181 lines
6.8 KiB
ReStructuredText
181 lines
6.8 KiB
ReStructuredText
Task Scheduling Interfaces
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**************************
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By default, NuttX performs strict priority scheduling: Tasks of higher
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priority have exclusive access to the CPU until they become blocked. At
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that time, the CPU is available to tasks of lower priority. Tasks of
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equal priority are scheduled FIFO.
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Optionally, a NuttX task or thread can be configured with round-robin or
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*sporadic* scheduler. The round-robin is similar to priority scheduling
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*except* that tasks with equal priority and share CPU time via
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*time-slicing*. The time-slice interval is a constant determined by the
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configuration setting ``CONFIG_RR_INTERVAL`` to a positive, non-zero
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value. Sporadic scheduling scheduling is more complex, varying the
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priority of a thread over a *replenishment* period. Support for sporadic
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scheduling is enabled by the configuration option
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``CONFIG_SCHED_SPORADIC``.
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The OS interfaces described in the following paragraphs provide a POSIX-
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compliant interface to the NuttX scheduler:
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- :c:func:`sched_setparam`
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- :c:func:`sched_getparam`
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- :c:func:`sched_setscheduler`
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- :c:func:`sched_getscheduler`
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- :c:func:`sched_yield`
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- :c:func:`sched_get_priority_max`
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- :c:func:`sched_get_priority_min`
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- :c:func:`sched_get_rr_interval`
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Functions
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---------
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.. c:function:: int sched_setparam(pid_t pid, FAR const struct sched_param *param)
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This function sets the priority of the task specified
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by pid input parameter.
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:param pid: The task ID of the task. If ``pid`` is zero, the priority of
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the calling task is set.
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:param param: A structure whose member ``sched_priority`` is the integer
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priority. The range of valid priority numbers is from
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``SCHED_PRIORITY_MIN`` through ``SCHED_PRIORITY_MAX``.
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:return: On success, sched_setparam() returns 0 (``OK``). On
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error, -1 (``ERROR``) is returned, and ```errno`` <#ErrnoAccess>`__ is
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set appropriately.
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- ``EINVAL``. The parameter ``param`` is invalid or does not make sense
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for the current scheduling policy.
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- ``EPERM``. The calling task does not have appropriate privileges.
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- ``ESRCH``. The task whose ID is ``pid`` could not be found.
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**POSIX Compatibility:** Comparable to the POSIX interface of the same
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name. Differences from the full POSIX implementation include:
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- The range of priority values for the POSIX call is 0 to 255.
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.. note:: Setting a task's priority to the same value has the similar effect
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to ``sched_yield()``: The task will be moved to after all other tasks
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with the same priority.
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.. c:function:: int sched_getparam(pid_t pid, FAR struct sched_param *param)
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This function gets the scheduling priority of the task
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specified by pid.
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:param pid: The task ID of the task. If pid is zero, the priority of the
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calling task is returned.
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:param param: A structure whose member ``sched_priority`` is the integer
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priority. The task's priority is copied to the ``sched_priority``
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element of this structure.
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:return: 0 (``OK``) if successful, otherwise -1 (``ERROR``).
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**POSIX Compatibility:** Comparable to the POSIX interface of the same
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name.
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.. c:function:: int sched_setscheduler (pid_t pid, int policy, const struct sched_param *param)
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``sched_setscheduler()`` sets both the scheduling
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policy and the priority for the task identified by ``pid``. If ``pid``
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equals zero, the scheduler of the calling thread will be set. The
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parameter ``param`` holds the priority of the thread under the new
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policy.
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:param pid: The task ID of the task. If ``pid`` is zero, the priority of
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the calling task is set.
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:param policy: Scheduling policy requested (either ``SCHED_FIFO`` or
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``SCHED_RR``).
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:param param: A structure whose member ``sched_priority`` is the integer
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priority. The range of valid priority numbers is from
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``SCHED_PRIORITY_MIN`` through ``SCHED_PRIORITY_MAX``.
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:return: On success, ``sched_setscheduler()`` returns ``OK``
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(zero). On error, ``ERROR`` (-1) is returned, and
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``errno`` is set appropriately:
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- ``EINVAL``: The scheduling ``policy`` is not one of the recognized
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policies.
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- ``ESRCH``: The task whose ID is ``pid`` could not be found.
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**POSIX Compatibility:** Comparable to the POSIX interface of the same
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name.
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.. c:function:: int sched_getscheduler (pid_t pid)
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``sched_getscheduler()`` returns the scheduling policy
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currently applied to the task identified by ``pid``. If ``pid`` equals
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zero, the policy of the calling process will be retrieved.
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:param pid: The task ID of the task to query. If ``pid`` is zero, the
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calling task is queried.
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:return: On success, ``sched_getscheduler()`` returns the policy for the task
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(either ``SCHED_FIFO`` or ``SCHED_RR``). On error, ``ERROR`` (-1) is
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returned, and ``errno`` is set appropriately:
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- ``ESRCH``: The task whose ID is pid could not be found.
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**POSIX Compatibility:** Comparable to the POSIX interface of the same
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name.
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.. c:function:: int sched_yield(void)
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This function forces the calling task to give up the
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CPU (only to other tasks at the same priority).
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:return: 0 (``OK``) or -1 (``ERROR``)
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**POSIX Compatibility:** Comparable to the POSIX interface of the same
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name.
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.. c:function:: int sched_get_priority_max (int policy)
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This function returns the value of the highest possible
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task priority for a specified scheduling policy.
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:param policy: Scheduling policy requested.
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:return: The maximum priority value or -1 (``ERROR``).
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**POSIX Compatibility:** Comparable to the POSIX interface of the same
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name.
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.. c:function:: int sched_get_priority_min (int policy)
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This function returns the value of the lowest possible
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task priority for a specified scheduling policy.
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:param policy: Scheduling policy requested.
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:return: The minimum priority value or -1 (``ERROR``)
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**POSIX Compatibility:** Comparable to the POSIX interface of the same
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name.
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.. c:function:: int sched_get_rr_interval (pid_t pid, struct timespec *interval)
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``sched_rr_get_interval()`` writes the timeslice
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interval for task identified by ``pid`` into the timespec structure
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pointed to by ``interval``. If pid is zero, the timeslice for the
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calling process is written into 'interval. The identified process should
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be running under the SCHED_RR scheduling policy.'
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:param pid: The task ID of the task. If pid is zero, the priority of the
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calling task is returned.
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:param interval: A structure used to return the time slice.
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:return: On success, sched_rr_get_interval() returns OK (0).
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On error, ERROR (-1) is returned, and ``errno`` is
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set to:
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- ``EFAULT``: Cannot copy to interval
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- ``EINVAL``: Invalid pid.
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- ``ENOSYS``: The system call is not yet implemented.
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- ``ESRCH``: The process whose ID is pid could not be found.
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**POSIX Compatibility:** Comparable to the POSIX interface of the same
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name.
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