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Date:   Mon, 15 Aug 2022 13:01:41 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Libo Chen <libo.chen@oracle.com>
Cc:     mingo@redhat.com, vincent.guittot@linaro.org,
        juri.lelli@redhat.com, dietmar.eggemann@arm.com, mgorman@suse.de,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/1] sched/fair: Fix inaccurate tally of ttwu_move_affine
Message-ID: <YvonlUOgMbla6dSh@worktop.programming.kicks-ass.net>
References: <20220810223313.386614-1-libo.chen@oracle.com>
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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;
>  
>  	schedstat_inc(sd->ttwu_move_affine);

This not only changes the accounting but also the placement, no?