On Wed, Jun 4, 2025 at 2:21 PM Yury Norov <yury.norov@gmail.com> wrote:
>
> From: Yury Norov [NVIDIA] <yury.norov@gmail.com>
>
> The current algorithm of picking a random CPU works OK for dence online
spelling nit: dence -> dense
> cpumask, but if cpumask is non-dence, the distribution of picked CPUs
same: non-dence -> non-dense
> 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 CPU with
> probability 1/4.
>
> Switch it to cpumask_random(), which has better statistical
> characteristics.
> Signed-off-by: Yury Norov [NVIDIA] <yury.norov@gmail.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 e400fe150f9d..0aef0e349e49 100644
> --- a/kernel/time/clocksource.c
> +++ b/kernel/time/clocksource.c
> @@ -340,10 +340,7 @@ 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);
> }
This looks ok to me. Again, just the smallest nit about the subject
line capitalization.
Acked-by: John Stultz <jstultz@google.com>
thanks
-john