linux-kernel - Re: [PATCH 3/6 v8] sched/fair: Prepare select_task_rq

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Message-ID: <CAKfTPtCOfAXOx_qHJEDH4RtjQTq5jvFyaLDKLrA+zXmGscC91A@mail.gmail.com>
Date: Mon, 9 Feb 2026 14:21:28 +0100
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Shrikanth Hegde <sshegde@...ux.ibm.com>
Cc: qyousef@...alina.io, hongyan.xia2@....com, christian.loehle@....com, 
	luis.machado@....com, mingo@...hat.com, peterz@...radead.org, 
	juri.lelli@...hat.com, dietmar.eggemann@....com, rostedt@...dmis.org, 
	bsegall@...gle.com, mgorman@...e.de, vschneid@...hat.com, 
	linux-kernel@...r.kernel.org, pierre.gondois@....com, kprateek.nayak@....com
Subject: Re: [PATCH 3/6 v8] sched/fair: Prepare select_task_rq_fair() to be
 called for new cases

On Sun, 7 Dec 2025 at 14:23, Shrikanth Hegde <sshegde@...ux.ibm.com> wrote:
>
>
>
> On 12/2/25 11:42 PM, Vincent Guittot wrote:
> > Update select_task_rq_fair() to be called out of the 3 current cases which
> > are :
> > - wake up
> > - exec
> > - fork
> >
> > We wants to select a rq in some new cases like pushing a runnable task on a
> > better CPU than the local one. In such case, it's not a wakeup , nor an
> > exec nor a fork. We make sure to not distrub these cases but still
>
> nit: s/distrub/disturb
>
> > go through EAS and fast-path.
> >
> > Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> > ---
> >   kernel/sched/fair.c | 22 ++++++++++++++--------
> >   1 file changed, 14 insertions(+), 8 deletions(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index f430ec890b72..80c4131fb35b 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -8518,6 +8518,7 @@ static int
> >   select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
> >   {
> >       int sync = (wake_flags & WF_SYNC) && !(current->flags & PF_EXITING);
> > +     int want_sibling = !(wake_flags & (WF_EXEC | WF_FORK));
> >       struct sched_domain *tmp, *sd = NULL;
> >       int cpu = smp_processor_id();
> >       int new_cpu = prev_cpu;
> > @@ -8535,16 +8536,21 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
> >               if ((wake_flags & WF_CURRENT_CPU) &&
> >                   cpumask_test_cpu(cpu, p->cpus_ptr))
> >                       return cpu;
> > +     }
> >
> > -             if (!is_rd_overutilized(this_rq()->rd)) {
> > -                     new_cpu = find_energy_efficient_cpu(p, prev_cpu);
> > -                     if (new_cpu >= 0)
> > -                             return new_cpu;
> > -                     new_cpu = prev_cpu;
> > -             }
> > +     /*
> > +      * We don't want EAS to be called for exec or fork but it should be
> > +      * called for any other case such as wake up or push callback.
> > +      */
> > +     if (!is_rd_overutilized(this_rq()->rd) && want_sibling) {
> > +             new_cpu = find_energy_efficient_cpu(p, prev_cpu);
> > +             if (new_cpu >= 0)
> > +                     return new_cpu;
> > +             new_cpu = prev_cpu;
> > +     }
> >
> > +     if (wake_flags & WF_TTWU)
> >               want_affine = !wake_wide(p) && cpumask_test_cpu(cpu, p->cpus_ptr);
> > -     }
> >
> >       rcu_read_lock();
> >       for_each_domain(cpu, tmp) {
> > @@ -8575,7 +8581,7 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
> >       if (unlikely(sd)) {
> >               /* Slow path */
> >               new_cpu = sched_balance_find_dst_cpu(sd, p, cpu, prev_cpu, sd_flag);
> > -     } else if (wake_flags & WF_TTWU) { /* XXX always ? */
> > +     } else if (want_sibling) {
>
> It is going to find a idle core withing LLC first. then idle sibling. right?
> So may need a better name than want_sibling.

it was a shortcut for want select_idle_sibling vs the larger space
search with sched_balance_find_dst_cpu but I will try to find a better
name


>
> >               /* Fast path */
> >               new_cpu = select_idle_sibling(p, prev_cpu, new_cpu);
> >       }
>