[tip: sched/core] sched: Fix the check of nr_running at queue wakelist

[tip-bot2 for Tianchen Ding]

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

Commit-ID:     28156108fecb1f808b21d216e8ea8f0d205a530c
Gitweb:        https://git.kernel.org/tip/28156108fecb1f808b21d216e8ea8f0d205a530c
Author:        Tianchen Ding <dtcccc@xxxxxxxxxxxxxxxxx>
AuthorDate:    Thu, 09 Jun 2022 07:34:11 +08:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Mon, 13 Jun 2022 10:30:01 +02:00

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@xxxxxxxxxxxxxxxxx/

Suggested-by: Valentin Schneider <vschneid@xxxxxxxxxx>
Signed-off-by: Tianchen Ding <dtcccc@xxxxxxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Valentin Schneider <vschneid@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20220608233412.327341-2-dtcccc@xxxxxxxxxxxxxxxxx
---
 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 bfa7452..294b918 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;