Messages in this thread Patch in this message | | | From | Valentin Schneider <> | Subject | Re: [PATCH v2] sched: Queue task on wakelist in the same llc if the wakee cpu is idle | Date | Tue, 31 May 2022 16:56:59 +0100 |
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On 31/05/22 14:55, 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. >
Thanks!
So I'm thinking we could first make that into
if ((wake_flags & WF_ON_CPU) && !cpu_rq(cpu)->nr_running)
Then building on this, we can generalize using the wakelist to any remote idle CPU (which on paper isn't as much as a clear win as just WF_ON_CPU, depending on how deeply idle the CPU is...)
We need the cpu != this_cpu check, as that's currently served by the WF_ON_CPU check (AFAIU we can only observe p->on_cpu in there for remote tasks).
--- diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 66c4e5922fe1..60038743f2f1 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -3830,13 +3830,20 @@ static inline bool ttwu_queue_cond(int cpu, int wake_flags) if (!cpus_share_cache(smp_processor_id(), cpu)) return true; + if (cpu == smp_processor_id()) + return false; + /* * If the task is descheduling and the only running task on the * 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 (!cpu_rq(cpu)->nr_running) return true; return false;
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