How to schedule a node on a different processor?

[Richi Dubey]

Hi,

Thanks for the help. I looked at the call trace and figured out what is
wrong.

As we had discussed earlier, We would be changing the passed variable
filter_base pointer (done here
<https://github.com/richidubey/rtems/blob/b94696c00c32bd9e5a74587af27a263794575919/cpukit/score/src/schedulerstrongapa.c#L408>)
to the function _Scheduler_strong_Get_lowest_scheduled because after
_Scheduler_SMP_Enqueue calls the former function, it either (see code here
<https://github.com/richidubey/rtems/blob/b94696c00c32bd9e5a74587af27a263794575919/cpukit/include/rtems/score/schedulersmpimpl.h#L892>)
preempts the lowest scheduled node and allocates the processor to node
(filter_base) or for the case lowest_scheduled has a higher priority (lower
priority number) it simply inserts the current node (or the filter_base)
into the ready queue.

When implementing the Strong_APA, the filter_node actually displaces the
higher priority node :

Ex :
A system with 2 CPUs, Tasks with lower task numbers have higher priority.

T1: Allocated CPU 1. Affinity to both CPUs
T3: Allocated CPU 2. Affinity to both CPUs

T2: Just Arrived. Affinity only to CPU1.

In this case, T2 would preempt T1 (A higher priority task) and T1 would
preempt T3.

So, to make sure this works with our SMP framework, we had decided to
change/make filter_base point to T1 and lowest_scheduled to T3 after
preempting T2 preempts T1, after which SMP_Enqueue would preempt T3 and
schedule T1.

But this is not working because when we change filter_base
from _Scheduler_strong_Get_lowest_scheduled (done here
<https://github.com/richidubey/rtems/blob/b94696c00c32bd9e5a74587af27a263794575919/cpukit/score/src/schedulerstrongapa.c#L408>),
the value of the node in SMP_Enqueue does not get changed (Because it is a
pass by value of pointers). Also, there is no way to change the
insert_priority variable in SMP_Enqueue either.

What should we do now? Can we change the SMP_Enqueue function to account
for the way Strong_APA behaves?




On Tue, Aug 18, 2020 at 9:40 PM Gedare Bloom <gedare at rtems.org> wrote:

> On Tue, Aug 18, 2020 at 9:21 AM Richi Dubey <richidubey at gmail.com> wrote:
> >
> > Hi,
> >
> > This question is wrt the use of Scheduler_SMP_Preempt here. This
> function eventually calls Allocate_Processor and while tracing with gdb,
> everything is going as I planned, i.e. the correct node is getting
> preempted and the caller to it is getting allocated the CPU. But, the test
> fails here, because eventually the heir is not changed.
> >
> > Do I have to explicitly change the heir when I am trying to schedule a
> node/thread on a different processor? Is doing SMP_Preempt not enough?
> Please suggest.
> >
> The heir should get set (in SMP scheduling) by call to
> _Thread_Dispatch_update_heir() from Allocate_Processor. Probably you
> need to look at your allocate_processor hook.
>
> > Thanks,
> > Richi.
> >
> > _______________________________________________
> > devel mailing list
> > devel at rtems.org
> > http://lists.rtems.org/mailman/listinfo/devel
>
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