linux-kernel - Re: [PATCH 2/2 RESEND] sched/topology: Optimize topology_span

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Date: Wed, 10 Apr 2024 09:34:40 +0200
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Kyle Meyer <kyle.meyer@....com>
Cc: linux-kernel@...r.kernel.org, yury.norov@...il.com, 
	andriy.shevchenko@...ux.intel.com, linux@...musvillemoes.dk, mingo@...hat.com, 
	peterz@...radead.org, juri.lelli@...hat.com, dietmar.eggemann@....com, 
	rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de, bristot@...hat.com, 
	vschneid@...hat.com, russ.anderson@....com, dimitri.sivanich@....com, 
	steve.wahl@....com
Subject: Re: [PATCH 2/2 RESEND] sched/topology: Optimize topology_span_sane()

On Tue, 9 Apr 2024 at 17:54, Kyle Meyer <kyle.meyer@....com> wrote:
>
> Optimize topology_span_sane() by removing duplicate comparisons.
>
> The total number of comparisons is reduced from N * (N - 1) to
> N * (N - 1) / 2 (per non-NUMA scheduling domain level).
>
> Signed-off-by: Kyle Meyer <kyle.meyer@....com>
> Reviewed-by: Yury Norov <yury.norov@...il.com>

Acked-by: Vincent Guittot <vincent.guittot@...aro.org>

> ---
>  kernel/sched/topology.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index 99ea5986038c..b6bcafc09969 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -2347,7 +2347,7 @@ static struct sched_domain *build_sched_domain(struct sched_domain_topology_leve
>  static bool topology_span_sane(struct sched_domain_topology_level *tl,
>                               const struct cpumask *cpu_map, int cpu)
>  {
> -       int i;
> +       int i = cpu + 1;
>
>         /* NUMA levels are allowed to overlap */
>         if (tl->flags & SDTL_OVERLAP)
> @@ -2359,9 +2359,7 @@ static bool topology_span_sane(struct sched_domain_topology_level *tl,
>          * breaking the sched_group lists - i.e. a later get_group() pass
>          * breaks the linking done for an earlier span.
>          */
> -       for_each_cpu(i, cpu_map) {
> -               if (i == cpu)
> -                       continue;
> +       for_each_cpu_from(i, cpu_map) {
>                 /*
>                  * We should 'and' all those masks with 'cpu_map' to exactly
>                  * match the topology we're about to build, but that can only
> --
> 2.44.0
>