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

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Date:   Wed, 10 Aug 2022 15:33:13 -0700
From:   Libo Chen <libo.chen@...cle.com>
To:     mingo@...hat.com, peterz@...radead.org, vincent.guittot@...aro.org,
        juri.lelli@...hat.com, dietmar.eggemann@....com, mgorman@...e.de
Cc:     linux-kernel@...r.kernel.org
Subject: [PATCH 1/1] sched/fair: Fix inaccurate tally of ttwu_move_affine

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