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SubjectRe: [PATCH 3/8] sched/fair: Tweak misfit-related capacity checks
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On 05/02/21 18:17, Vincent Guittot wrote:
> On Fri, 5 Feb 2021 at 18:00, Valentin Schneider
>> >> @@ -8253,7 +8260,7 @@ check_cpu_capacity(struct rq *rq, struct sched_domain *sd)
>> >> static inline int check_misfit_status(struct rq *rq, struct sched_domain *sd)
>> >> {
>> >> return rq->misfit_task_load &&
>> >> - (rq->cpu_capacity_orig < rq->rd->max_cpu_capacity ||
>> >> + (capacity_greater(rq->rd->max_cpu_capacity, rq->cpu_capacity_orig) ||
>> >
>> > Why do you add a margin here whereas there was no margin before ?
>> >
>>
>> Comparing capacities without any sort of filter can lead to ping-ponging
>> tasks around (capacity values very easily fluctuate by +/- 1, if not more).
>
> max_cpu_capacity reflects the max of the cpu_capacity_orig values
> don't aim to change and can be considered as static values.
> It would be better to fix this rounding problem (if any) in
> topology_get_cpu_scale instead of computing a margin every time it's
> used
>

That's embarrassing, I was convinced we had something updating
rd->max_cpu_capacity with actual rq->capacity values... But as you point
out that's absolutely not the case, it's all based on rq->capacity_orig,
which completely invalidates patch 5/8.

Welp.

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);

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 :/

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