linux-kernel - Re: [PATCH] sched/fair: improve spreading of utilization

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Date:   Fri, 13 Mar 2020 11:28:55 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Valentin Schneider <valentin.schneider@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched/fair: improve spreading of utilization


On Fri, Mar 13 2020, Vincent Guittot wrote:
>> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> > index 3c8a379c357e..97a0307312d9 100644
>> > --- a/kernel/sched/fair.c
>> > +++ b/kernel/sched/fair.c
>> > @@ -9025,6 +9025,14 @@ static struct rq *find_busiest_queue(struct lb_env *env,
>> >               case migrate_util:
>> >                       util = cpu_util(cpu_of(rq));
>> >
>> > +                     /*
>> > +                      * Don't try to pull utilization from a CPU with one
>> > +                      * running task. Whatever its utilization, we will fail
>> > +                      * detach the task.
>> > +                      */
>> > +                     if (nr_running <= 1)
>> > +                             continue;
>> > +
>>
>> Doesn't this break misfit? If the busiest group is group_misfit_task, it
>> is totally valid for the runqueues to have a single running task -
>> that's the CPU-bound task we want to upmigrate.
>
>  group_misfit_task has its dedicated migrate_misfit case
>

Doh, yes, sorry. I think my rambling on ASYM_PACKING / reduced capacity
migration is still relevant, though.

>>
>> If the busiest rq has only a single running task, we'll skip the
>> detach_tasks() block and go straight to the active balance bits.
>> Misfit balancing totally relies on this, and IMO ASYM_PACKING does
>> too. Looking at voluntary_active_balance(), it seems your change also
>> goes against the one added by
>>   1aaf90a4b88a ("sched: Move CFS tasks to CPUs with higher capacity")
>>
>> The bandaid here would be gate this 'continue' with checks against the
>> busiest_group_type, but that's only a loose link wrt
>> voluntary_active_balance().
>>
>> >                       if (busiest_util < util) {
>> >                               busiest_util = util;
>> >                               busiest = rq;