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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 <akpm@linux-foundation.org>
Acked-by: John Stultz <jstultz@google.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: "Yury Norov [NVIDIA]" <yury.norov@gmail.com>
This commit is contained in:
Yury Norov [NVIDIA] 2025-06-19 14:26:25 -04:00 committed by Yury Norov
parent 012b104342
commit 8557c8628c
1 changed files with 1 additions and 4 deletions
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5
kernel/time/clocksource.c
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@ -342,10 +342,7 @@ static void clocksource_verify_choose_cpus(void)
* CPUs that are currently online. * CPUs that are currently online.
*/ */
for (i = 1; i < n; i++) { for (i = 1; i < n; i++) {
cpu = get_random_u32_below(nr_cpu_ids); cpu = cpumask_random(cpu_online_mask);
cpu = cpumask_next(cpu - 1, cpu_online_mask);
if (cpu >= nr_cpu_ids)
cpu = cpumask_first(cpu_online_mask);
if (!WARN_ON_ONCE(cpu >= nr_cpu_ids)) if (!WARN_ON_ONCE(cpu >= nr_cpu_ids))
cpumask_set_cpu(cpu, &cpus_chosen); cpumask_set_cpu(cpu, &cpus_chosen);
} }