linux-kernel - Re: [PATCH v2 8/9] sched/fair: Split select_task_rq_fair want

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Date:   Thu, 19 Mar 2020 12:06:48 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        peterz@...radead.org, vincent.guittot@...aro.org
Subject: Re: [PATCH v2 8/9] sched/fair: Split select_task_rq_fair want_affine logic


On Thu, Mar 19 2020, Dietmar Eggemann wrote:
>> +	 */
>>      for_each_domain(cpu, tmp) {
>> -		/*
>> -		 * If both 'cpu' and 'prev_cpu' are part of this domain,
>> -		 * cpu is a valid SD_WAKE_AFFINE target.
>> -		 */
>> -		if (want_affine && (tmp->flags & SD_WAKE_AFFINE) &&
>> +		if ((tmp->flags & SD_WAKE_AFFINE) &&
>>                  cpumask_test_cpu(prev_cpu, sched_domain_span(tmp))) {
>>                      if (cpu != prev_cpu)
>>                              new_cpu = wake_affine(tmp, p, cpu, prev_cpu, sync);
>>
>> -			sd = NULL; /* Prefer wake_affine over balance flags */
>> +			/* Prefer wake_affine over SD lookup */
>
> I assume that 'balance flags' stands for (wakeup) load balance, i.e.
> find_idlest_xxx() path. So why change it?
>
>

You mean the comment part, right? I was hoping to clarify it a bit - if
we go through the want_affine condition, we'll override whatever SD we
picked with the highest_flag_domain() lookup (and the cached version in
9/9). Hence me referring to the SD lookup there.

> [...]