linux-kernel - Re: [PATCH v3] sched/fair: bring back select_idle

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Message-ID: <CAKfTPtC2Cnr=SoD9t+dLh01ux_bc8PAcHTXxxiBqbqykLphBNw@mail.gmail.com>
Date:   Tue, 6 Apr 2021 17:31:03 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Rik van Riel <riel@...riel.com>
Cc:     Mel Gorman <mgorman@...hsingularity.net>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Kernel Team <kernel-team@...com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Valentin Schneider <valentin.schneider@....com>
Subject: Re: [PATCH v3] sched/fair: bring back select_idle_smt, but differently

On Tue, 6 Apr 2021 at 17:26, Rik van Riel <riel@...riel.com> wrote:
>
> On Tue, 2021-04-06 at 17:10 +0200, Vincent Guittot wrote:
> > On Fri, 26 Mar 2021 at 20:19, Rik van Riel <riel@...riel.com> wrote:
> >
> > > -static int select_idle_cpu(struct task_struct *p, struct
> > > sched_domain *sd, int target)
> > > +static int select_idle_cpu(struct task_struct *p, struct
> > > sched_domain *sd, int prev, int target)
> > >  {
> > >         struct cpumask *cpus =
> > > this_cpu_cpumask_var_ptr(select_idle_mask);
> > >         int i, cpu, idle_cpu = -1, nr = INT_MAX;
> > > @@ -6136,23 +6163,32 @@ static int select_idle_cpu(struct
> > > task_struct *p, struct sched_domain *sd, int t
> > >
> > >         cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
> > >
> > > -       if (sched_feat(SIS_PROP) && !smt) {
> > > -               u64 avg_cost, avg_idle, span_avg;
> > > +       if (!smt) {
> > > +               if (cpus_share_cache(prev, target)) {
> >
> > Have you checked the impact on no smt system ? would worth a static
> > branch.
> >
> > Also, this doesn't need to be in select_idle_cpu() which aims to loop
> > the sched_domain becaus you only compare  target and prev. So you can
> > move this call to select_idle_smt() in select_idle_sibling()
>
> After Mel's rewrite, there no longer are calls to
> select_idle_core() or select_idle_smt() in select_idle_sibling().

select_idle_smt() had even disappeared that why it was not in
select_idle_sibling

>
> Everything got folded into one single loop in select_idle_cpu()

but this is done completely out of the loop so we don't need to
complify the function with unrelated stuff

>
> I would be happy to pull the static branch out of select_idle_smt()
> and place it into this if condition, though. You are right that
> would save some overhead on non-smt systems.
>
> Peter, would you prefer a follow-up patch for that or a version 4
> of the patch?
>
> --
> All Rights Reversed.