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

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Message-ID: <jhjsg6a1doz.mognet@arm.com>
Date:   Fri, 05 Feb 2021 20:07:08 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Qais Yousef <qais.yousef@....com>,
        Quentin Perret <qperret@...gle.com>,
        Pavan Kondeti <pkondeti@...eaurora.org>,
        Rik van Riel <riel@...riel.com>
Subject: Re: [PATCH 3/8] sched/fair: Tweak misfit-related capacity checks

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 :/