linux-kernel - [PATCH v2 2/2] sched/topology: Optimize topology_span

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Date: Wed, 10 Apr 2024 16:33:11 -0500
From: Kyle Meyer <kyle.meyer@....com>
To: 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,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, vschneid@...hat.com
Cc: russ.anderson@....com, dimitri.sivanich@....com, steve.wahl@....com,
        Kyle Meyer <kyle.meyer@....com>
Subject: [PATCH v2 2/2] sched/topology: Optimize topology_span_sane()

Optimize topology_span_sane() by removing duplicate comparisons.

Since topology_span_sane() is called inside of for_each_cpu(), each
pervious CPU has already been compared against every other CPU. The
current CPU only needs to be compared against higher-numbered CPUs.

The total number of comparisons is reduced from N * (N - 1) to
N * (N - 1) / 2 on each 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