How to schedule a node on a different processor?

[Gedare Bloom]

On Tue, Aug 18, 2020 at 11:20 AM Richi Dubey <richidubey at gmail.com> wrote:
>
> 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) to the function _Scheduler_strong_Get_lowest_scheduled because after _Scheduler_SMP_Enqueue calls the former function, it either (see code here) 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), 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?
>

No. Try to think through if you can solve this a different way. Maybe
you can store something in the context?

>
>
>
> 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.
>> >
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