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From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Fri, 20 Mar 2020 17:54:57 +0100
Message-ID: <CAKfTPtBixZKDES_i3Lnsj1eAa_kVi-zHv-0uE8uTsKOBcjmkYg@mail.gmail.com>
Subject: Re: [PATCH 4/4] sched/fair: Track possibly overloaded domains and
 abort a scan if necessary
To:     Mel Gorman <mgorman@techsingularity.net>
Cc:     Ingo Molnar <mingo@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Phil Auld <pauld@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>
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On Fri, 20 Mar 2020 at 17:44, Mel Gorman <mgorman@techsingularity.net> wrote:
>
> On Fri, Mar 20, 2020 at 04:48:39PM +0100, Vincent Guittot wrote:
> > > ---
> > >  include/linux/sched/topology.h |  1 +
> > >  kernel/sched/fair.c            | 65 +++++++++++++++++++++++++++++++++++++++---
> > >  kernel/sched/features.h        |  3 ++
> > >  3 files changed, 65 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
> > > index af9319e4cfb9..76ec7a54f57b 100644
> > > --- a/include/linux/sched/topology.h
> > > +++ b/include/linux/sched/topology.h
> > > @@ -66,6 +66,7 @@ struct sched_domain_shared {
> > >         atomic_t        ref;
> > >         atomic_t        nr_busy_cpus;
> > >         int             has_idle_cores;
> > > +       int             is_overloaded;
> >
> > Can't nr_busy_cpus compared to sd->span_weight give you similar status  ?
> >
>
> It's connected to nohz balancing and I didn't see how I could use that
> for detecting overload. Also, I don't think it ever can be larger than
> the sd weight and overload is based on the number of running tasks being
> greater than the number of available CPUs. Did I miss something obvious?

IIUC you try to estimate if there is a chance to find an idle cpu
before starting the loop and scanning the domain and abort early if
the possibility is low.

if nr_busy_cpus equals to sd->span_weight it means that there is no
free cpu so there is no need to scan

>
> --
> Mel Gorman
> SUSE Labs