Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <20210119112211.3196-1-mgorman@techsingularity.net>
 <20210119112211.3196-6-mgorman@techsingularity.net> <20210120083018.GA14462@in.ibm.com>
 <20210120091235.GT3592@techsingularity.net>
In-Reply-To: <20210120091235.GT3592@techsingularity.net>
From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Wed, 20 Jan 2021 10:21:47 +0100
Message-ID: <CAKfTPtBYuxKywCPeEd=vYCu31Ni0=uXoJa4v3ZV_T9J0TsVyhg@mail.gmail.com>
Subject: Re: [PATCH 5/5] sched/fair: Merge select_idle_core/cpu()
To:     Mel Gorman <mgorman@techsingularity.net>
Cc:     Gautham R Shenoy <ego@linux.vnet.ibm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Li Aubrey <aubrey.li@linux.intel.com>,
        Qais Yousef <qais.yousef@arm.com>,
        LKML <linux-kernel@vger.kernel.org>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Wed, 20 Jan 2021 at 10:12, Mel Gorman <mgorman@techsingularity.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