Messages in this thread | | | Date | Tue, 31 May 2022 14:55:32 +0100 | From | Mel Gorman <> | Subject | Re: [PATCH v2] sched: Queue task on wakelist in the same llc if the wakee cpu is idle |
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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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