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

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Message-ID: <20240328213331.2vmuub3qud7nfx6t@airbuntu>
Date: Thu, 28 Mar 2024 21:33:31 +0000
From: Qais Yousef <qyousef@...alina.io>
To: Vincent Guittot <vincent.guittot@...aro.org>
Cc: Ingo Molnar <mingo@...nel.org>, Shrikanth Hegde <sshegde@...ux.ibm.com>,
	peterz@...radead.org, dietmar.eggemann@....com,
	linux-kernel@...r.kernel.org, vschneid@...hat.com
Subject: Re: [PATCH] sched/fair: Combine EAS check with overutilized access

On 03/26/24 21:15, Vincent Guittot wrote:
> 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

I do think it is better this way too.

> 
> >
> > 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.

But not sure about moving is_rd_overutilized() call out of
find_energy_efficient_cpu() is a good call here. Are we trying to save the
function call? Looking at disassembly the function was completely inlined into
select_task_rq_fair() on my recent compilation on Apple M1 system (6.8 kernel).

I see it is applied now. I agree it is a matter of PoV. So no big deal.