linux-kernel - Re: [PATCH 1/6] sched/fair: remove update of blocked load from newidle

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Date:   Tue, 9 Feb 2021 14:20:37 +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>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Qais Yousef <qais.yousef@....com>
Subject: Re: [PATCH 1/6] sched/fair: remove update of blocked load from newidle_balance

On Tue, 9 Feb 2021 at 14:09, Valentin Schneider
<valentin.schneider@....com> wrote:
>
> On 05/02/21 12:48, Vincent Guittot wrote:
> > @@ -10517,16 +10499,11 @@ static void nohz_newidle_balance(struct rq *this_rq)
> >           time_before(jiffies, READ_ONCE(nohz.next_blocked)))
> >               return;
> >
>
> I was wondering whether all the conditions above were still relevant. I
> think they are, but this one:
>
>         /* Will wake up very soon. No time for doing anything else*/
>         if (this_rq->avg_idle < sysctl_sched_migration_cost)
>                 return;
>
> should have its comment updated to something like:
>
>         /*
>          * Will wake up very soon. Blocked load will be updated
>          * periodically, no need to wake an idle CPU.
>          */
>
> given kick_ilb() isn't the costliest of things.
>
> > -     raw_spin_unlock(&this_rq->lock);
> >       /*
> > -      * This CPU is going to be idle and blocked load of idle CPUs
> > -      * need to be updated. Run the ilb locally as it is a good
> > -      * candidate for ilb instead of waking up another idle CPU.
> > -      * Kick an normal ilb if we failed to do the update.
> > +      * Blocked load of idle CPUs need to be updated.
> > +      * Kick an ILB to update statistics.
> >        */
> > -     if (!_nohz_idle_balance(this_rq, NOHZ_STATS_KICK, CPU_NEWLY_IDLE))
> > -             kick_ilb(NOHZ_STATS_KICK);
> > -     raw_spin_lock(&this_rq->lock);
>
> With this change, the return value of _nohz_idle_balance() is no longer
> used. This means we could get rid of the tracking of whether it iterated
> over all nohz CPUs or not.

Yeah, the return is useless now

>
> > +     kick_ilb(NOHZ_STATS_KICK);
> >  }
> >
> >  #else /* !CONFIG_NO_HZ_COMMON */