linux-kernel - Re: [PATCH v2 2/3] sched/fair: Change likelyhood of nohz.nr

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Message-ID: <f5c5d44f-8ec3-4d49-ac89-0abf706c19de@amd.com>
Date: Mon, 5 Jan 2026 09:22:58 +0530
From: K Prateek Nayak <kprateek.nayak@....com>
To: Shrikanth Hegde <sshegde@...ux.ibm.com>, <mingo@...nel.org>,
	<peterz@...radead.org>, <vincent.guittot@...aro.org>
CC: <linux-kernel@...r.kernel.org>, <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 2/3] sched/fair: Change likelyhood of nohz.nr_cpus and
 do stats update if its due

Hello Shrikanth,

On 1/2/2026 6:17 PM, Shrikanth Hegde wrote:
> These days most of the system have multi cores. The likelyhood of
> at least one or more CPUs in nohz (idle state) is higher.
> So move likely to unlikely.
> 
> Allow stats balancing to complete when there are no nr_cpus as the check
> happens later. This may do an additional stats based load balancing
> which would reset has_blocked_load. Code also looks saner by removing
> that uncharactiristic return in between.
> 
> Signed-off-by: Shrikanth Hegde <sshegde@...ux.ibm.com>
> ---
>  kernel/sched/fair.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index cd1c78d2c272..5ceb9126d441 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -12456,10 +12456,10 @@ static void nohz_balancer_kick(struct rq *rq)
>  
>  	/*
>  	 * None are in tickless mode and hence no need for NOHZ idle load
> -	 * balancing:
> +	 * balancing, do stats update if its due
>  	 */
> -	if (likely(!atomic_read(&nohz.nr_cpus)))
> -		return;
> +	if (unlikely(!atomic_read(&nohz.nohz_balancer_kick)))
> +		goto out;

Since we are sure that "nohz.nr_cpus" is 0, there is a good chance
find_new_ilb() in kick_ilb() will not find any CPU to run balance on, so
why not just retain that return?

The "flags" can only be set to (NOHZ_NEXT_KICK | NOHZ_STATS_KICK) on
this path and kick_ilb() will simply return early without updating
"nohz.next_balance" when it doesn't see NOHZ_BALANCE_KICK and fails to
find any CPU. Might as well keep the early return.

>  
>  	if (rq->nr_running >= 2) {
>  		flags = NOHZ_STATS_KICK | NOHZ_BALANCE_KICK;

-- 
Thanks and Regards,
Prateek