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Message-ID: <d387ede1-d000-37aa-6ede-237b65b59ce5@linux.alibaba.com>
Date:   Tue, 31 May 2022 23:38:32 +0800
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Subject: Re: [PATCH v2] sched: Queue task on wakelist in the same llc if the
 wakee cpu is idle
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To:     Mel Gorman <mgorman@suse.de>,
        Valentin Schneider <vschneid@redhat.com>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        linux-kernel@vger.kernel.org
References: <20220527090544.527411-1-dtcccc@linux.alibaba.com>
 <xhsmhleuj7zve.mognet@vschneid.remote.csb>
 <1d0eb8f4-e474-86a9-751a-7c2e1788df85@linux.alibaba.com>
 <xhsmhilpl9azq.mognet@vschneid.remote.csb> <20220531135532.GA3332@suse.de>
From:   Tianchen Ding <dtcccc@linux.alibaba.com>
In-Reply-To: <20220531135532.GA3332@suse.de>
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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?