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

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Message-ID: <CAKfTPtCQZMOz9HzdiWg5g9O+W=hC5E-fiG8YVHWCcODjFRfefQ@mail.gmail.com>
Date:   Fri, 13 Mar 2020 12:24:05 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     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, 13 Mar 2020 at 12:00, Valentin Schneider
<valentin.schneider@....com> wrote:
>
>
> On Thu, Mar 12 2020, Vincent Guittot wrote:
> >  kernel/sched/fair.c | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> >
> > 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

>
> 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;