From a6a901205b15a61d87dab54bfe01997f2d9f993a Mon Sep 17 00:00:00 2001
From: Tianchen Ding <dtcccc@linux.alibaba.com>
Date: Thu, 9 Jun 2022 07:34:11 +0800
Subject: [PATCH] sched: Fix the check of nr_running at queue wakelist

References: bnc#1189999 (Scheduler functional and performance backports)
Patch-mainline: v6.0-rc1
Git-commit: 28156108fecb1f808b21d216e8ea8f0d205a530c

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@redhat.com>
Signed-off-by: Tianchen Ding <dtcccc@linux.alibaba.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Valentin Schneider <vschneid@redhat.com>
Link: https://lore.kernel.org/r/20220608233412.327341-2-dtcccc@linux.alibaba.com
Signed-off-by: Mel Gorman <mgorman@suse.de>
---
 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 f60c30baed77..f504b951b591 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3592,8 +3592,12 @@ ttwu_queue_cond(struct task_struct *p, 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;