linux-kernel - Re: [PATCH] sched/fair: ignore cache hotness for SMT migration

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Date:   Tue, 4 Aug 2020 12:24:43 -0700
From:   Josh Don <joshdon@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched/fair: ignore cache hotness for SMT migration

On Tue, Aug 4, 2020 at 3:56 AM <peterz@...radead.org> wrote:
>
> On Mon, Aug 03, 2020 at 05:06:14PM -0700, Josh Don wrote:
> > SMT siblings share caches, so cache hotness should be irrelevant for
> > cross-sibling migration.
> >
> > Proposed-by: Venkatesh Pallipadi <venki@...gle.com>
> > Signed-off-by: Josh Don <joshdon@...gle.com>
> > ---
> >  kernel/sched/fair.c | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 2ba8f230feb9..5b203b55bcb2 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -7402,6 +7402,9 @@ static int task_hot(struct task_struct *p, struct lb_env *env)
> >       if (unlikely(task_has_idle_policy(p)))
> >               return 0;
> >
> > +     if ((env->sd->flags & cpu_smt_flags()) == cpu_smt_flags())
> > +             return 0;
>
> I think that wants to be:
>
>         if (env->sd->flags & SD_SHARE_CPUCAPACITY)
>

Agreed

> Also, perhaps stick a comment on top with the rationale for this.
>
> > +
> >       /*
> >        * Buddy candidates are cache hot:
> >        */
> > --
> > 2.28.0.163.g6104cc2f0b6-goog
> >