About is_non_preempt_mode_supported

[Joel Sherrill]

On Wed, Mar 17, 2021 at 1:54 AM Richi Dubey <richidubey at gmail.com> wrote:

> Hi,
>
> I am debugging tm19 running on Strong APA scheduler. It gives the
> following error:
> rtems_signal_catch FAILED -- expected (RTEMS_SUCCESSFUL) got
> (RTEMS_NOT_IMPLEMENTED)
>
> which is due to line 167
> <https://git.rtems.org/rtems/tree/testsuites/tmtests/tm19/task1.c#n167>.
> This arises because the variable is_non_preempt_mode_supported in
> _Scheduler_Control of SMP Strong APA scheduler is set to false.
>
> On further checking, I can see that almost all the SMP schedulers have
> this variable set as false in cpukit/include/rtems/scheduler.h. Please let
> me know why this is the case. What would I need to do to support non
> preempt tasks in the Strong APA scheduler?
>

No preempt does not make sense in an SMP environment. The idea with
no preempt is that by setting it, the thread runs without another thread
(only
interrupts) preempting it. This works fine on uniprocessor systems but since
there are always multiple threads running in SMP, the assumption is
violated.

No preempt is an old RTOS feature and sometimes has a name like
scheduler lock. It is just one of the features/APIs not safe for SMP that
we removed or made inoperable in SMP mode. This section in the
manual covers these features:

https://docs.rtems.org/branches/master/c-user/symmetric_multiprocessing_services.html#application-issues

This is definitely an area where you have to think a bit at the
conceptual purpose of the API/feature to realize why it can't
work or is unsafe in SMP mode.

--joel

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