linux-kernel - Re: [PATCH] sched/fair: Optimize select_idle

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Date:   Fri, 6 Dec 2019 14:27:17 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Rik van Riel <riel@...riel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Valentin Schneider <valentin.schneider@....com>
Subject: Re: [PATCH] sched/fair: Optimize select_idle_core

On Fri, 6 Dec 2019 at 09:17, Srikar Dronamraju
<srikar@...ux.vnet.ibm.com> wrote:
>
> * Vincent Guittot <vincent.guittot@...aro.org> [2019-12-05 19:52:40]:
>
> > On Thu, 5 Dec 2019 at 18:52, Srikar Dronamraju
> > <srikar@...ux.vnet.ibm.com> wrote:
> > >
> > > * Vincent Guittot <vincent.guittot@...aro.org> [2019-12-05 18:27:51]:
> > >
> > > > Hi Srikar,
> > > >
> > > > On Thu, 5 Dec 2019 at 18:23, Srikar Dronamraju
> > > > <srikar@...ux.vnet.ibm.com> wrote:
> > > > >
> > > > > Currently we loop through all threads of a core to evaluate if the core
> > > > > is idle or not. This is unnecessary. If a thread of a core is not
> > > > > idle, skip evaluating other threads of a core.
> > > >
> > > > I think that the goal is also to clear all CPUs of the core from the
> > > > cpumask  of the loop above so it will not try the same core next time
> > > >
> > > > >
> > >
> > > That goal we still continue to maintain by the way of cpumask_andnot.
> > > i.e instead of clearing CPUs one at a time, we clear all the CPUs in the
> > > core at one shot.
> >
> > ah yes sorry, I have been to quick and overlooked the cpumask_andnot line
> >
>
> Just to reiterate why this is necessary.
> Currently, even if the first thread of a core is not idle, we iterate
> through all threads of the core and individually clear the CPU from the core
> mask.
>
> Collecting ticks on a Power 9 SMT 8 system around select_idle_core
> while running schbench shows us that
>
> (units are in ticks, hence lesser is better)
> Without patch
>     N           Min           Max        Median           Avg        Stddev
> x 130           151          1083           284     322.72308     144.41494
>
>
> With patch
>     N           Min           Max        Median           Avg        Stddev   Improvement
> x 164            88           610           201     225.79268     106.78943        30.03%

Thanks for the figures. Might be good to include them in the commit message

Reviewed-by: Vincent Guittot <vincent.guittot@...aro.org>


>
> --
> Thanks and Regards
> Srikar Dronamraju
>