sched: Fix the check of nr_running at queue wakelist

The commit 5378097a078d ("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

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index bfa7452ca92e7950fe99923a070e8cef9a540594..294b9184dfe1a2f10c0b2d71ea7784c66116f3e6 100644 (file)
--- 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;