.. SPDX-License-Identifier: CC-BY-SA-4.0

.. Copyright (C) 2020 embedded brains GmbH (http://www.embedded-brains.de)
.. Copyright (C) 2010 Gedare Bloom
.. Copyright (C) 1988, 2008 On-Line Applications Research Corporation (OAR)

General Scheduler Configuration
===============================

This section describes configuration options related to selecting a
scheduling algorithm for an application.  A scheduler configuration is optional
and only necessary in very specific circumstances.  A normal application
configuration does not need any of the configuration options described in this
section.

By default, the :ref:`Deterministic Priority Scheduler <SchedulerPriority>`
algorithm is used in uniprocessor configurations.  In case SMP is enabled and
the configured maximum processors
(:ref:`CONFIGURE_MAXIMUM_PROCESSORS <CONFIGURE_MAXIMUM_PROCESSORS>`) is greater
than one, then the :ref:`Earliest Deadline First (EDF) SMP Scheduler
<SchedulerSMPEDF>` is selected as the default scheduler algorithm.

For the :ref:`schedulers built into
RTEMS <SchedulingConcepts>`, the configuration is straightforward.  All that is
required is to define the configuration option which specifies which scheduler
you want for in your application.

The pluggable scheduler interface also enables the user to provide their own
scheduling algorithm.  If you choose to do this, you must define multiple
configuration option.

.. index:: CONFIGURE_CBS_MAXIMUM_SERVERS

.. _CONFIGURE_CBS_MAXIMUM_SERVERS:

CONFIGURE_CBS_MAXIMUM_SERVERS
-----------------------------

CONSTANT:
    ``CONFIGURE_CBS_MAXIMUM_SERVERS``

OPTION TYPE:
    This configuration option is an integer define.

DEFAULT VALUE:
    The default value is :ref:`CONFIGURE_MAXIMUM_TASKS`.

VALUE CONSTRAINTS:
    The value of this configuration option shall satisfy all of the following
    constraints:

    * It shall be greater than or equal to 0.

    * It shall be less than or equal to ``SIZE_MAX``.

    * It shall be less than or equal to a
      BSP-specific and application-specific value which depends on the size of the
      memory available to the application.

DESCRIPTION:
    The value of this configuration option defines the maximum number Constant
    Bandwidth Servers that can be concurrently active.

NOTES:
    This configuration option is only evaluated if the configuration option
    :ref:`CONFIGURE_SCHEDULER_CBS` is defined.

.. index:: CONFIGURE_MAXIMUM_PRIORITY
.. index:: maximum priority
.. index:: number of priority levels

.. _CONFIGURE_MAXIMUM_PRIORITY:

CONFIGURE_MAXIMUM_PRIORITY
--------------------------

CONSTANT:
    ``CONFIGURE_MAXIMUM_PRIORITY``

OPTION TYPE:
    This configuration option is an integer define.

DEFAULT VALUE:
    The default value is 255.

VALUE CONSTRAINTS:
    The value of this configuration option shall be
    an element of {3, 7, 31, 63, 127, 255}.

DESCRIPTION:
    For the following schedulers

    * :ref:`SchedulerPriority`, which is the default in uniprocessor
      configurations and can be configured through the
      :ref:`CONFIGURE_SCHEDULER_PRIORITY` configuration option,

    * :ref:`SchedulerSMPPriority` which can be configured through the
      :ref:`CONFIGURE_SCHEDULER_PRIORITY_SMP` configuration option, and

    * :ref:`SchedulerSMPPriorityAffinity` which can be configured through the
      :ref:`CONFIGURE_SCHEDULER_PRIORITY_AFFINITY_SMP` configuration option

    this configuration option specifies the maximum numeric priority of any task
    for these schedulers and one less that the number of priority levels for
    these schedulers.  For all other schedulers provided by RTEMS, this
    configuration option has no effect.

NOTES:
    The numerically greatest priority is the logically lowest priority in the
    system and will thus be used by the IDLE task.

    Priority zero is reserved for internal use by RTEMS and is not available to
    applications.

    Reducing the number of priorities through this configuration option reduces
    the amount of memory allocated by the schedulers listed above.  These
    schedulers use a chain control structure per priority and this structure
    consists of three pointers.  On a 32-bit architecture, the allocated memory
    is 12 bytes * (``CONFIGURE_MAXIMUM_PRIORITY`` + 1), e.g. 3072 bytes for 256
    priority levels (default), 48 bytes for 4 priority levels
    (``CONFIGURE_MAXIMUM_PRIORITY == 3``).

    The default value is 255, because RTEMS shall support 256 priority levels to
    be compliant with various standards.  These priorities range from 0 to 255.

.. index:: CONFIGURE_SCHEDULER_ASSIGNMENTS

.. _CONFIGURE_SCHEDULER_ASSIGNMENTS:

CONFIGURE_SCHEDULER_ASSIGNMENTS
-------------------------------

CONSTANT:
    ``CONFIGURE_SCHEDULER_ASSIGNMENTS``

OPTION TYPE:
    This configuration option is an initializer define.

DEFAULT VALUE:
    The default value of this configuration option is computed so that the
    default scheduler is assigned to each configured processor (up to 32).

VALUE CONSTRAINTS:
    The value of this configuration option shall satisfy all of the following
    constraints:

    * It shall be a list of the following
      macros:

      * ``RTEMS_SCHEDULER_ASSIGN( processor_index, attributes )``

      * :c:macro:`RTEMS_SCHEDULER_ASSIGN_NO_SCHEDULER`

    * It shall be a list of exactly
      :ref:`CONFIGURE_MAXIMUM_PROCESSORS` elements.

DESCRIPTION:
    The value of this configuration option is used to initialize the initial
    scheduler to processor assignments.

NOTES:
    This configuration option is only evaluated in SMP configurations.

    This is an advanced configuration option, see
    :ref:`ConfigurationSchedulersClustered`.

.. index:: CONFIGURE_SCHEDULER_CBS

.. _CONFIGURE_SCHEDULER_CBS:

CONFIGURE_SCHEDULER_CBS
-----------------------

CONSTANT:
    ``CONFIGURE_SCHEDULER_CBS``

OPTION TYPE:
    This configuration option is a boolean feature define.

DEFAULT CONFIGURATION:
    If this configuration option is undefined, then the described feature is not
    enabled.

DESCRIPTION:
    In case this configuration option is defined, then
    :ref:`Constant Bandwidth Server (CBS) Scheduler <SchedulerCBS>`
    algorithm is made available to the application.

NOTES:
    This scheduler configuration option is an advanced configuration option.
    Think twice before you use it.

    In case no explicit :ref:`clustered scheduler configuration
    <ConfigurationSchedulersClustered>` is present, then it is used as the
    scheduler for exactly one processor.

.. index:: CONFIGURE_SCHEDULER_EDF

.. _CONFIGURE_SCHEDULER_EDF:

CONFIGURE_SCHEDULER_EDF
-----------------------

CONSTANT:
    ``CONFIGURE_SCHEDULER_EDF``

OPTION TYPE:
    This configuration option is a boolean feature define.

DEFAULT CONFIGURATION:
    If this configuration option is undefined, then the described feature is not
    enabled.

DESCRIPTION:
    In case this configuration option is defined, then
    :ref:`Earliest Deadline First (EDF) Scheduler <SchedulerEDF>`
    algorithm is made available to the application.

NOTES:
    This scheduler configuration option is an advanced configuration option.
    Think twice before you use it.

    In case no explicit :ref:`clustered scheduler configuration
    <ConfigurationSchedulersClustered>` is present, then it is used as the
    scheduler for exactly one processor.

.. index:: CONFIGURE_SCHEDULER_EDF_SMP

.. _CONFIGURE_SCHEDULER_EDF_SMP:

CONFIGURE_SCHEDULER_EDF_SMP
---------------------------

CONSTANT:
    ``CONFIGURE_SCHEDULER_EDF_SMP``

OPTION TYPE:
    This configuration option is a boolean feature define.

DEFAULT CONFIGURATION:
    If this configuration option is undefined, then the described feature is not
    enabled.

DESCRIPTION:
    In case this configuration option is defined, then
    :ref:`Earliest Deadline First (EDF) SMP Scheduler <SchedulerSMPEDF>`
    algorithm is made available to the application.

NOTES:
    This scheduler configuration option is an advanced configuration option.
    Think twice before you use it.

    This scheduler algorithm is only available when RTEMS is built with SMP
    support enabled.

    In case no explicit :ref:`clustered scheduler configuration
    <ConfigurationSchedulersClustered>` is present, then it is used as the
    scheduler for up to 32 processors.

    This scheduler algorithm is the default in SMP configurations if
    :ref:`CONFIGURE_MAXIMUM_PROCESSORS` is
    greater than one.

.. index:: CONFIGURE_SCHEDULER_NAME

.. _CONFIGURE_SCHEDULER_NAME:

CONFIGURE_SCHEDULER_NAME
------------------------

CONSTANT:
    ``CONFIGURE_SCHEDULER_NAME``

OPTION TYPE:
    This configuration option is an integer define.

DEFAULT VALUE:
    The default value is

    * ``"MEDF"`` for the :ref:`EDF SMP Scheduler <SchedulerSMPEDF>`,

    * ``"MPA "`` for the :ref:`Arbitrary Processor Affinity Priority SMP Scheduler <SchedulerSMPPriorityAffinity>`,

    * ``"MPD "`` for the :ref:`Deterministic Priority SMP Scheduler <SchedulerSMPPriority>`,

    * ``"MPS "`` for the :ref:`Simple Priority SMP Scheduler <SchedulerSMPPrioritySimple>`,

    * ``"UCBS"`` for the :ref:`Uniprocessor CBS Scheduler <SchedulerCBS>`,

    * ``"UEDF"`` for the :ref:`Uniprocessor EDF Scheduler <SchedulerEDF>`,

    * ``"UPD "`` for the :ref:`Uniprocessor Deterministic Priority Scheduler <SchedulerPriority>`, and

    * ``"UPS "`` for the :ref:`Uniprocessor Simple Priority Scheduler <SchedulerPrioritySimple>`.

VALUE CONSTRAINTS:
    The value of this configuration option shall be a valid integer of type
    ``rtems_name``.

DESCRIPTION:
    The value of this configuration option defines the name of the default
    scheduler.

NOTES:
    This scheduler configuration option is an advanced configuration option.
    Think twice before you use it.

    Schedulers can be identified via c:func:`rtems_scheduler_ident`.

    Use :c:func:`rtems_build_name` to define the scheduler name.

.. index:: CONFIGURE_SCHEDULER_PRIORITY

.. _CONFIGURE_SCHEDULER_PRIORITY:

CONFIGURE_SCHEDULER_PRIORITY
----------------------------

CONSTANT:
    ``CONFIGURE_SCHEDULER_PRIORITY``

OPTION TYPE:
    This configuration option is a boolean feature define.

DEFAULT CONFIGURATION:
    If this configuration option is undefined, then the described feature is not
    enabled.

DESCRIPTION:
    In case this configuration option is defined, then
    :ref:`Deterministic Priority Scheduler <SchedulerPriority>`
    algorithm is made available to the application.

NOTES:
    This scheduler configuration option is an advanced configuration option.
    Think twice before you use it.

    In case no explicit :ref:`clustered scheduler configuration
    <ConfigurationSchedulersClustered>` is present, then it is used as the
    scheduler for exactly one processor.

    This scheduler algorithm is the default when
    :ref:`CONFIGURE_MAXIMUM_PROCESSORS` is
    exactly one.

    The memory allocated for this scheduler depends on the
    :ref:`CONFIGURE_MAXIMUM_PRIORITY` configuration option.

.. index:: CONFIGURE_SCHEDULER_PRIORITY_AFFINITY_SMP

.. _CONFIGURE_SCHEDULER_PRIORITY_AFFINITY_SMP:

CONFIGURE_SCHEDULER_PRIORITY_AFFINITY_SMP
-----------------------------------------

CONSTANT:
    ``CONFIGURE_SCHEDULER_PRIORITY_AFFINITY_SMP``

OPTION TYPE:
    This configuration option is a boolean feature define.

DEFAULT CONFIGURATION:
    If this configuration option is undefined, then the described feature is not
    enabled.

DESCRIPTION:
    In case this configuration option is defined, then
    :ref:`Arbitrary Processor Affinity SMP Scheduler <SchedulerSMPPriorityAffinity>`
    algorithm is made available to the application.

NOTES:
    This scheduler configuration option is an advanced configuration option.
    Think twice before you use it.

    This scheduler algorithm is only available when RTEMS is built with SMP
    support enabled.

    In case no explicit :ref:`clustered scheduler configuration
    <ConfigurationSchedulersClustered>` is present, then it is used as the
    scheduler for up to 32 processors.

    The memory allocated for this scheduler depends on the
    :ref:`CONFIGURE_MAXIMUM_PRIORITY` configuration option.

.. index:: CONFIGURE_SCHEDULER_PRIORITY_SMP

.. _CONFIGURE_SCHEDULER_PRIORITY_SMP:

CONFIGURE_SCHEDULER_PRIORITY_SMP
--------------------------------

CONSTANT:
    ``CONFIGURE_SCHEDULER_PRIORITY_SMP``

OPTION TYPE:
    This configuration option is a boolean feature define.

DEFAULT CONFIGURATION:
    If this configuration option is undefined, then the described feature is not
    enabled.

DESCRIPTION:
    In case this configuration option is defined, then
    :ref:`Deterministic Priority SMP Scheduler <SchedulerSMPPriority>`
    algorithm is made available to the application.

NOTES:
    This scheduler configuration option is an advanced configuration option.
    Think twice before you use it.

    This scheduler algorithm is only available when RTEMS is built with SMP
    support enabled.

    In case no explicit :ref:`clustered scheduler configuration
    <ConfigurationSchedulersClustered>` is present, then it is used as the
    scheduler for up to 32 processors.

    The memory allocated for this scheduler depends on the
    :ref:`CONFIGURE_MAXIMUM_PRIORITY` configuration option.

.. index:: CONFIGURE_SCHEDULER_SIMPLE

.. _CONFIGURE_SCHEDULER_SIMPLE:

CONFIGURE_SCHEDULER_SIMPLE
--------------------------

CONSTANT:
    ``CONFIGURE_SCHEDULER_SIMPLE``

OPTION TYPE:
    This configuration option is a boolean feature define.

DEFAULT CONFIGURATION:
    If this configuration option is undefined, then the described feature is not
    enabled.

DESCRIPTION:
    In case this configuration option is defined, then
    :ref:`Simple Priority Scheduler <SchedulerPrioritySimple>`
    algorithm is made available to the application.

NOTES:
    This scheduler configuration option is an advanced configuration option.
    Think twice before you use it.

    In case no explicit :ref:`clustered scheduler configuration
    <ConfigurationSchedulersClustered>` is present, then it is used as the
    scheduler for exactly one processor.

.. index:: CONFIGURE_SCHEDULER_SIMPLE_SMP

.. _CONFIGURE_SCHEDULER_SIMPLE_SMP:

CONFIGURE_SCHEDULER_SIMPLE_SMP
------------------------------

CONSTANT:
    ``CONFIGURE_SCHEDULER_SIMPLE_SMP``

OPTION TYPE:
    This configuration option is a boolean feature define.

DEFAULT CONFIGURATION:
    If this configuration option is undefined, then the described feature is not
    enabled.

DESCRIPTION:
    In case this configuration option is defined, then
    :ref:`Simple Priority SMP Scheduler <SchedulerSMPPrioritySimple>`
    algorithm is made available to the application.
    application.

NOTES:
    This scheduler configuration option is an advanced configuration option.
    Think twice before you use it.

    This scheduler algorithm is only available when RTEMS is built with SMP
    support enabled.

    In case no explicit :ref:`clustered scheduler configuration
    <ConfigurationSchedulersClustered>` is present, then it is used as the
    scheduler for up to 32 processors.

.. index:: CONFIGURE_SCHEDULER_STRONG_APA

.. _CONFIGURE_SCHEDULER_STRONG_APA:

CONFIGURE_SCHEDULER_STRONG_APA
------------------------------

CONSTANT:
    ``CONFIGURE_SCHEDULER_STRONG_APA``

OPTION TYPE:
    This configuration option is a boolean feature define.

DEFAULT CONFIGURATION:
    If this configuration option is undefined, then the described feature is not
    enabled.

DESCRIPTION:
    In case this configuration option is defined, then Strong APA algorithm is
    made available to the application.

NOTES:
    This scheduler configuration option is an advanced configuration option.
    Think twice before you use it.

    This scheduler algorithm is only available when RTEMS is built with SMP
    support enabled.

    This scheduler algorithm is not correctly implemented.  Do not use it.

.. index:: CONFIGURE_SCHEDULER_USER

.. _CONFIGURE_SCHEDULER_USER:

CONFIGURE_SCHEDULER_USER
------------------------

CONSTANT:
    ``CONFIGURE_SCHEDULER_USER``

OPTION TYPE:
    This configuration option is a boolean feature define.

DEFAULT CONFIGURATION:
    If this configuration option is undefined, then the described feature is not
    enabled.

DESCRIPTION:
    In case this configuration option is defined, then the user shall provide a
    scheduler algorithm to the application.

NOTES:
    This scheduler configuration option is an advanced configuration option.
    Think twice before you use it.

    RTEMS allows the application to provide its own task/thread scheduling
    algorithm. In order to do this, one shall define
    ``CONFIGURE_SCHEDULER_USER`` to indicate the application provides its own
    scheduling algorithm. If ``CONFIGURE_SCHEDULER_USER`` is defined then the
    following additional macros shall be defined:

    * ``CONFIGURE_SCHEDULER`` shall be defined to a static definition of
      the scheduler data structures of the user scheduler.

    * ``CONFIGURE_SCHEDULER_TABLE_ENTRIES`` shall be defined to a scheduler
      table entry initializer for the user scheduler.

    * ``CONFIGURE_SCHEDULER_USER_PER_THREAD`` shall be defined to the type of
      the per-thread information of the user scheduler.

    At this time, the mechanics and requirements for writing a new scheduler
    are evolving and not fully documented.  It is recommended that you look at
    the existing Deterministic Priority Scheduler in
    ``cpukit/score/src/schedulerpriority*.c`` for guidance.  For guidance on
    the configuration macros, please examine ``cpukit/sapi/include/confdefs.h``
    for how these are defined for the Deterministic Priority Scheduler.
