linux-kernel - Re: [PATCH] sched/fair: Force idle aware load balancing

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Message-ID: <CAKfTPtBkdp2GcK9yLR3_rJrW7AH_hFpjfMjMFQKCngLbXHBPRw@mail.gmail.com>
Date: Fri, 28 Nov 2025 14:50:10 +0100
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Fernand Sieber <sieberf@...zon.com>, mingo@...hat.com, linux-kernel@...r.kernel.org, 
	juri.lelli@...hat.com, dietmar.eggemann@....com, rostedt@...dmis.org, 
	bsegall@...gle.com, mgorman@...e.de, vschneid@...hat.com, 
	kprateek.nayak@....com, dwmw@...zon.co.uk, jschoenh@...zon.de, 
	liuyuxua@...zon.com, abusse@...zon.com, gmazz@...zon.com, rkagan@...zon.com
Subject: Re: [PATCH] sched/fair: Force idle aware load balancing

On Fri, 28 Nov 2025 at 12:14, Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Thu, Nov 27, 2025 at 10:27:17PM +0200, Fernand Sieber wrote:
>
> > @@ -11123,7 +11136,8 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
> >               return;
> >       }
> >
> > -     if (busiest->group_type == group_smt_balance) {
> > +     if (busiest->group_type == group_smt_balance ||
> > +         busiest->forceidle_weight) {
>
> Should we not instead make it so that we select group_smt_balance in
> this case?

Why do we need this test ? We have already removed forced idle cpus
from statistics ?

I suppose Fernand wants to cover cases where there is 1 task per CPUs
so we are balanced but one CPU is forced idle and we want to force
migrating a task to then try to move back another one ? In this case
it should be detected early and become group_imbalanced type
Also what happens if we could migrate more than one task


>
> Anyway, the patch doesn't seem horrible to me. Vincent?
>
> >               /* Reduce number of tasks sharing CPU capacity */
> >               env->migration_type = migrate_task;
> >               env->imbalance = 1;