMS_INVALID_PRIORITY`` + - Invalid task priority. + +DESCRIPTION: + Maps a task priority to the corresponding POSIX thread priority. + +NOTES: + None. + +.. raw:: latex + + \clearpage + +.. _rtems_scheduler_map_priority_from_posix: + +SCHEDULER_MAP_PRIORITY_FROM_POSIX - Map POSIX thread prority to task priority +----------------------------------------------------------------------------- + +CALLING SEQUENCE: + .. code-block:: c + + rtems_status_code rtems_scheduler_map_priority_from_posix( + rtems_id scheduler_id, + int posix_priority, + rtems_task_priority *priority + ); + +DIRECTIVE STATUS CODES: + .. list-table:: + :class: rtems-table + + * - ``RTEMS_SUCCESSFUL`` + - Successful operation. + * - ``RTEMS_INVALID_ADDRESS`` + - The ``priority`` parameter is ``NULL``. + * - ``RTEMS_INVALID_ID`` + - Invalid scheduler instance identifier. + * - ``RTEMS_INVALID_PRIORITY`` + - Invalid POSIX thread priority. + +DESCRIPTION: + Maps a POSIX thread priority to the corresponding task priority. + +NOTES: + None. + +.. raw:: latex + + \clearpage + +.. _rtems_scheduler_get_processor: + +SCHEDULER_GET_PROCESSOR - Get current processor index +----------------------------------------------------- + +CALLING SEQUENCE: + .. code-block:: c + + uint32_t rtems_scheduler_get_processor( void ); + +DIRECTIVE STATUS CODES: + This directive returns the index of the current processor. + +DESCRIPTION: + In uniprocessor configurations, a value of zero will be returned. + + In 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. + +.. raw:: latex + + \clearpage + +.. _rtems_scheduler_get_processor_maximum: + +SCHEDULER_GET_PROCESSOR_MAXIMUM - Get processor maximum +------------------------------------------------------- + +CALLING SEQUENCE: + .. code-block:: c + + uint32_t rtems_scheduler_get_processor_maximum( void ); + +DIRECTIVE STATUS CODES: + This directive returns the processor maximum supported by the system. + +DESCRIPTION: + In uniprocessor configurations, a value of one will be returned. + + In SMP configurations, this directive returns the minimum of the processors + (physically or virtually) available by the platform and the configured + processor maximum. Not all processors in the range from processor index + zero to the last processor index (which is the processor maximum minus one) + may be configured to be used by a scheduler or online (online processors + have a scheduler assigned). + +NOTES: + None. + +.. raw:: latex + + \clearpage + +.. _rtems_scheduler_get_processor_set: + +SCHEDULER_GET_PROCESSOR_SET - Get processor set of a scheduler +-------------------------------------------------------------- + +CALLING SEQUENCE: + .. code-block:: c + + rtems_status_code rtems_scheduler_get_processor_set( + rtems_id scheduler_id, + size_t cpusetsize, + cpu_set_t *cpuset + ); + +DIRECTIVE STATUS CODES: + .. list-table:: + :class: rtems-table + + * - ``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. + +.. raw:: latex + + \clearpage + +.. _rtems_scheduler_add_processor: + +SCHEDULER_ADD_PROCESSOR - Add processor to a scheduler +------------------------------------------------------ + +CALLING SEQUENCE: + .. code-block:: c + + rtems_status_code rtems_scheduler_add_processor( + rtems_id scheduler_id, + uint32_t cpu_index + ); + +DIRECTIVE STATUS CODES: + .. list-table:: + :class: rtems-table + + * - ``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. + +.. raw:: latex + + \clearpage + +.. _rtems_scheduler_remove_processor: + +SCHEDULER_REMOVE_PROCESSOR - Remove processor from a scheduler +-------------------------------------------------------------- + +CALLING SEQUENCE: + .. code-block:: c + + rtems_status_code rtems_scheduler_remove_processor( + rtems_id scheduler_id, + uint32_t cpu_index + ); + +DIRECTIVE STATUS CODES: + .. list-table:: + :class: rtems-table + + * - ``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. + * - ``RTEMS_RESOURCE_IN_USE`` + - A task with a restricted processor affinity exists that uses this + scheduler instance as its home scheduler instance and it would be no + longer possible to allocate a processor for this task after the + removal of this processor. + +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. diff --git a/c-user/scheduling-concepts/index.rst b/c-user/scheduling-concepts/index.rst new file mode 100644 index 0000000..6d06d91 --- /dev/null +++ b/c-user/scheduling-concepts/index.rst @@ -0,0 +1,19 @@ +.. SPDX-License-Identifier: CC-BY-SA-4.0 + +.. Copyright (C) 2020 embedded brains GmbH (http://www.embedded-brains.de) + +.. index:: scheduling +.. index:: task scheduling + +.. _SchedulingConcepts: + +Scheduling Concepts +******************* + +.. toctree:: + + introduction + background + uniprocessor-schedulers + smp-schedulers + directives diff --git a/c-user/scheduling-concepts/introduction.rst b/c-user/scheduling-concepts/introduction.rst new file mode 100644 index 0000000..92da6de --- /dev/null +++ b/c-user/scheduling-concepts/introduction.rst @@ -0,0 +1,42 @@ +.. SPDX-License-Identifier: CC-BY-SA-4.0 + +.. Copyright (C) 1988, 2008 On-Line Applications Research Corporation (OAR) + +Introduction +============ + +The concept of scheduling in real-time systems dictates the ability to provide +an immediate response to specific external events, particularly the necessity of +scheduling tasks to run within a specified time limit after the occurrence of +an event. For example, software embedded in life-support systems used to +monitor hospital patients must take instant action if a change in the patient's +status is detected. + +The component of RTEMS responsible for providing this capability is +appropriately called the scheduler. The scheduler's sole purpose is to +allocate the all important resource of processor time to the various tasks +competing for attention. + +The directives provided by the scheduler manager are: + +- :ref:`rtems_scheduler_ident` + +- :ref:`rtems_scheduler_ident_by_processor` + +- :ref:`rtems_scheduler_ident_by_processor_set` + +- :ref:`rtems_scheduler_get_maximum_priority` + +- :ref:`rtems_scheduler_map_priority_to_posix` + +- :ref:`rtems_scheduler_map_priority_from_posix` + +- :ref:`rtems_scheduler_get_processor` + +- :ref:`rtems_scheduler_get_processor_maximum` + +- :ref:`rtems_scheduler_get_processor_set` + +- :ref:`rtems_scheduler_add_processor` + +- :ref:`rtems_scheduler_remove_processor` diff --git a/c-user/scheduling-concepts/smp-schedulers.rst b/c-user/scheduling-concepts/smp-schedulers.rst new file mode 100644 index 0000000..cfab04f --- /dev/null +++ b/c-user/scheduling-concepts/smp-schedulers.rst @@ -0,0 +1,69 @@ +.. SPDX-License-Identifier: CC-BY-SA-4.0 + +.. Copyright (C) 2011 Petr Benes +.. Copyright (C) 1988, 2008 On-Line Applications Research Corporation (OAR) + +SMP Schedulers +============== + +All SMP schedulers included in RTEMS are priority based. The processors +managed by a scheduler instance are allocated to the highest priority tasks +allowed to run. + +.. _SchedulerSMPEDF: + +Earliest Deadline First SMP Scheduler +------------------------------------- + +This is a job-level fixed-priority scheduler using the Earliest Deadline First +(EDF) method. By convention, the maximum priority level is +:math:`min(INT\_MAX, 2^{62} - 1)` for background tasks. Tasks without an +active deadline are background tasks. In case deadlines are not used, then the +EDF scheduler behaves exactly like a fixed-priority scheduler. The tasks with +an active deadline have a higher priority than the background tasks. This +scheduler supports :ref:`task processor affinities ` +of one-to-one and one-to-all, e.g., a task can execute on exactly one processor +or all processors managed by the scheduler instance. The processor affinity +set of a task must contain all online processors to select the one-to-all +affinity. This is to avoid pathological cases if processors are added/removed +to/from the scheduler instance at run-time. In case the processor affinity set +contains not all online processors, then a one-to-one affinity will be used +selecting the processor with the largest index within the set of processors +currently owned by the scheduler instance. This scheduler algorithm supports +:ref:`thread pinning `. The ready queues use a red-black tree +with the task priority as the key. + +This scheduler algorithm is the default scheduler in SMP configurations if more +than one processor is configured (:ref:`CONFIGURE_MAXIMUM_PROCESSORS +`). + +.. _SchedulerSMPPriority: + +Deterministic Priority SMP Scheduler +------------------------------------ + +A fixed-priority scheduler which uses a table of chains with one chain per +priority level for the ready tasks. The maximum priority level is +configurable. By default, the maximum priority level is 255 (256 priority +levels). + +.. _SchedulerSMPPrioritySimple: + +Simple Priority SMP Scheduler +----------------------------- + +A fixed-priority scheduler which uses a sorted chain for the ready tasks. By +convention, the maximum priority level is 255. The implementation limit is +actually :math:`2^{63} - 1`. + +.. _SchedulerSMPPriorityAffinity: + +Arbitrary Processor Affinity Priority SMP Scheduler +--------------------------------------------------- + +A fixed-priority scheduler which uses a table of chains with one chain per +priority level for the ready tasks. The maximum priority level is +configurable. By default, the maximum priority level is 255 (256 priority +levels). This scheduler supports arbitrary task processor affinities. The +worst-case run-time complexity of some scheduler operations exceeds +:math:`O(n)` while :math:`n` is the count of ready tasks. diff --git a/c-user/scheduling-concepts/uniprocessor-schedulers.rst b/c-user/scheduling-concepts/uniprocessor-schedulers.rst new file mode 100644 index 0000000..34969bc --- /dev/null +++ b/c-user/scheduling-concepts/uniprocessor-schedulers.rst @@ -0,0 +1,113 @@ +.. SPDX-License-Identifier: CC-BY-SA-4.0 + +.. Copyright (C) 2011 Petr Benes +.. Copyright (C) 1988, 2008 On-Line Applications Research Corporation (OAR) + +Uniprocessor Schedulers +======================= + +All uniprocessor schedulers included in RTEMS are priority based. The +processor is allocated to the highest priority task allowed to run. + +.. _SchedulerPriority: + +Deterministic Priority Scheduler +-------------------------------- + +This is the scheduler implementation which has always been in RTEMS. After the +4.10 release series, it was factored into a pluggable scheduler selection. It +schedules tasks using a priority based algorithm which takes into account +preemption. It is implemented using an array of FIFOs with a FIFO per +priority. It maintains a bitmap which is used to track which priorities have +ready tasks. + +This algorithm is deterministic (e.g., predictable and fixed) in execution time. +This comes at the cost of using slightly over three (3) kilobytes of RAM on a +system configured to support 256 priority levels. + +This scheduler is only aware of a single core. + +.. _SchedulerPrioritySimple: + +Simple Priority Scheduler +------------------------- + +This scheduler implementation has the same behaviour as the Deterministic +Priority Scheduler but uses only one linked list to manage all ready tasks. +When a task is readied, a linear search of that linked list is performed to +determine where to insert the newly readied task. + +This algorithm uses much less RAM than the Deterministic Priority Scheduler but +is *O(n)* where *n* is the number of ready tasks. In a small system with a +small number of tasks, this will not be a performance issue. Reducing RAM +consumption is often critical in small systems that are incapable of +supporting a large number of tasks. + +This scheduler is only aware of a single core. + +.. index:: earliest deadline first scheduling + +.. _SchedulerEDF: + +Earliest Deadline First Scheduler +--------------------------------- + +This is an alternative scheduler in RTEMS for single-core applications. The +primary EDF advantage is high total CPU utilization (theoretically up to +100%). It assumes that tasks have priorities equal to deadlines. + +This EDF is initially preemptive, however, individual tasks may be declared +not-preemptive. Deadlines are declared using only Rate Monotonic manager whose +goal is to handle periodic behavior. Period is always equal to the deadline. All +ready tasks reside in a single ready queue implemented using a red-black tree. + +This implementation of EDF schedules two different types of task priority types +while each task may switch between the two types within its execution. If a +task does have a deadline declared using the Rate Monotonic manager, the task +is deadline-driven and its priority is equal to deadline. On the contrary, if a +task does not have any deadline or the deadline is cancelled using the Rate +Monotonic manager, the task is considered a background task with priority equal +to that assigned upon initialization in the same manner as for priority +scheduler. Each background task is of lower importance than each +deadline-driven one and is scheduled when no deadline-driven task and no higher +priority background task is ready to run. + +Every deadline-driven scheduling algorithm requires means for tasks to claim a +deadline. The Rate Monotonic Manager is responsible for handling periodic +execution. In RTEMS periods are equal to deadlines, thus if a task announces a +period, it has to be finished until the end of this period. The call of +``rtems_rate_monotonic_period`` passes the scheduler the length of an oncoming +deadline. Moreover, the ``rtems_rate_monotonic_cancel`` and +``rtems_rate_monotonic_delete`` calls clear the deadlines assigned to the task. + +.. index:: constant bandwidth server scheduling + +.. _SchedulerCBS: + +Constant Bandwidth Server Scheduling (CBS) +------------------------------------------ + +This is an alternative scheduler in RTEMS for single-core applications. The +CBS is a budget aware extension of EDF scheduler. The main goal of this +scheduler is to ensure temporal isolation of tasks meaning that a task's +execution in terms of meeting deadlines must not be influenced by other tasks +as if they were run on multiple independent processors. + +Each task can be assigned a server (current implementation supports only one +task per server). The server is characterized by period (deadline) and +computation time (budget). The ratio budget/period yields bandwidth, which is +the fraction of CPU to be reserved by the scheduler for each subsequent period. + +The CBS is equipped with a set of rules applied to tasks attached to servers +ensuring that deadline miss because of another task cannot occur. In case a +task breaks one of the rules, its priority is pulled to background until the +end of its period and then restored again. The rules are: + +- Task cannot exceed its registered budget, + +- Task cannot be unblocked when a ratio between remaining budget and remaining + deadline is higher than declared bandwidth. + +The CBS provides an extensive API. Unlike EDF, the +``rtems_rate_monotonic_period`` does not declare a deadline because it is +carried out using CBS API. This call only announces next period. -- cgit v1.2.3