linux-kernel - Re: [PATCH] sched/fair: Combine EAS check with overutilized access

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Date: Tue, 26 Mar 2024 21:15:07 +0100
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Ingo Molnar <mingo@...nel.org>
Cc: Shrikanth Hegde <sshegde@...ux.ibm.com>, peterz@...radead.org, dietmar.eggemann@....com, 
	qyousef@...alina.io, linux-kernel@...r.kernel.org, vschneid@...hat.com
Subject: Re: [PATCH] sched/fair: Combine EAS check with overutilized access

On Tue, 26 Mar 2024 at 20:32, Ingo Molnar <mingo@...nel.org> wrote:
>
>
> * Shrikanth Hegde <sshegde@...ux.ibm.com> wrote:
>
> > Access to overutilized is always used with sched_energy_enabled in
> > the pattern:
> >
> > if (sched_energy_enabled && !overutilized)
> >        do something
> >
> > So modify the helper function to return this pattern. This is more
> > readable code as it would say, do something when root domain is not
> > overutilized. This function always return true when EAS is disabled.
> >
> > No change in functionality intended.
> >
> > Suggested-by: Vincent Guittot <vincent.guittot@...aro.org>
> > Signed-off-by: Shrikanth Hegde <sshegde@...ux.ibm.com>
> > ---
> >  kernel/sched/fair.c | 20 +++++++-------------
> >  1 file changed, 7 insertions(+), 13 deletions(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 24a7530a7d3f..e222e3ad4cfe 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -6686,12 +6686,11 @@ static inline bool cpu_overutilized(int cpu)
> >  }
> >
> >  /*
> > - * Ensure that caller can do EAS. overutilized value
> > - * make sense only if EAS is enabled
> > + * overutilized value make sense only if EAS is enabled
> >   */
> >  static inline int is_rd_overutilized(struct root_domain *rd)
> >  {
> > -     return READ_ONCE(rd->overutilized);
> > +     return !sched_energy_enabled() || READ_ONCE(rd->overutilized);
> >  }
> >
> >  static inline void set_rd_overutilized_status(struct root_domain *rd,
> > @@ -6710,8 +6709,6 @@ static inline void check_update_overutilized_status(struct rq *rq)
> >        * overutilized field is used for load balancing decisions only
> >        * if energy aware scheduler is being used
> >        */
> > -     if (!sched_energy_enabled())
> > -             return;
> >
> >       if (!is_rd_overutilized(rq->rd) && cpu_overutilized(rq->cpu))
> >               set_rd_overutilized_status(rq->rd, SG_OVERUTILIZED);
>
> On a second thought, I'm not sure removing the open-coded
> sched_energy_enabled() branches is a good idea: the current code makes it
> really, really clear when we are within EAS code paths.
>
> Hiding it within is_rd_overutilized() makes it a lot less obvious IMO.

That's probably a matter of pov. I prefer embedding everything to make
sure to have useless access to rd->overutilized

>
> And this one:
>
> > @@ -8202,7 +8199,7 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
> >                   cpumask_test_cpu(cpu, p->cpus_ptr))
> >                       return cpu;
> >
> > -             if (sched_energy_enabled()) {
> > +             if (!is_rd_overutilized(this_rq()->rd)) {
> >                       new_cpu = find_energy_efficient_cpu(p, prev_cpu);
> >                       if (new_cpu >= 0)
> >                               return new_cpu;
>
> Didn't have a root_domain::overutilized check before?

This is the one that was in find_energy_efficient_cpu() before.

>
> Thanks,
>
>         Ingo