linux-kernel - Re: [PATCH 06/10] sched/fair: Clear the target CPU from the cpumask of CPUs searched

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Date:   Fri, 4 Dec 2020 14:17:20 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     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 06/10] sched/fair: Clear the target CPU from the cpumask
 of CPUs searched

On Fri, 4 Dec 2020 at 14:13, Vincent Guittot <vincent.guittot@...aro.org> wrote:
>
> On Fri, 4 Dec 2020 at 12:30, Mel Gorman <mgorman@...hsingularity.net> wrote:
> >
> > On Fri, Dec 04, 2020 at 11:56:36AM +0100, Vincent Guittot wrote:
> > > > The intent was that the sibling might still be an idle candidate. In
> > > > the current draft of the series, I do not even clear this so that the
> > > > SMT sibling is considered as an idle candidate. The reasoning is that if
> > > > there are no idle cores then an SMT sibling of the target is as good an
> > > > idle CPU to select as any.
> > >
> > > Isn't the purpose of select_idle_smt ?
> > >
> >
> > Only in part.
> >
> > > select_idle_core() looks for an idle core and opportunistically saves
> > > an idle CPU candidate to skip select_idle_cpu. In this case this is
> > > useless loops for select_idle_core() because we are sure that the core
> > > is not idle
> > >
> >
> > If select_idle_core() finds an idle candidate other than the sibling,
> > it'll use it if there is no idle core -- it picks a busy sibling based
> > on a linear walk of the cpumask. Similarly, select_idle_cpu() is not
>
> My point is that it's a waste of time to loop the sibling cpus of
> target in select_idle_core because it will not help to find an idle
> core. The sibling  cpus will then be check either by select_idle_cpu
> of select_idle_smt

also, while looping the cpumask, the sibling cpus of not idle cpu are
removed and will not be check

>
> > guaranteed to scan the sibling first (ordering) or even reach the sibling
> > (throttling). select_idle_smt() is a last-ditch effort.
> >
> > --
> > Mel Gorman
> > SUSE Labs