Messages in this thread | | | From | Valentin Schneider <> | Subject | Re: [PATCH v3 1/2] sched: Fix the check of nr_running at queue wakelist | Date | Mon, 06 Jun 2022 11:39:02 +0100 |
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On 02/06/22 12:06, Tianchen Ding wrote: > The commit 2ebb17717550 ("sched/core: Offload wakee task activation if it > the wakee is descheduling") checked nr_running <= 1 to avoid task > stacking when WF_ON_CPU. Consider the order of p->on_rq and p->on_cpu, > if we have WF_ON_CPU here then we must have !p->on_rq, so the deactivate > has happened, thus the task being alone on the rq implies nr_running==0. > Change the check to !cpu_rq(cpu)->nr_running to fix it. >
I'd flesh this out a bit as in the below:
""" 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.
Change the check to !cpu_rq(cpu)->nr_running. """
Also, this is lacking some mention of tests that have been run to verify this isn't causing a regression. This does however make sense to me, so as long as nothing gets hurt by the change:
Reviewed-by: Valentin Schneider <vschneid@redhat.com>
> Signed-off-by: Tianchen Ding <dtcccc@linux.alibaba.com> > --- > kernel/sched/core.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index bfa7452ca92e..a9efe134fbe5 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -3830,7 +3830,7 @@ static inline bool ttwu_queue_cond(int cpu, int wake_flags) > * the soon-to-be-idle CPU as the current CPU is likely busy. > * nr_running is checked to avoid unnecessary task stacking. > */ > - 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; > -- > 2.27.0
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