linux-kernel - [tip: sched/core] sched/fair: Fix inaccurate tally of ttwu_move

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Date:   Thu, 06 Apr 2023 10:05:19 -0000
From:   "tip-bot2 for Libo Chen" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Daniel Jordan <daniel.m.jordan@...cle.com>,
        Libo Chen <libo.chen@...cle.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        "Gautham R. Shenoy" <gautham.shenoy@....com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: sched/core] sched/fair: Fix inaccurate tally of ttwu_move_affine

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     39afe5d6fc59237ff7738bf3ede5a8856822d59d
Gitweb:        https://git.kernel.org/tip/39afe5d6fc59237ff7738bf3ede5a8856822d59d
Author:        Libo Chen <libo.chen@...cle.com>
AuthorDate:    Wed, 10 Aug 2022 15:33:13 -07:00
Committer:     Peter Zijlstra <peterz@...radead.org>
CommitterDate: Wed, 05 Apr 2023 09:58:48 +02:00

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>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Reviewed-by: Gautham R. Shenoy <gautham.shenoy@....com>
Link: https://lore.kernel.org/r/20220810223313.386614-1-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 bc358dc..f5da01a 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6582,7 +6582,7 @@ 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);