Re: [PATCH 1/1] sched/fair: Fix inaccurate tally of ttwu_move_affine

Gautham R. Shenoy
2022-08-25T07:30:43Z

On Wed, Aug 10, 2022 at 03:33:13PM -0700, Libo Chen wrote:
> There are scenarios where non-affine wakeups are incorrectly counted as
> affine wakeups by schedstats.
>
> When wake_affine_idle() returns prev_cpu which doesn't equal to
> nr_cpumask_bits, it will slip through the check: target == nr_cpumask_bits
> in wake_affine() and be counted as if target == this_cpu in schedstats.
>
> Replace target == nr_cpumask_bits with target != this_cpu to make sure
> affine wakeups are accurately tallied.
>
> Fixes: 806486c377e33 (sched/fair: Do not migrate if the prev_cpu is idle)
> Suggested-by: Daniel Jordan <daniel.m.jordan@oracle.com>
> Signed-off-by: Libo Chen <libo.chen@oracle.com>
> ---
> kernel/sched/fair.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index da388657d5ac..b179da4f8105 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6114,7 +6114,7 @@ static int wake_affine(struct sched_domain *sd, struct task_struct *p,
> target = wake_affine_weight(sd, p, this_cpu, prev_cpu, sync);
>
> schedstat_inc(p->stats.nr_wakeups_affine_attempts);
> - if (target == nr_cpumask_bits)
> + if (target != this_cpu)
> return prev_cpu;

This seems to be the right thing to do. However,..

if this_cpu and prev_cpu were in the same LLC and we pick prev_cpu,
technically is it still not an affine wakeup?

>
> schedstat_inc(sd->ttwu_move_affine);
> --
> 2.31.1
>

--
Thanks and Regards
gautham.