linux-kernel - Re: [PATCH] sched, fair: Allow a small load imbalance between low utilisation SD

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Message-ID: <CAKfTPtB_9uf+f8ZWFW-=-orS3KGKDGPwY3YrPSFn54z0C=7o6g@mail.gmail.com>
Date:   Tue, 7 Jan 2020 09:44:23 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Hillf Danton <hdanton@...a.com>
Cc:     Rik van Riel <riel@...riel.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Phil Auld <pauld@...hat.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        Quentin Perret <quentin.perret@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <Morten.Rasmussen@....com>,
        Parth Shah <parth@...ux.ibm.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched, fair: Allow a small load imbalance between low
 utilisation SD_NUMA domains v3

On Tue, 7 Jan 2020 at 02:51, Hillf Danton <hdanton@...a.com> wrote:
>
>
> Hi Folks
>
> On Mon, 06 Jan 2020 11:44:57 -0500 Rik van Riel wrote:
> > On Mon, 2020-01-06 at 16:33 +0000, Mel Gorman wrote:
> > > On Mon, Jan 06, 2020 at 10:47:18AM -0500, Rik van Riel wrote:
> > > > > +                       imbalance_adj = (100 / (env->sd->imbalance_pct - 100)) - 1;
> > > > > +
> > > > > +                       /*
> > > > > +                        * Allow small imbalances when the busiest group has
> > > > > +                        * low utilisation.
> > > > > +                        */
> > > > > +                       imbalance_max = imbalance_adj << 1;
> > > > > +                       if (busiest->sum_nr_running < imbalance_max)
> > > > > +                               env->imbalance -= min(env->imbalance, imbalance_adj);
> > > > > +               }
> > > > > +
> > > >
> > > > Wait, so imbalance_max is a function only of
> > > > env->sd->imbalance_pct, and it gets compared
> > > > against busiest->sum_nr_running, which is related
> > > > to the number of CPUs in the node?
> > > >
> > >
> > > It's not directly related to the number of CPUs in the node. Are you
> > > thinking of busiest->group_weight?
> >
> > I am, because as it is right now that if condition
> > looks like it might never be true for imbalance_pct 115.
> >
> > Presumably you put that check there for a reason, and
> > would like it to trigger when the amount by which a node
> > is busy is less than 2 * (imbalance_pct - 100).
>
>
> If three per cent can make any sense in helping determine utilisation
> low then the busy load has to meet
>
>         busiest->sum_nr_running < max(3, cpus in the node / 32);

I agree that we should take into account the size or the utilization
of the node in some way

>
> And we can't skip pulling tasks from a numa node without comparing it
> to the local load
>
>         local->sum_nr_running * env->sd->imbalance_pct <
>         busiest->sum_nr_running * 100;

A similar test has already been done in fbg and we call calculate
imbalance only if busiest is busier than local

>
> with imbalance_pct taken into account.
>
> Hillf
>