linux-kernel - [PATCH v2 4/6] lib/group_cpus: optimize outer loop in grp_spread_init

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Date:   Thu,  7 Dec 2023 12:38:58 -0800
From:   Yury Norov <yury.norov@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org
Cc:     Yury Norov <yury.norov@...il.com>, Ming Lei <ming.lei@...hat.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>
Subject: [PATCH v2 4/6] lib/group_cpus: optimize outer loop in grp_spread_init_one()

Similarly to the inner loop, in the outer loop we can use for_each_cpu()
macro, and skip CPUs that have been copied.

With this patch, the function becomes O(1), despite that it's a
double-loop.

Signed-off-by: Yury Norov <yury.norov@...il.com>
---
 lib/group_cpus.c | 12 ++++--------
 1 file changed, 4 insertions(+), 8 deletions(-)

diff --git a/lib/group_cpus.c b/lib/group_cpus.c
index 7ac94664230f..cded3c8ea63b 100644
--- a/lib/group_cpus.c
+++ b/lib/group_cpus.c
@@ -17,16 +17,11 @@ static void grp_spread_init_one(struct cpumask *irqmsk, struct cpumask *nmsk,
 	const struct cpumask *siblmsk;
 	int cpu, sibl;
 
-	for ( ; cpus_per_grp > 0; ) {
-		cpu = cpumask_first(nmsk);
-
-		/* Should not happen, but I'm too lazy to think about it */
-		if (cpu >= nr_cpu_ids)
-			return;
-
+	for_each_cpu(cpu, nmsk) {
 		__cpumask_clear_cpu(cpu, nmsk);
 		__cpumask_set_cpu(cpu, irqmsk);
-		cpus_per_grp--;
+		if (cpus_per_grp-- == 0)
+			return;
 
 		/* If the cpu has siblings, use them first */
 		siblmsk = topology_sibling_cpumask(cpu);
@@ -37,6 +32,7 @@ static void grp_spread_init_one(struct cpumask *irqmsk, struct cpumask *nmsk,
 			__cpumask_set_cpu(sibl, irqmsk);
 			if (cpus_per_grp-- == 0)
 				return;
+			cpu = sibl + 1;
 		}
 	}
 }
-- 
2.40.1