Re: [PATCH v2 1/8] sched/fair: Update affine statistics when needed

[Valentin Schneider]

On 06/05/21 22:15, Srikar Dronamraju wrote:
> wake_affine_idle() can return prev_cpu. Even in such a scenario,
> scheduler was going ahead and updating schedstats related to wake
> affine. i.e even if the task is not moved across LLC domains,
> schedstats would have accounted.
>
> Hence add a check before updating schedstats.
>

I briefly glanced at the git history but didn't find any proper description
of that stat. As it stands, it counts the number of times wake_affine()
purposedly steered a task towards a particular CPU (waker or wakee's prev),
so nr_wakeups_affine / nr_wakeups_affine_attempts is your wake_affine()
"success rate" - how often could it make a choice with the available data.

I could see a point in only incrementing the count if wake_affine() steers
towards the waker rather than the wakee (i.e. don't increment if choice is
prev), but then that has no link with LLC spans

> Cc: LKML <linux-kernel@xxxxxxxxxxxxxxx>
> Cc: Gautham R Shenoy <ego@xxxxxxxxxxxxxxxxxx>
> Cc: Parth Shah <parth@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Valentin Schneider <valentin.schneider@xxxxxxx>
> Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> Cc: Rik van Riel <riel@xxxxxxxxxxx>
> Signed-off-by: Srikar Dronamraju <srikar@xxxxxxxxxxxxxxxxxx>
> ---
>  kernel/sched/fair.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 794c2cb945f8..a258a84cfdfd 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5884,8 +5884,10 @@ static int wake_affine(struct sched_domain *sd, struct task_struct *p,
>       if (target == nr_cpumask_bits)
>               return prev_cpu;
>
> -	schedstat_inc(sd->ttwu_move_affine);
> -	schedstat_inc(p->se.statistics.nr_wakeups_affine);
> +	if (!cpus_share_cache(prev_cpu, target)) {

Per the above, why? Why not just if(target == this_cpu) ?

> +		schedstat_inc(sd->ttwu_move_affine);
> +		schedstat_inc(p->se.statistics.nr_wakeups_affine);
> +	}
>       return target;
>  }
>
> --
> 2.18.2