linux-kernel - Re: [PATCH v2 1/3] sched/fair: Move checking for nohz cpus after time check

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Message-ID: <20260106155010.GC3707837@noisy.programming.kicks-ass.net>
Date: Tue, 6 Jan 2026 16:50:10 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Shrikanth Hegde <sshegde@...ux.ibm.com>
Cc: mingo@...nel.org, vincent.guittot@...aro.org,
	linux-kernel@...r.kernel.org, kprateek.nayak@....com,
	juri.lelli@...hat.com, vschneid@...hat.com, tglx@...utronix.de,
	dietmar.eggemann@....com, anna-maria@...utronix.de,
	frederic@...nel.org, wangyang.guo@...el.com
Subject: Re: [PATCH v2 1/3] sched/fair: Move checking for nohz cpus after
 time check

On Fri, Jan 02, 2026 at 06:17:42PM +0530, Shrikanth Hegde wrote:
> Idle load balancer is kicked off only after time check. So move
> the atomic read after the time check to avoid the overhead.

Presumably this is because the nr_cpus field is more contended than the
next_balance field; but aren't they in the same cacheline? That is,
*what* !??!

> If there are no nohz CPUs and next_blocked has passed, then there
> will be one additional stats based load balancing which would set the
> has_blocked_load to 0. It shouldn't make a difference.
> 
> Signed-off-by: Shrikanth Hegde <sshegde@...ux.ibm.com> 
> ---
>  kernel/sched/fair.c | 14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 7377f9117501..cd1c78d2c272 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -12447,13 +12447,6 @@ static void nohz_balancer_kick(struct rq *rq)
>  	 */
>  	nohz_balance_exit_idle(rq);
>  
> -	/*
> -	 * None are in tickless mode and hence no need for NOHZ idle load
> -	 * balancing:
> -	 */
> -	if (likely(!atomic_read(&nohz.nr_cpus)))
> -		return;
> -
>  	if (READ_ONCE(nohz.has_blocked_load) &&
>  	    time_after(now, READ_ONCE(nohz.next_blocked)))
>  		flags = NOHZ_STATS_KICK;
> @@ -12461,6 +12454,13 @@ static void nohz_balancer_kick(struct rq *rq)
>  	if (time_before(now, nohz.next_balance))
>  		goto out;
>  
> +	/*
> +	 * None are in tickless mode and hence no need for NOHZ idle load
> +	 * balancing:
> +	 */
> +	if (likely(!atomic_read(&nohz.nr_cpus)))
> +		return;
> +
>  	if (rq->nr_running >= 2) {
>  		flags = NOHZ_STATS_KICK | NOHZ_BALANCE_KICK;
>  		goto out;
> -- 
> 2.47.3
>