linux-kernel - [PATCH v3 1/2] sched: Fix the check of nr

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Date:   Thu,  2 Jun 2022 12:06:44 +0800
From:   Tianchen Ding <dtcccc@...ux.alibaba.com>
To:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>
Cc:     linux-kernel@...r.kernel.org
Subject: [PATCH v3 1/2] sched: Fix the check of nr_running at queue wakelist

The commit 2ebb17717550 ("sched/core: Offload wakee task activation if it
the wakee is descheduling") checked nr_running <= 1 to avoid task
stacking when WF_ON_CPU. Consider the order of p->on_rq and p->on_cpu,
if we have WF_ON_CPU here then we must have !p->on_rq, so the deactivate
has happened, thus the task being alone on the rq implies nr_running==0.
Change the check to !cpu_rq(cpu)->nr_running to fix it.

Signed-off-by: Tianchen Ding <dtcccc@...ux.alibaba.com>
---
 kernel/sched/core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index bfa7452ca92e..a9efe134fbe5 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3830,7 +3830,7 @@ static inline bool ttwu_queue_cond(int cpu, int wake_flags)
 	 * the soon-to-be-idle CPU as the current CPU is likely busy.
 	 * nr_running is checked to avoid unnecessary task stacking.
 	 */
-	if ((wake_flags & WF_ON_CPU) && cpu_rq(cpu)->nr_running <= 1)
+	if ((wake_flags & WF_ON_CPU) && !cpu_rq(cpu)->nr_running)
 		return true;
 
 	return false;
-- 
2.27.0