linux-kernel - Re: [PATCH] sched/psi: Skip CPUs with zero non-idle jiffies in per-cpu aggregation

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Message-ID: <aYImYitSfznXIcsC@cmpxchg.org>
Date: Tue, 3 Feb 2026 11:46:26 -0500
From: Johannes Weiner <hannes@...xchg.org>
To: Zhan Xusheng <zhanxusheng1024@...il.com>
Cc: linux-kernel@...r.kernel.org, linux-sched@...r.kernel.org,
	Zhan Xusheng <zhanxusheng@...omi.com>
Subject: Re: [PATCH] sched/psi: Skip CPUs with zero non-idle jiffies in
 per-cpu aggregation

On Tue, Feb 03, 2026 at 06:00:07PM +0800, Zhan Xusheng wrote:
> PSI per-cpu aggregation weights each CPU's contribution by its
> non-idle time converted to jiffies. CPUs with zero non-idle jiffies
> do not contribute to the weighted result, but are still processed in
> the current implementation.
> 
> Skip CPUs with zero non-idle jiffies early to avoid unnecessary
> per-cpu arithmetic during aggregation.
> 
> No functional change intended.
> 
> Signed-off-by: Zhan Xusheng <zhanxusheng@...omi.com>

Makes sense.

> ---
>  kernel/sched/psi.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
> index 59fdb7ebbf22..ce2321793a67 100644
> --- a/kernel/sched/psi.c
> +++ b/kernel/sched/psi.c
> @@ -387,6 +387,13 @@ static void collect_percpu_times(struct psi_group *group,
>  		changed_states |= cpu_changed_states;
>  
>  		nonidle = nsecs_to_jiffies(times[PSI_NONIDLE]);
> +		/*
> +		 * A CPU with zero non-idle jiffies does not contribute to the
> +		 * weighted per-CPU aggregation. There is no need to include it
> +		 * in deltas or total accumulation.
> +		 */
> +		if (!nonidle)
> +			continue;

You could save the nsecs_to_jiffies() dance as well by doing:

		if (!(cpu_changed_states & (1 << PSI_NONIDLE)))
			coninue;