linux-kernel - [PATCH 3/3] clocksource: Improve randomness in clocksource_verify_choose

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Message-ID: <20250608194536.28130-4-yury.norov@gmail.com>
Date: Sun,  8 Jun 2025 15:45:35 -0400
From: Yury Norov <yury.norov@...il.com>
To: linux-kernel@...r.kernel.org,
	Yury Norov <yury.norov@...il.com>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	John Stultz <jstultz@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Stephen Boyd <sboyd@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: [PATCH 3/3] clocksource: Improve randomness in clocksource_verify_choose_cpus()

From: "Yury Norov [NVIDIA]" <yury.norov@...il.com>

The current algorithm of picking a random CPU works OK for dense online
cpumask, but if cpumask is non-dense, the distribution of picked CPUs
is skewed.

For example, on 8-CPU board with CPUs 4-7 offlined, the probability of
selecting CPU 0 is 5/8. Accordingly, cpus 1, 2 and 3 are chosen with
probability 1/8 each. The proper algorithm should pick each online CPU
with probability 1/4.

Switch it to cpumask_random(), which has better statistical
characteristics.

Acked-by: John Stultz <jstultz@...gle.com>
Signed-off-by: Yury Norov [NVIDIA] <yury.norov@...il.com>
---
 kernel/time/clocksource.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index 6a8bc7da9062..4b005b2f3ef5 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -342,10 +342,7 @@ static void clocksource_verify_choose_cpus(void)
 	 * CPUs that are currently online.
 	 */
 	for (i = 1; i < n; i++) {
-		cpu = get_random_u32_below(nr_cpu_ids);
-		cpu = cpumask_next(cpu - 1, cpu_online_mask);
-		if (cpu >= nr_cpu_ids)
-			cpu = cpumask_first(cpu_online_mask);
+		cpu = cpumask_random(cpu_online_mask);
 		if (!WARN_ON_ONCE(cpu >= nr_cpu_ids))
 			cpumask_set_cpu(cpu, &cpus_chosen);
 	}
-- 
2.43.0