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

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Message-ID: <20251114094901.GH3245006@noisy.programming.kicks-ass.net>
Date: Fri, 14 Nov 2025 10:49:01 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Shrikanth Hegde <sshegde@...ux.ibm.com>
Cc: vincent.guittot@...aro.org, 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 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.