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doc: Add deadline scheduling information

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Adds information to the kernel scheduling documentation explaining
how a thread's deadline is used to determine the thread's relative
priority.

Signed-off-by: Peter Mitsis <peter.mitsis@intel.com>
This commit is contained in:
Peter Mitsis 2021-09-29 15:51:05 -04:00 • committed by Anas Nashif
commit 0153b7dc50
1 changed files with 9 additions and 0 deletions
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9
doc/reference/kernel/scheduling/index.rst
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@ -39,6 +39,15 @@ The kernel's scheduler selects the highest priority ready thread
to be the current thread. When multiple ready threads of the same priority
exist, the scheduler chooses the one that has been waiting longest.
A thread's relative priority is primarily determined by its static priority.
However, when both earliest-deadline-first scheduling is enabled
(:kconfig:`CONFIG_SCHED_DEADLINE`) and a choice of threads have equal
static priority, then the thread with the earlier deadline is considered
to have the higher priority. Thus, when earliest-deadline-first scheduling is
enabled, two threads are only considered to have the same priority when both
their static priorities and deadlines are equal. The routine
:c:func:`k_thread_deadline_set` is used to set a thread's deadline.
.. note::
Execution of ISRs takes precedence over thread execution,
so the execution of the current thread may be replaced by an ISR