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.

6 files changed, 994 insertions, 0 deletions

diff --git a/c-user/scheduling-concepts/background.rst b/c-user/scheduling-concepts/background.rst
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+.. 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
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+.. 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
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+.. 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 <rtems_task_set_affinity>`
+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 <ThreadPinning>`.  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
+<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.