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authorTianchen Ding <dtcccc@linux.alibaba.com>2022-06-09 02:34:11 +0300
committerPeter Zijlstra <peterz@infradead.org>2022-06-13 11:30:01 +0300
commit28156108fecb1f808b21d216e8ea8f0d205a530c (patch)
tree00eeddc6d084d0059f632d9c7b7bebb4ace1839c /kernel/sched
parent792b9f65a568f48c50b3175536db9cde5a1edcc0 (diff)
downloadlinux-28156108fecb1f808b21d216e8ea8f0d205a530c.tar.xz
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@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
Diffstat (limited to 'kernel/sched')
-rw-r--r--kernel/sched/core.c6
1 files changed, 5 insertions, 1 deletions
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index bfa7452ca92e..294b9184dfe1 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;