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

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Message-ID: <20240807190522.432388-3-yury.norov@gmail.com>
Date: Wed,  7 Aug 2024 12:05:21 -0700
From: Yury Norov <yury.norov@...il.com>
To: linux-kernel@...r.kernel.org
Cc: Yury Norov <yury.norov@...il.com>,
	Chen Yu <yu.c.chen@...el.com>,
	Christophe JAILLET <christophe.jaillet@...adoo.fr>,
	Leonardo Bras <leobras@...hat.com>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>,
	Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>
Subject: [PATCH 2/2] sched/topology: optimize topology_span_sane()

The function may call cpumask_equal with tl->mask(cpu) == tl->mask(i),
even though cpu != i. In such case, cpumask_equal() would always return
true, and we can proceed to the next iteration immediately.

Comment is provided by Valentin Schneider.

Reviewed-by: Valentin Schneider <vschneid@...hat.com>
Signed-off-by: Yury Norov <yury.norov@...il.com>
---
 kernel/sched/topology.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 8af3b48da458..3661d4173d1f 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -2370,6 +2370,18 @@ static bool topology_span_sane(struct sched_domain_topology_level *tl,
 	 */
 	for_each_cpu_from(i, cpu_map) {
 		mi = tl->mask(i);
+		/*
+		 * Some topology levels (e.g. PKG in default_topology[])
+		 * have a sched_domain_mask_f implementation that reuses
+		 * the same mask for several CPUs (in PKG's case, one mask
+		 * for all CPUs in the same NUMA node).
+		 *
+		 * For such topology levels, repeating cpumask_equal()
+		 * checks is wasteful. Instead, we first check that the
+		 * tl->mask(i) pointers aren't the same.
+		 */
+		if (mi == mc)
+			continue;
 
 		/*
 		 * We should 'and' all those masks with 'cpu_map' to exactly
-- 
2.43.0