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

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Message-ID: <20251128135825.GC4067720@noisy.programming.kicks-ass.net>
Date: Fri, 28 Nov 2025 14:58:25 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Vincent Guittot <vincent.guittot@...aro.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, Nov 28, 2025 at 02:55:36PM +0100, Vincent Guittot wrote:
> On Thu, 27 Nov 2025 at 21:28, Fernand Sieber <sieberf@...zon.com> wrote:

> > @@ -10135,15 +10136,15 @@ static inline int sg_imbalanced(struct sched_group *group)
> >  static inline bool
> >  group_has_capacity(unsigned int imbalance_pct, struct sg_lb_stats *sgs)
> >  {
> > -       if (sgs->sum_nr_running < sgs->group_weight)
> > +       if (sgs->sum_nr_running < (sgs->group_weight - sgs->forceidle_weight))
> >                 return true;
> >
> > -       if ((sgs->group_capacity * imbalance_pct) <
> > -                       (sgs->group_runnable * 100))
> > +       if ((sgs->group_capacity * imbalance_pct * (sgs->group_weight - sgs->forceidle_weight)) <
> > +                       (sgs->group_runnable * 100 * sgs->group_weight))
> 
> so you apply a ratio on group capacity based on the number of forced
> idle but what if you have heterogeneous systems ?

Ah, good point. I suppose tracking force_idle_capacity in
update_sg_lb_stats() should be possible, and then subtract that from
group_capacity or so.