linux-kernel - Re: [PATCH 3/3] sched/fair: Do not raise overutilized for idle CPUs

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Date:   Mon, 10 Jan 2022 16:40:27 +0000
From:   Vincent Donnefort <vincent.donnefort@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     peterz@...radead.org, mingo@...hat.com,
        linux-kernel@...r.kernel.org, dietmar.eggemann@....com,
        Valentin.Schneider@....com, Morten.Rasmussen@....com,
        qperret@...gle.com
Subject: Re: [PATCH 3/3] sched/fair: Do not raise overutilized for idle CPUs

On Wed, Dec 22, 2021 at 09:20:17AM +0100, Vincent Guittot wrote:
> On Mon, 20 Dec 2021 at 12:43, Vincent Donnefort
> <vincent.donnefort@....com> wrote:
> >
> > During a migration, the lock for the previous runqueue is not taken and
> > hence, the task contribution isn't directly removed from that runqueue
> > utilization but instead temporarily saved, until the next PELT signals
> > update where it would be accounted. There is then a window in which a
> > CPU can ben idle be nonetheless overutilized.
> >
> > The load balancer wouldn't be able to do anything to help a sleeping CPU,
> > it brings then no gain to raise overutilized there, only the risk of
> > spuriously doing it.
> 
> But how do you make the difference between a very short idle time of
> an overutilized CPU and a idle cpu with outdated utilization

No distinction here, but if the CPU is idle there's nothing to pull, so the load
balance wouldn't do anything with this information.

> 
> Being idle is not a good reason for not being overutilized (ie ~80% of
> average utilisation)
> 
> >
> > Signed-off-by: Vincent Donnefort <vincent.donnefort@....com>
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 51f6f55abb37..37f737c5f0b8 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -8641,26 +8641,28 @@ static inline void update_sg_lb_stats(struct lb_env *env,
> >
> >                 nr_running = rq->nr_running;
> >                 sgs->sum_nr_running += nr_running;
> > -
> > -               if (nr_running > 1)
> > -                       *sg_status |= SG_OVERLOAD;
> > -
> > -               if (cpu_overutilized(i))
> > -                       *sg_status |= SG_OVERUTILIZED;
> > -
> >  #ifdef CONFIG_NUMA_BALANCING
> >                 sgs->nr_numa_running += rq->nr_numa_running;
> >                 sgs->nr_preferred_running += rq->nr_preferred_running;
> >  #endif
> > +               if (nr_running > 1)
> > +                       *sg_status |= SG_OVERLOAD;
> 
> Why do you move this code related to overload ?

This was a cosmetic change to put the NUMA related stats next to the other ones.

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