From: Yury Norov [NVIDIA] Date: Thu, 19 Jun 2025 18:26:25 +0000 (-0400) Subject: clocksource: Improve randomness in clocksource_verify_choose_cpus() X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=8557c8628cf3cf8ebd3b32601ccdde550bbf6c54;p=thirdparty%2Fkernel%2Flinux.git clocksource: Improve randomness in clocksource_verify_choose_cpus() 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. CC: Andrew Morton Acked-by: John Stultz Reviewed-by: Thomas Gleixner Signed-off-by: "Yury Norov [NVIDIA]" --- diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c index 6a8bc7da90626..4b005b2f3ef55 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); }