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SubjectRe: [PATCH v2] sched: Queue task on wakelist in the same llc if the wakee cpu is idle
On Tue, May 31, 2022 at 12:50:49PM +0100, Valentin Schneider wrote:
> >> With all that in mind, I'm curious whether your patch is functionaly close
> >> to the below.
> >>
> >> ---
> >> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> >> index 66c4e5922fe1..ffd43264722a 100644
> >> --- a/kernel/sched/core.c
> >> +++ b/kernel/sched/core.c
> >> @@ -3836,7 +3836,7 @@ 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 (cpu_rq(cpu)->nr_running <= 1)
> >> return true;
> >>
> >> return false;
> >
> > It's a little different. This may bring extra IPIs when nr_running == 1
> > and the current task on wakee cpu is not the target wakeup task (i.e.,
> > rq->curr == another_task && rq->curr != p). Then this another_task may
> > be disturbed by IPI which is not expected. So IMO the promise by
> > WF_ON_CPU is necessary.
>
> You're right, actually taking a second look at that WF_ON_CPU path,
> shouldn't the existing condition be:
>
> if ((wake_flags & WF_ON_CPU) && !cpu_rq(cpu)->nr_running)
>
> ? Per the p->on_rq and p->on_cpu ordering, 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.
>
> @Mel, do you remember why you went for <=1 here? I couldn't find any clues
> on the original posting.
>

I don't recall exactly why I went with <= 1 there but I may not have
considered the memory ordering of on_rq and nr_running and the comment
above it is literally what I was thinking at the time. I think you're
right and that check can be !cpu_rq(cpu)->nr_running.

--
Mel Gorman
SUSE Labs

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