linux-kernel - Re: [PATCH v4 1/3] sched/fair: Move checking for nohz cpus after time check

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Message-ID: <CAKfTPtANCdWEptRSY=XK8Z8v1YJZrpJtpYKP7D-4uOJ-jDVf4w@mail.gmail.com>
Date: Tue, 13 Jan 2026 10:07:38 +0100
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Shrikanth Hegde <sshegde@...ux.ibm.com>
Cc: mingo@...nel.org, peterz@...radead.org, linux-kernel@...r.kernel.org, 
	kprateek.nayak@....com, juri.lelli@...hat.com, vschneid@...hat.com, 
	tglx@...nel.org, dietmar.eggemann@....com, anna-maria@...utronix.de, 
	frederic@...nel.org, wangyang.guo@...el.com
Subject: Re: [PATCH v4 1/3] sched/fair: Move checking for nohz cpus after time check

On Mon, 12 Jan 2026 at 06:05, Shrikanth Hegde <sshegde@...ux.ibm.com> wrote:
>
> NOHZ idle load balancer is kicked off only after time check. So move
> the atomic read after the time check to access it only when needed.
>
> When there are no idle CPUs(100% busy), even if the flag gets set to
> NOHZ_STATS_KICK | NOHZ_NEXT_KICK, find_new_ilb will fail and
> there will be no NOHZ idle balance. The current behaviour is retained.
>
> Note: This patch doesn't solve any cacheline overheads. No improvement
> in performance apart from saving a few cycles of atomic_read.

But won't these cycles be then wasted by calling needlessly kick_ilb

>
> Signed-off-by: Shrikanth Hegde <sshegde@...ux.ibm.com>
> ---
>  kernel/sched/fair.c | 18 +++++++++++-------
>  1 file changed, 11 insertions(+), 7 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 9743fc0b225c..17e4e8ac5fca 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -12451,20 +12451,24 @@ 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;
>
> +       /*
> +        * If none are in tickless mode, though flag maybe set,
> +        * idle load balancing is not done as find_new_ilb fails
> +        */
>         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
>