linux-kernel - Re: [PATCH 1/3] sched/fair: Make cpu

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Message-ID: <20211221090444.GA2747@ubiquitous>
Date:   Tue, 21 Dec 2021 10:09:08 +0100
From:   Vincent Donnefort <vincent.donnefort@....com>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     peterz@...radead.org, mingo@...hat.com, vincent.guittot@...aro.org,
        linux-kernel@...r.kernel.org, dietmar.eggemann@....com,
        morten.rasmussen@....com, qperret@...gle.com
Subject: Re: [PATCH 1/3] sched/fair: Make cpu_overutilized() EAS dependent

On Mon, Dec 20, 2021 at 05:17:09PM +0000, Valentin Schneider wrote:
> On 20/12/21 12:43, Vincent Donnefort wrote:
> > On a system with Energy Aware Scheduling (EAS), tasks are placed according
> > to their energy consumption estimation and load balancing is disabled to
> > not break that energy biased placement. If the system becomes
> > overutilized, i.e. one of the CPU has too much utilization, energy
> > placement would then be disabled, in favor of Capacity-Aware Scheduling
> > (CAS), including load balancing. This is the sole usage for
> > rd->overutilized. Hence, there is no need to raise it for !EAS systems.
> >
> > Fixes: 2802bf3cd936 ("sched/fair: Add over-utilization/tipping point indicator")
> 
> I'm not sure a Fixes: is warranted, this does not fix any misbehaviour or
> performance regression (even if this might gain us a few extra IPS by not
> writing 1's to rd->overutilized on SMP systems, note that this still gives
> us writes of 0's).

I put the tag to make sure this patch would be taken for stable releases
with:

  [PATCH 2/3] sched/fair: Fix newidle_balance() for overutilized systems

Without the EAS requirement for cpu_overutilized() (patch 1/3), patch 2/3
could lead to useless newidle_balance() for !EAS systems.

Maybe that means in the end 2/3 and 1/3 should be squashed?

> 
> Regardless:
> 
> Reviewed-by: Valentin Schneider <valentin.schneider@....com>
> 
> > Signed-off-by: Vincent Donnefort <vincent.donnefort@....com>
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 095b0aa378df..e2f6fa14e5e7 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -5511,7 +5511,8 @@ static inline void hrtick_update(struct rq *rq)
> >  #ifdef CONFIG_SMP
> >  static inline bool cpu_overutilized(int cpu)
> >  {
> > -	return !fits_capacity(cpu_util_cfs(cpu), capacity_of(cpu));
> > +	return sched_energy_enabled() &&
> > +	       !fits_capacity(cpu_util_cfs(cpu), capacity_of(cpu));
> >  }
> >
> >  static inline void update_overutilized_status(struct rq *rq)
> > --
> > 2.25.1