linux-kernel - Re: [PATCH v2] sched: Queue task on wakelist in the same llc if the wakee cpu is idle

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Date:   Tue, 31 May 2022 16:56:59 +0100
From:   Valentin Schneider <vschneid@...hat.com>
To:     Mel Gorman <mgorman@...e.de>
Cc:     Tianchen Ding <dtcccc@...ux.alibaba.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] sched: Queue task on wakelist in the same llc if the
 wakee cpu is idle

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;