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

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Date:   Fri, 05 Feb 2021 16:59:55 +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 15:31, Vincent Guittot wrote:
> On Thu, 28 Jan 2021 at 19:32, Valentin Schneider
> <valentin.schneider@....com> wrote:
>>
>> During load-balance, groups classified as group_misfit_task are filtered
>> out if they do not pass
>>
>>   group_smaller_max_cpu_capacity(<candidate group>, <local group>);
>
> group_smaller_max_cpu_capacity and  group_smaller_max_cpu_capacity are
> removed in the next patch. Merge this and the next and directly remove
> them
>

OK.

>> @@ -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).
I'm guilty of doing two things at once here: replace existing users, and
convert callsites that should be existing users. I can split the conversion
in a separate patch.