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score: Implement forced thread migration

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The current implementation of task migration in RTEMS has some
implications with respect to the interrupt latency. It is crucial to
preserve the system invariant that a task can execute on at most one
processor in the system at a time. This is accomplished with a boolean
indicator in the task context. The processor architecture specific
low-level task context switch code will mark that a task context is no
longer executing and waits that the heir context stopped execution
before it restores the heir context and resumes execution of the heir
task. So there is one point in time in which a processor is without a
task. This is essential to avoid cyclic dependencies in case multiple
tasks migrate at once. Otherwise some supervising entity is necessary to
prevent life-locks. Such a global supervisor would lead to scalability
problems so this approach is not used. Currently the thread dispatch is
performed with interrupts disabled. So in case the heir task is
currently executing on another processor then this prolongs the time of
disabled interrupts since one processor has to wait for another
processor to make progress.

It is difficult to avoid this issue with the interrupt latency since
interrupts normally store the context of the interrupted task on its
stack. In case a task is marked as not executing we must not use its
task stack to store such an interrupt context. We cannot use the heir
stack before it stopped execution on another processor. So if we enable
interrupts during this transition we have to provide an alternative task
independent stack for this time frame. This issue needs further
investigation.
This commit is contained in:
Sebastian Huber
2014-05-02 10:31:09 +02:00
parent 58444f7795
commit 38b59a6d30
43 changed files with 800 additions and 153 deletions
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46
doc/user/smp.t
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@@ -211,6 +211,52 @@ affinity. Although the behavior is scheduler specific, if the scheduler
does not support affinity, it is likely to ignore all attempts to set
affinity.
@subsection Task Migration
@cindex task migration
@cindex thread migration
With more than one processor in the system tasks can migrate from one processor
to another. There are three reasons why tasks migrate in RTEMS.
@itemize @bullet
@item The scheduler changes explicitly via @code{rtems_task_set_scheduler()} or
similar directives.
@item The task resumes execution after a blocking operation. On a priority
based scheduler it will evict the lowest priority task currently assigned to a
processor in the processor set managed by the scheduler instance.
@item The task moves temporarily to another scheduler instance due to locking
protocols like @cite{Migratory Priority Inheritance} or the
@cite{Multiprocessor Resource Sharing Protocol}.
@end itemize
Task migration should be avoided so that the working set of a task can stay on
the most local cache level.
The current implementation of task migration in RTEMS has some implications
with respect to the interrupt latency. It is crucial to preserve the system
invariant that a task can execute on at most one processor in the system at a
time. This is accomplished with a boolean indicator in the task context. The
processor architecture specific low-level task context switch code will mark
that a task context is no longer executing and waits that the heir context
stopped execution before it restores the heir context and resumes execution of
the heir task. So there is one point in time in which a processor is without a
task. This is essential to avoid cyclic dependencies in case multiple tasks
migrate at once. Otherwise some supervising entity is necessary to prevent
life-locks. Such a global supervisor would lead to scalability problems so
this approach is not used. Currently the thread dispatch is performed with
interrupts disabled. So in case the heir task is currently executing on
another processor then this prolongs the time of disabled interrupts since one
processor has to wait for another processor to make progress.
It is difficult to avoid this issue with the interrupt latency since interrupts
normally store the context of the interrupted task on its stack. In case a
task is marked as not executing we must not use its task stack to store such an
interrupt context. We cannot use the heir stack before it stopped execution on
another processor. So if we enable interrupts during this transition we have
to provide an alternative task independent stack for this time frame. This
issue needs further investigation.
@subsection Critical Section Techniques and SMP
As discussed earlier, SMP systems have opportunities for true parallelism