linux-kernel - Re: [PATCH 1/1] sched/fair: Fix inaccurate tally of ttwu_move

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Message-ID: <Ywck+3E0DyNdUJRE@BLR-5CG11610CF.amd.com>
Date:   Thu, 25 Aug 2022 13:00:03 +0530
From:   "Gautham R. Shenoy" <gautham.shenoy@....com>
To:     Libo Chen <libo.chen@...cle.com>
Cc:     mingo@...hat.com, peterz@...radead.org, vincent.guittot@...aro.org,
        juri.lelli@...hat.com, dietmar.eggemann@....com, mgorman@...e.de,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] sched/fair: Fix inaccurate tally of ttwu_move_affine

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@...cle.com>
> Signed-off-by: Libo Chen <libo.chen@...cle.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.