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 14:47:48 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     "Li, Aubrey" <aubrey.li@...ux.intel.com>
Cc:     Mel Gorman <mgorman@...hsingularity.net>,
        LKML <linux-kernel@...r.kernel.org>,
        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 14:40, Li, Aubrey <aubrey.li@...ux.intel.com> wrote:
>
> On 2020/12/4 21:17, Vincent Guittot wrote:
> > 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
> >
>
> IIUC, select_idle_core and select_idle_cpu share the same cpumask(select_idle_mask)?
> If the target's sibling is removed from select_idle_mask from select_idle_core(),
> select_idle_cpu() will lose the chance to pick it up?

This is only relevant for patch 10 which is not to be included IIUC
what mel said in cover letter : "Patches 9 and 10 are stupid in the
context of this series."

>
> Thanks,
> -Aubrey