linux-kernel - Re: [PATCH] sched/fair: prefer prev cpu in asymmetric wakeup path

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Message-ID: <2a2699ee-e62f-1590-c846-009af0478c59@arm.com>
Date:   Fri, 23 Oct 2020 19:14:54 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>,
        Valentin Schneider <valentin.schneider@....com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Morten Rasmussen <morten.rasmussen@....com>
Subject: Re: [PATCH] sched/fair: prefer prev cpu in asymmetric wakeup path

On 22/10/2020 17:33, Vincent Guittot wrote:
> On Thu, 22 Oct 2020 at 16:53, Valentin Schneider
> <valentin.schneider@....com> wrote:
>>
>>
>> Hi Vincent,
>>
>> On 22/10/20 14:43, Vincent Guittot wrote:

[...]

>>>  static int
>>> -select_idle_capacity(struct task_struct *p, struct sched_domain *sd, int target)
>>> +select_idle_capacity(struct task_struct *p, struct sched_domain *sd, int prev, int target)
>>>  {
>>>       unsigned long best_cap = 0;
>>>       int cpu, best_cpu = -1;
>>> @@ -6178,9 +6178,22 @@ select_idle_capacity(struct task_struct *p, struct sched_domain *sd, int target)
>>>
>>>       sync_entity_load_avg(&p->se);
>>>
>>> +     if ((available_idle_cpu(target) || sched_idle_cpu(target)) &&
>>> +         task_fits_capacity(p, capacity_of(target)))
>>> +             return target;
>>> +
>>
>> I think we still need to check for CPU affinity here.
> 
> yes good point

We don't check CPU affinity on target and prev in the symmetric case.

I always thought that since we:

(1) check 'want_affine = ... && cpumask_test_cpu(cpu, p->cpus_ptr);' in
    select_task_rq_fair() and

(2) we have the select_fallback_rq() in select_task_rq() for prev

that this would be sufficient?

[...]