linux-kernel - Re: [PATCH 2/4] sched/fair: Small cleanup to sched_balance

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Message-ID: <CAKfTPtAk=GAKBDdJve-V-FuMgGPD5hNA0rTL5v4SV0G5ha3a+g@mail.gmail.com>
Date: Fri, 14 Nov 2025 11:22:48 +0100
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Shrikanth Hegde <sshegde@...ux.ibm.com>, 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, 
	Chris Mason <clm@...a.com>, Joseph Salisbury <joseph.salisbury@...cle.com>, 
	Adam Li <adamli@...amperecomputing.com>, 
	Hazem Mohamed Abuelfotoh <abuehaze@...zon.com>, Josh Don <joshdon@...gle.com>
Subject: Re: [PATCH 2/4] sched/fair: Small cleanup to sched_balance_newidle()

On Fri, 14 Nov 2025 at 10:49, Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Wed, Nov 12, 2025 at 08:58:23PM +0530, Shrikanth Hegde wrote:
>
> > > @@ -12865,6 +12869,8 @@ static int sched_balance_newidle(struct
> > >     if (!cpu_active(this_cpu))
> > >             return 0;
> > > +   __sched_balance_update_blocked_averages(this_rq);
> > > +
> >
> > is this done only when sd == null ?
>
> Its done always.
>
> > >     /*
> > >      * This is OK, because current is on_cpu, which avoids it being picked
> > >      * for load-balance and preemption/IRQs are still disabled avoiding
> > > @@ -12891,7 +12897,6 @@ static int sched_balance_newidle(struct
> > >     raw_spin_rq_unlock(this_rq);
> > >     t0 = sched_clock_cpu(this_cpu);
> > > -   sched_balance_update_blocked_averages(this_cpu);
> > >     rcu_read_lock();
> > >     for_each_domain(this_cpu, sd) {
> >
> > Referring to commit,
> > 9d783c8dd112a (sched/fair: Skip update_blocked_averages if we are defering load balance)
> > I think vincent added the max_newidle_lb_cost check since sched_balance_update_blocked_averages is costly.
>
> That seems to suggest we only should do
> sched_balance_update_blocked_averages() when we're going to do
> balancing and so skipping when !sd is fine.

sched_balance_update_blocked_averages() can be costly so we want to
include it in the sd->max_idle_balance_cost so we skip newidle lb and
update blocked load if avg_idle is shorter than doin the update and
the newidle lb of the 1st level

when sd is null we should still skip
sched_balance_update_blocked_averages() is its cost is higher than
avg_idle but it seems that we don't.

>
>