Participation in GSoC 2016

[Darshit Shah]

On 18 March 2016 at 14:48, Sebastian Huber <
sebastian.huber at embedded-brains.de> wrote:

>
>
> On 18/03/16 14:38, Darshit Shah wrote:
>
>> > "schedulable task sets would miss their deadlines since it does not
>> consider
>> > migrating high priority tasks"
>> >
>> > I think this is not correct. The current APA scheduler should implement
>> > strong APA. However, it may iterate several times over all (!) ready
>> tasks.
>> > This is the actual problem.
>> >
>> I may be wrong, but I think the current implementation does indeed not
>> consider strong APA guarantees.
>>
>> The `_Scheduler_priority_affinity_SMP_Get_lowest_scheduled` method tries
>> to find a thread that needs to be migrated to another core in order to
>> schedule the selected thread. In this method, on line 274 is the comment:
>>
>>       /*
>>          * If we didn't find a thread which is of equal or lower
>>     importance
>>          * than filter thread is, then we can't schedule the filter thread
>>          * to execute.
>>          */
>>
>>
>> If I understand this comment correctly, the scheduler does not consider
>> threads for migration if they are not of "equal or lower importance" than
>> the filter thread.
>>
>
> In contrast to the Linux push/pull scheduler you search here in the set of
> all scheduled/ready threads and not just in your local queue. I didn't look
> into the details, but I think this is enough for strong APA. See also
> _Scheduler_priority_affinity_SMP_Check_for_migrations().
>
>
That is true. Unlike the linux scheduler, all currently scheduled threads
are considered for migration, but then it seems we only decide to migrate a
thread if its priority is lower than the thread being dispatched. In case
I'm wrong then the current implementation is indeed Strong APA, just with a
bad time complexity. Else, we still have some form of a weak APA constrain
in the scheduler.

>
> --
> Sebastian Huber, embedded brains GmbH
>
> Address : Dornierstr. 4, D-82178 Puchheim, Germany
> Phone   : +49 89 189 47 41-16
> Fax     : +49 89 189 47 41-09
> E-Mail  : sebastian.huber at embedded-brains.de
> PGP     : Public key available on request.
>
> Diese Nachricht ist keine geschäftliche Mitteilung im Sinne des EHUG.
>
>


-- 
Thanking You,
Darshit Shah
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