Re: [PATCH 3/8] sched/fair: Tweak misfit-related capacity checks

[Valentin Schneider <valentin.schneider@arm.com>]

On 08/02/21 16:29, Vincent Guittot wrote:
> On Fri, 5 Feb 2021 at 21:07, Valentin Schneider
> <valentin.schneider@arm.com> wrote:
>>
>> Perhaps I can still keep 5/8 with something like
>>
>>   if (!rq->misfit_task_load)
>>           return false;
>>
>>   do {
>>           if (capacity_greater(group->sgc->max_capacity, rq->cpu_capacity))
>>                   return true;
>>
>>           group = group->next;
>>   } while (group != sd->groups);
>
> I don't catch what you want to achieve with this  while loop compared
> to the original condition which is :
> trigger a load_balance :
> - if there is CPU with higher original capacity
> - or if the capacity of this cpu has significantly reduced because of
> pressure and there is maybe others with more capacity even if it's one
> with highest original capacity
>

If we had a root-domain-wide (dynamic) capacity maximum, we could make
check_misfit_status() return false if the CPU *is* pressured but there is
no better alternative - e.g. if all other CPUs are pressured even worse.

This isn't a correctness issue as the nohz load-balance will just not
migrate the misfit task, but it would be nice to prevent the nohz kick
altogether.

I might ditch this for now and revisit it later.

>>
>>   return false;
>>
>> This works somewhat well for big.LITTLE, but for DynamIQ systems under a
>> single L3 this ends up iterating over all the CPUs :/