From: Tianchen Ding dtcccc@linux.alibaba.com
[ Upstream commit 28156108fecb1f808b21d216e8ea8f0d205a530c ]
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.aliba...
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 Signed-off-by: Sasha Levin sashal@kernel.org --- 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 5c7937b504d2..892c06ff9dd0 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -3735,8 +3735,12 @@ 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;