linux-kernel - Re: [PATCH v3] sched/fair: Use sched_domain_span() for topology_span

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Message-ID: <xhsmhikk1pl8v.mognet@vschneid-thinkpadt14sgen2i.remote.csb>
Date: Wed, 09 Jul 2025 15:05:52 +0200
From: Valentin Schneider <vschneid@...hat.com>
To: K Prateek Nayak <kprateek.nayak@....com>, Ingo Molnar
 <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>, Juri Lelli
 <juri.lelli@...hat.com>, Vincent Guittot <vincent.guittot@...aro.org>,
 Leon Romanovsky <leon@...nel.org>, linux-kernel@...r.kernel.org
Cc: Steve Wahl <steve.wahl@....com>, Borislav Petkov <bp@...en8.de>, Dietmar
 Eggemann <dietmar.eggemann@....com>, Steven Rostedt <rostedt@...dmis.org>,
 Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>
Subject: Re: [PATCH v3] sched/fair: Use sched_domain_span() for
 topology_span_sane()

On 09/07/25 09:31, K Prateek Nayak wrote:
>
> How about the following instead of cpumask_empty() to avoid two cpumask
> operation in the common case where sd is initialized:
>
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index e682bf991ce6..e889ae012c17 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -2423,19 +2423,13 @@ static bool topology_span_sane(const struct cpumask *cpu_map)
>                       struct cpumask *sd_span = sched_domain_span(sd);
>                       int id;
>
> -			/*
> -			 * If a child level for a CPU has already covered
> -			 * the entire cpumap, build_sched_domain() for the
> -			 * domains above is skipped. Use sd->private to detect
> -			 * levels that have not been initialized in the CPU's
> -			 * hierarchy and skip them.
> -			 */
> -			if (!sd->private)
> -				continue;
> -
>                       /* lowest bit set in this mask is used as a unique id */
>                       id = cpumask_first(sd_span);
>
> +			/* Skip if span is empty */
> +			if (id >= nr_cpu_ids)
> +				continue;
> +

Oh yeah, even better. Just slap a comment like the below and ship it!

/*
 * Span can be empty if that topology level won't be used for this CPU,
 * i.e. a lower level already fully describes the topology and
 * build_sched_domain() stopped there.
 */

>                       if (cpumask_test_cpu(id, id_seen)) {
>                               /* First CPU has already been seen, ensure identical sd spans */
>                               sd = *per_cpu_ptr(sdd->sd, id);
> ---
>
> --
> Thanks and Regards,
> Prateek