linux-kernel - Re: [PATCH 1/4] sched/fair: Remove SIS_AVG

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Date:   Tue, 8 Dec 2020 13:53:58 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Vincent Guittot <vincent.guittot@...aro.org>
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, Dec 08, 2020 at 02:43:10PM +0100, Vincent Guittot wrote:
> On Tue, 8 Dec 2020 at 14:36, Mel Gorman <mgorman@...hsingularity.net> wrote:
> >
> > On Tue, Dec 08, 2020 at 02:24:32PM +0100, Vincent Guittot wrote:
> > > > > 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 ?
> > >
> >
> > It's outside the scope of the series but why not. This?
> >
> > --8<--
> > sched/fair: Move avg_scan_cost calculations under SIS_PROP
> >
> > As noted by Vincent Guittot, avg_scan_costs are calculated for SIS_PROP
> > even if SIS_PROP is disabled. Move the time calculations under a SIS_PROP
> > check and while we are at it, exclude the cost of initialising the CPU
> > mask from the average scan cost.
> >
> > Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 19ca0265f8aa..0fee53b1aae4 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -6176,10 +6176,10 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
> >                         nr = 4;
> >         }
> >
> > -       time = cpu_clock(this);
> 
> I would move it in the if (sched_feat(SIS_PROP)) above.
> 

I considered it but made the choice to exclude the cost of cpumask_and()
from the avg_scan_cost instead. It's minor but when doing the original
prototype, I didn't think it was appropriate to count the cpumask
clearing as part of the scan cost as it's not directly related.

-- 
Mel Gorman
SUSE Labs