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| author | Tianchen Ding <dtcccc@linux.alibaba.com> | 2022-06-09 02:34:11 +0300 |
|---|---|---|
| committer | Peter Zijlstra <peterz@infradead.org> | 2022-06-13 11:30:01 +0300 |
| commit | 28156108fecb1f808b21d216e8ea8f0d205a530c (patch) | |
| tree | 00eeddc6d084d0059f632d9c7b7bebb4ace1839c /kernel/sched/core.c | |
| parent | 792b9f65a568f48c50b3175536db9cde5a1edcc0 (diff) | |
| download | linux-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/core.c')
| -rw-r--r-- | kernel/sched/core.c | 6 |
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; |