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From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Fri, 13 Mar 2020 13:55:20 +0100
Message-ID: <CAKfTPtD67EKA46i12FHpJQT4gTzaH=ASAyb2dhv4=owPHBRSdQ@mail.gmail.com>
Subject: Re: [PATCH] sched/fair: improve spreading of utilization
To:     Valentin Schneider <valentin.schneider@arm.com>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        linux-kernel <linux-kernel@vger.kernel.org>
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On Fri, 13 Mar 2020 at 13:42, Valentin Schneider
<valentin.schneider@arm.com> wrote:
>
>
> On Fri, Mar 13 2020, Valentin Schneider wrote:
> > 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.
> >
>
> And with more coffee that's another Doh, ASYM_PACKING would end up as
> migrate_task. So this only affects the reduced capacity migration, which

yes  ASYM_PACKING uses migrate_task and the case of reduced capacity
would use it too and would not be impacted by this patch. I say
"would" because the original rework of load balance got rid of this
case. I'm going to prepare a separate fix  for this

> might be hard to notice in benchmarks.