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

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Message-ID: <20251112144241.GP278048@noisy.programming.kicks-ass.net>
Date: Wed, 12 Nov 2025 15:42:41 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Shrikanth Hegde <sshegde@...ux.ibm.com>
Cc: mingo@...hat.com, juri.lelli@...hat.com, vincent.guittot@...aro.org,
	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 08:07:24PM +0530, Shrikanth Hegde wrote:
> 
> 
> On 11/7/25 9:36 PM, Peter Zijlstra wrote:
> > Pull out the !sd check to simplify code.
> > 
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > ---
> >   kernel/sched/fair.c |   10 ++++++----
> >   1 file changed, 6 insertions(+), 4 deletions(-)
> > 
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -12811,14 +12811,16 @@ static int sched_balance_newidle(struct
> >   	rcu_read_lock();
> >   	sd = rcu_dereference_check_sched_domain(this_rq->sd);
> > +	if (!sd) {
> > +		rcu_read_unlock();
> > +		goto out;
> > +	}
> 
> when is sd null? on isol_cpus ?

Yeah, partitions (cpuset/isol_cpus) but also spuriously during
domain-rebuild IIRC.

> if sd is null, i think we are skipping these compared to earlier.
> 
>         t0 = sched_clock_cpu(this_cpu);
>         sched_balance_update_blocked_averages(this_cpu);

let me pull that sched_balance_update_blocked_averages() thing up a few
lines.

Thanks!