linux-kernel - Re: [PATCH 7/9] sched/fair: Do not try to migrate delayed dequeue task

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Message-ID: <CAKfTPtB5LaZf6joAEDePxudi8kU_4vxZ1Was9VZjF+t+btzaXg@mail.gmail.com>
Date: Thu, 28 Nov 2024 11:03:44 +0100
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: mingo@...hat.com, juri.lelli@...hat.com, dietmar.eggemann@....com, 
	rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de, vschneid@...hat.com, 
	linux-kernel@...r.kernel.org, kprateek.nayak@....com, pauld@...hat.com, 
	efault@....de, luis.machado@....com
Subject: Re: [PATCH 7/9] sched/fair: Do not try to migrate delayed dequeue task

On Thu, 28 Nov 2024 at 10:49, Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Thu, Nov 28, 2024 at 10:27:48AM +0100, Vincent Guittot wrote:
> > Migrating a delayed dequeued task doesn't help in balancing the number
> > of runnable tasks in the system.
>
> But it can help balance the weight; furthermore, by moving them to a
> lighter queue, they'll get picked sooner and disappear sooner.

When groups are not overloaded, we don't compare load but only running
tasks t balance them across cpus

It's only when both src and dst groups are overloaded that we look at
the load and the weight

>
> Perhaps make it: p->se.sched_delayed && !env->sd->nr_balance_failed ?

So we could take into account which type of migration with
env->migration_type == migrate_load

In this case, migrating a delayed dequeue task would help

>
> > Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> > ---
> >  kernel/sched/fair.c | 12 ++++++++----
> >  1 file changed, 8 insertions(+), 4 deletions(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 1b4f1b610543..9d80f3a61082 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -9405,11 +9405,15 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
> >
> >       /*
> >        * We do not migrate tasks that are:
> > -      * 1) throttled_lb_pair, or
> > -      * 2) cannot be migrated to this CPU due to cpus_ptr, or
> > -      * 3) running (obviously), or
> > -      * 4) are cache-hot on their current CPU.
> > +      * 1) delayed dequeued, or
> > +      * 2) throttled_lb_pair, or
> > +      * 3) cannot be migrated to this CPU due to cpus_ptr, or
> > +      * 4) running (obviously), or
> > +      * 5) are cache-hot on their current CPU.
> >        */
> > +     if (p->se.sched_delayed)
> > +             return 0;
> > +
> >       if (throttled_lb_pair(task_group(p), env->src_cpu, env->dst_cpu))
> >               return 0;
> >
> > --
> > 2.43.0
> >