linux-kernel - Re: [PATCH 06/10] sched/fair: Clear the target CPU from the cpumask of CPUs searched

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Date:   Fri, 4 Dec 2020 16:43:05 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     "Li, Aubrey" <aubrey.li@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Barry Song <song.bao.hua@...ilicon.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Ziljstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Linux-ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 06/10] sched/fair: Clear the target CPU from the cpumask
 of CPUs searched

On Fri, 4 Dec 2020 at 16:40, Mel Gorman <mgorman@...hsingularity.net> wrote:
>
> On Fri, Dec 04, 2020 at 04:23:48PM +0100, Vincent Guittot wrote:
> > On Fri, 4 Dec 2020 at 15:31, Mel Gorman <mgorman@...hsingularity.net> wrote:
> > >
> > > On Fri, Dec 04, 2020 at 02:47:48PM +0100, Vincent Guittot wrote:
> > > > > IIUC, select_idle_core and select_idle_cpu share the same cpumask(select_idle_mask)?
> > > > > If the target's sibling is removed from select_idle_mask from select_idle_core(),
> > > > > select_idle_cpu() will lose the chance to pick it up?
> > > >
> > > > This is only relevant for patch 10 which is not to be included IIUC
> > > > what mel said in cover letter : "Patches 9 and 10 are stupid in the
> > > > context of this series."
> > > >
> > >
> > > Patch 10 was stupid in the context of the prototype because
> > > select_idle_core always returned a CPU. A variation ended up being
> > > reintroduced at the end of the Series Yet To Be Posted so that SMT siblings
> > > are cleared during select_idle_core() but select_idle_cpu() still has a
> > > mask with unvisited CPUs to consider if no idle cores are found.
> > >
> > > As far as I know, this would still be compatible with Aubrey's idle
> > > cpu mask as long as it's visited and cleared between select_idle_core
> > > and select_idle_cpu. It relaxes the contraints on Aubrey to some extent
> > > because the idle cpu mask would be a hint so if the information is out
> > > of date, an idle cpu may still be found the normal way.
> >
> > But even without patch 10, just replacing sched_domain_span(sd) by
> > sds_idle_cpus(sd->shared) will ensure that sis loops only on cpus that
> > get a chance to be idle so select_idle_core is likely to return an
> > idle_candidate
> >
>
> Yes but if the idle mask is out of date for any reason then idle CPUs might

In fact it's the opposite, a cpu in idle mask might not be idle but
all cpus that enter idle will be set

> be missed -- hence the intent to maintain a mask of CPUs visited and use
> the idle cpu mask as a hint to prioritise CPUs that are likely idle but
> fall back to a normal scan if none of the "idle cpu mask" CPUs are
> actually idle.
>
> --
> Mel Gorman
> SUSE Labs