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

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Date:   Fri, 6 Dec 2019 13:46:54 +0530
From:   Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
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

* 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 and Regards
Srikar Dronamraju