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

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Date:   Mon, 08 Feb 2021 17:49:00 +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 08/02/21 16:29, Vincent Guittot wrote:
> On Fri, 5 Feb 2021 at 21:07, Valentin Schneider
> <valentin.schneider@....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 :/