linux-kernel - Re: [External] Re: [PATCH 1/2] sched/numa: Stop an exhastive search if an idle core is found

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Date:   Wed, 26 Oct 2022 10:30:43 +0800
From:   Hao Jia <jiahao.os@...edance.com>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Mel Gorman <mgorman@...e.de>, mingo@...hat.com,
        peterz@...radead.org, mingo@...nel.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, bristot@...hat.com,
        vschneid@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [External] Re: [PATCH 1/2] sched/numa: Stop an exhastive search
 if an idle core is found



On 2022/10/25 Mel Gorman wrote:
> On Tue, Oct 25, 2022 at 07:10:22PM +0800, Hao Jia wrote:
>> On 2022/10/25 Mel Gorman wrote:
>>> On Tue, Oct 25, 2022 at 11:16:29AM +0800, Hao Jia wrote:
>>>>> Remove the change in the first hunk and call break in the second hunk
>>>>> after updating ns->idle_cpu.
>>>>>
>>>>
>>>> Yes, thanks for your review.
>>>> If I understand correctly, some things might look like this.
>>>>
>>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>>> index e4a0b8bd941c..dfcb620bfe50 100644
>>>> --- a/kernel/sched/fair.c
>>>> +++ b/kernel/sched/fair.c
>>>> @@ -1792,7 +1792,7 @@ static void update_numa_stats(struct task_numa_env
>>>> *env,
>>>>                   ns->nr_running += rq->cfs.h_nr_running;
>>>>                   ns->compute_capacity += capacity_of(cpu);
>>>>
>>>> -               if (find_idle && !rq->nr_running && idle_cpu(cpu)) {
>>>> +               if (find_idle && idle_core < 0 && !rq->nr_running &&
>>>> idle_cpu(cpu)) {
>>>>                           if (READ_ONCE(rq->numa_migrate_on) ||
>>>>                               !cpumask_test_cpu(cpu, env->p->cpus_ptr))
>>>>                                   continue;
>>>>
>>>
>>> I meant more like the below but today I wondered why did I not do this in
>>> the first place? The answer is because it's wrong and broken in concept.
>>>
>>> The full loop is needed to calculate approximate NUMA stats at a
>>> point in time. For example, the src and dst nr_running is needed by
>>> task_numa_find_cpu. The search for an idle CPU or core in update_numa_stats
>>> is simply taking advantage of the fact we are scanning anyway to keep
>>> track of an idle CPU or core to avoid a second search as per ff7db0bf24db
>>> ("sched/numa: Prefer using an idle CPU as a migration target instead of
>>> comparing tasks")
>>>
>>> The patch I had in mind is below but that said, for both your version and
>>> my initial suggestion
>>>
>>> Naked-by: Mel Gorman <mgorman@...e.de>
>>>
>>> For the record, this is what I was suggesting initially because it's more
>>> efficient but it's wrong, don't do it.
>>>
>>
>> Thanks for the detailed explanation, maybe my commit message misled you.
>>
> 
> Yes, I did end up confusing myself. The title and changelog referred to
> stopping a search which made me think of terms of "this whole loop can
> terminate early" which it can't but it *can* stop checking for a new idle
> core. If an idle core has been found, it follows that an idle CPU has also
> been found. While numa_idle_core checks this explicitly, your patch avoids
> an unnecessary cpumask_test_cpu so it has value.
> 

Thank you for your review, I will change the commit message and send 
patch v2.


>> Yes, we can't stop the whole loop of scanning the CPU because we have a lot
>> of NUMA information to count.
>>
>> But we can stop looking for the next idle core or idle cpu after finding an
>> idle core.
>>
>> So, please review the previous code.
>>
> 
> You're right and sorry for the noise.
> 
> Acked-by: Mel Gorman <mgorman@...hsingularity.net>

Thanks!

> 
Thanks,
Hao