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
Date: Mon, 1 Nov 2021 08:56:33 +0000 [thread overview]
Message-ID: <20211101085633.GW3959@techsingularity.net> (raw)
In-Reply-To: <CAKfTPtB-fJ7Pd6eYPDrHB8Ts0o7SCbN7nniAD9PSoF4Pf+xB3w@mail.gmail.com>
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
next prev parent reply other threads:[~2021-11-01 8:56 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-10-28 9:48 [PATCH v4 0/2] Reduce stacking and overscheduling Mel Gorman
2021-10-28 9:48 ` [PATCH 1/2] sched/fair: Couple wakee flips with heavy wakers Mel Gorman
2021-10-28 16:19 ` Tao Zhou
2021-10-29 8:42 ` Mel Gorman
2021-11-10 9:53 ` Tao Zhou
2021-11-10 15:40 ` Mike Galbraith
2021-10-29 15:17 ` Vincent Guittot
2021-10-30 3:11 ` Mike Galbraith
2021-10-30 4:12 ` Mike Galbraith
2021-11-01 8:56 ` Mel Gorman [this message]
2021-10-28 9:48 ` [PATCH 2/2] sched/fair: Increase wakeup_gran if current task has not executed the minimum granularity Mel Gorman
2021-10-29 16:07 ` Vincent Guittot
-- strict thread matches above, loose matches on Subject: below --
2021-10-21 14:56 [PATCH 0/2] Reduce stacking and overscheduling Mel Gorman
2021-10-21 14:56 ` [PATCH 1/2] sched/fair: Couple wakee flips with heavy wakers Mel Gorman
2021-10-22 10:26 ` Mike Galbraith
2021-10-22 11:05 ` Mel Gorman
2021-10-22 12:00 ` Mike Galbraith
2021-10-25 6:35 ` Mike Galbraith
2021-10-26 8:18 ` Mel Gorman
2021-10-26 10:15 ` Mike Galbraith
2021-10-26 10:41 ` Mike Galbraith
2021-10-26 11:57 ` Mel Gorman
2021-10-26 12:13 ` Mike Galbraith
2021-10-27 2:09 ` Mike Galbraith
2021-10-27 9:00 ` Mel Gorman
2021-10-27 10:18 ` Mike Galbraith
2021-11-09 11:56 ` Peter Zijlstra
2021-11-09 12:55 ` Mike Galbraith
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