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;
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