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Date:   Tue, 8 Dec 2020 14:24:32 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Dietmar Eggemann <dietmar.eggemann@....com>,
        LKML <linux-kernel@...r.kernel.org>,
        Aubrey Li <aubrey.li@...ux.intel.com>,
        Barry Song <song.bao.hua@...ilicon.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Ziljstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Linux-ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 1/4] sched/fair: Remove SIS_AVG_CPU

On Tue, 8 Dec 2020 at 11:59, Mel Gorman <mgorman@...hsingularity.net> wrote:
>
> On Tue, Dec 08, 2020 at 11:07:19AM +0100, Dietmar Eggemann wrote:
> > On 07/12/2020 10:15, Mel Gorman wrote:
> > > SIS_AVG_CPU was introduced as a means of avoiding a search when the
> > > average search cost indicated that the search would likely fail. It
> > > was a blunt instrument and disabled by 4c77b18cf8b7 ("sched/fair: Make
> > > select_idle_cpu() more aggressive") and later replaced with a proportional
> > > search depth by 1ad3aaf3fcd2 ("sched/core: Implement new approach to
> > > scale select_idle_cpu()").
> > >
> > > While there are corner cases where SIS_AVG_CPU is better, it has now been
> > > disabled for almost three years. As the intent of SIS_PROP is to reduce
> > > the time complexity of select_idle_cpu(), lets drop SIS_AVG_CPU and focus
> > > on SIS_PROP as a throttling mechanism.
> > >
> > > Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>
> > > ---
> > >  kernel/sched/fair.c     | 3 ---
> > >  kernel/sched/features.h | 1 -
> > >  2 files changed, 4 deletions(-)
> > >
> > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > index 98075f9ea9a8..23934dbac635 100644
> > > --- a/kernel/sched/fair.c
> > > +++ b/kernel/sched/fair.c
> > > @@ -6161,9 +6161,6 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
> > >     avg_idle = this_rq()->avg_idle / 512;
> > >     avg_cost = this_sd->avg_scan_cost + 1;
> > >
> > > -   if (sched_feat(SIS_AVG_CPU) && avg_idle < avg_cost)
> > > -           return -1;
> > > -
> > >     if (sched_feat(SIS_PROP)) {
> > >             u64 span_avg = sd->span_weight * avg_idle;
> > >             if (span_avg > 4*avg_cost)
> >
> > Nitpick:
> >
> > Since now avg_cost and avg_idle are only used w/ SIS_PROP, they could go
> > completely into the SIS_PROP if condition.
> >
>
> Yeah, I can do that. In the initial prototype, that happened in a
> separate patch that split out SIS_PROP into a helper function and I
> never merged it back. It's a trivial change.

while doing this, should you also put the update of
this_sd->avg_scan_cost under the SIS_PROP feature ?

>
> Thanks.
>
> --
> Mel Gorman
> SUSE Labs

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