Commit 8557c862 authored by Yury Norov [NVIDIA]'s avatar Yury Norov [NVIDIA] Committed by Yury Norov
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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: default avatarJohn Stultz <jstultz@google.com>
Reviewed-by: default avatarThomas Gleixner <tglx@linutronix.de>
Signed-off-by: default avatar"Yury Norov [NVIDIA]" <yury.norov@gmail.com>
parent 012b1043

kernel/time/clocksource.c

+1 −4
Original line number Diff line number Diff line
@@ -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);

	}