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From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Wed, 7 Apr 2021 11:57:40 +0200
Message-ID: <CAKfTPtB7bSkQttZoOPfvsHmN6yG6BrYTs0r0xBO4Xs-A+UhH6w@mail.gmail.com>
Subject: Re: [PATCH v3] sched/fair: bring back select_idle_smt, but differently
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Rik van Riel <riel@surriel.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Kernel Team <kernel-team@fb.com>,
        Ingo Molnar <mingo@kernel.org>,
        Valentin Schneider <valentin.schneider@arm.com>
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On Wed, 7 Apr 2021 at 11:55, Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Wed, Apr 07, 2021 at 11:42:17AM +0200, Vincent Guittot wrote:
> > I would really prefer to keep that out of select_idle_cpu which aims to merge in one
> > single loop the walk through sd_llc. In the case of select_idle_smt, this is done outside
> > the loop:
>
> Fair enough.
>
> > @@ -6317,11 +6339,21 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
> >               }
> >       }
> >
> > +     if (static_branch_likely(&sched_smt_present)) {
> > +             smt = test_idle_cores(target, false);
> > +             if (!smt && cpus_share_cache(prev, target)) {
> > +                     /* No idle core. Check if prev has an idle sibling. */
> > +                     i = select_idle_smt(p, sd, prev);
> > +                     if ((unsigned int)i < nr_cpumask_bits)
> > +                             return i;
> > +             }
> > +     }
> > +
> >       sd = rcu_dereference(per_cpu(sd_llc, target));
> >       if (!sd)
> >               return target;
>
> It needs to be here, otherwise you're using @sd uninitialized.

argh yes...

>
> > -     i = select_idle_cpu(p, sd, target);
> > +     i = select_idle_cpu(p, sd, smt, target);
> >       if ((unsigned)i < nr_cpumask_bits)
> >               return i;
>
> Let me have another poke at it.