Messages in this thread | | | Date | Tue, 31 May 2022 23:38:32 +0800 | Subject | Re: [PATCH v2] sched: Queue task on wakelist in the same llc if the wakee cpu is idle | From | Tianchen Ding <> |
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On 2022/5/31 21:55, Mel Gorman wrote: > 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. >
If the check becomes !cpu_rq(cpu)->nr_running My patch would change, too. Shall we remove WF_ON_CPU completely?
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