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 11:56:36 +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 Thu, 3 Dec 2020 at 18:52, Mel Gorman <mgorman@...hsingularity.net> wrote:
>
> On Thu, Dec 03, 2020 at 05:38:03PM +0100, Vincent Guittot wrote:
> > On Thu, 3 Dec 2020 at 15:11, Mel Gorman <mgorman@...hsingularity.net> wrote:
> > >
> > > The target CPU is definitely not idle in both select_idle_core and
> > > select_idle_cpu. For select_idle_core(), the SMT is potentially
> > > checked unnecessarily as the core is definitely not idle if the
> > > target is busy. For select_idle_cpu(), the first CPU checked is
> > > simply a waste.
> >
> > >
> > > Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>
> > > ---
> > >  kernel/sched/fair.c | 2 ++
> > >  1 file changed, 2 insertions(+)
> > >
> > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > index 68dd9cd62fbd..1d8f5c4b4936 100644
> > > --- a/kernel/sched/fair.c
> > > +++ b/kernel/sched/fair.c
> > > @@ -6077,6 +6077,7 @@ static int select_idle_core(struct task_struct *p, struct sched_domain *sd, int
> > >                 return -1;
> > >
> > >         cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
> > > +       __cpumask_clear_cpu(target, cpus);
> >
> > should clear cpu_smt_mask(target) as we are sure that the core will not be idle
> >
>
> 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 ?

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


>
> --
> Mel Gorman
> SUSE Labs