linux-kernel - Re: [PATCH 5/5] sched/fair: Merge select_idle

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Date:   Wed, 20 Jan 2021 10:21:47 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Gautham R Shenoy <ego@...ux.vnet.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Li Aubrey <aubrey.li@...ux.intel.com>,
        Qais Yousef <qais.yousef@....com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/5] sched/fair: Merge select_idle_core/cpu()

On Wed, 20 Jan 2021 at 10:12, Mel Gorman <mgorman@...hsingularity.net> wrote:
>
> On Wed, Jan 20, 2021 at 02:00:18PM +0530, Gautham R Shenoy wrote:
> > > @@ -6157,18 +6169,31 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
> > >     }
> > >
> > >     for_each_cpu_wrap(cpu, cpus, target) {
> > > -           if (!--nr)
> > > -                   return -1;
> > > -           if (available_idle_cpu(cpu) || sched_idle_cpu(cpu))
> > > -                   break;
> > > +           if (smt) {
> > > +                   i = select_idle_core(p, cpu, cpus, &idle_cpu);
> > > +                   if ((unsigned int)i < nr_cpumask_bits)
> > > +                           return i;
> > > +
> > > +           } else {
> > > +                   if (!--nr)
> > > +                           return -1;
> > > +                   i = __select_idle_cpu(cpu);
> > > +                   if ((unsigned int)i < nr_cpumask_bits) {
> > > +                           idle_cpu = i;
> > > +                           break;
> > > +                   }
> > > +           }
> > >     }
> > >
> > > -   if (sched_feat(SIS_PROP)) {
> > > +   if (smt)
> > > +           set_idle_cores(this, false);
> >
> > Shouldn't we set_idle_cores(false) only if this was the last idle
> > core in the LLC ?
> >
>
> That would involve rechecking the cpumask bits that have not been
> scanned to see if any of them are an idle core. As the existance of idle
> cores can change very rapidly, it's not worth the cost.

But don't we reach this point only if we scanned all CPUs and didn't
find an idle core ?
If there is an idle core, it returns early
And in case of smt == true, nr is set to INt_MAX and we loop on all CPUs/cores

>
> --
> Mel Gorman
> SUSE Labs