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Date:   Mon, 1 Nov 2021 08:56:33 +0000
From:   Mel Gorman <mgorman@techsingularity.net>
To:     Vincent Guittot <vincent.guittot@linaro.org>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        Valentin Schneider <Valentin.Schneider@arm.com>,
        Aubrey Li <aubrey.li@linux.intel.com>,
        Barry Song <song.bao.hua@hisilicon.com>,
        Mike Galbraith <efault@gmx.de>,
        Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 1/2] sched/fair: Couple wakee flips with heavy wakers
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On Fri, Oct 29, 2021 at 05:17:38PM +0200, Vincent Guittot wrote:
> > index ff69f245b939..d00af3b97d8f 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -5865,6 +5865,14 @@ static void record_wakee(struct task_struct *p)
> >         }
> >
> >         if (current->last_wakee != p) {
> > +               int min = __this_cpu_read(sd_llc_size) << 1;
> > +               /*
> > +                * Couple the wakee flips to the waker for the case where it
> > +                * doesn't accrue flips, taking care to not push the wakee
> > +                * high enough that the wake_wide() heuristic fails.
> > +                */
> > +               if (current->wakee_flips > p->wakee_flips * min)
> > +                       p->wakee_flips++;
> 
> I have a hard time understanding the rationale behind these changes
> and the one below. Could you provide more details about why to
> increase p->wakee_flips here ? Also would be good to add such
> explanation in the commit message


The changelog covers it in the first two paragraphs but would the
following be better as a comment?

/*
 * Couple the wakee flips to the waker for the case where the
 * wakee doesn't accrue any flips during a short interval where
 * there are many wakeups without cpu load average being updated.
 * Otherwise, it is possible for wake_wide to not trigger followed
 * by an affine wake stacking multiple tasks on the same CPU due
 * to a stale cpu_load() value checked in wake_affine_weight.
 * This heuristic reduces excessive stacking of tasks while taking
 * care to not push the wakee high enough that the wake_wide
 * heuristic fails differently.
 */

Is that any better? I know this is a heuristic that is a bit on the
fuzzy side as it's trying to clamp the worst of a corner case. Ideally
"wake_wide" would be replaced with a more straight-forward heuristic but
I'm not aware of any alternatives being proposed (and I don't have one
of my own).

-- 
Mel Gorman
SUSE Labs