linux-kernel - [PATCH v4 1/2] sched: Fix the check of nr

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Message-Id: <20220608163518.324276-2-dtcccc@linux.alibaba.com>
Date:   Thu,  9 Jun 2022 00:35:17 +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 v4 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 rq->nr_running <= 1 to avoid task
stacking when WF_ON_CPU.

Per the ordering of writes to p->on_rq and p->on_cpu, observing p->on_cpu
(WF_ON_CPU) in ttwu_queue_cond() implies !p->on_rq, IOW p has gone through
the deactivate_task() in __schedule(), thus p has been accounted out of
rq->nr_running. As such, the task being the only runnable task on the rq
implies reading rq->nr_running == 0 at that point.

The benchmark result is in [1].

[1] https://lore.kernel.org/all/e34de686-4e85-bde1-9f3c-9bbc86b38627@linux.alibaba.com/

Suggested-by: Valentin Schneider <vschneid@...hat.com>
Signed-off-by: Tianchen Ding <dtcccc@...ux.alibaba.com>
Reviewed-by: Valentin Schneider <vschneid@...hat.com>
---
 kernel/sched/core.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 21db6816a7bd..a4bdb2b95976 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3829,8 +3829,12 @@ static inline bool ttwu_queue_cond(int cpu, int wake_flags)
 	 * CPU then use the wakelist to offload the task activation to
 	 * the soon-to-be-idle CPU as the current CPU is likely busy.
 	 * nr_running is checked to avoid unnecessary task stacking.
+	 *
+	 * Note that we can only get here with (wakee) p->on_rq=0,
+	 * p->on_cpu can be whatever, we've done the dequeue, so
+	 * the wakee has been accounted out of ->nr_running.
 	 */
-	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