linux-kernel - Re: [PATCH] sched/fair: Rearrange select_task_rq

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Date:   Tue, 24 Apr 2018 08:47:35 -0700
From:   Joel Fernandes <joel.opensrc@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Valentin Schneider <valentin.schneider@....com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Ingo Molnar <mingo@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Quentin Perret <quentin.perret@....com>,
        Joel Fernandes <joelaf@...gle.com>
Subject: Re: [PATCH] sched/fair: Rearrange select_task_rq_fair() to optimize it

On Tue, Apr 24, 2018 at 8:46 AM, Joel Fernandes <joel.opensrc@...il.com> wrote:
> On Tue, Apr 24, 2018 at 5:35 AM, Peter Zijlstra <peterz@...radead.org> wrote:
>> On Tue, Apr 24, 2018 at 12:19:07PM +0100, Valentin Schneider wrote:
>>> On 24/04/18 11:43, Peter Zijlstra wrote:
>>> > On Tue, Apr 24, 2018 at 11:02:26AM +0100, Valentin Schneider wrote:
>>> >> I'd argue making things easier to read is a non-negligible part as well.
>>> >
>>> > Right, so I don't object to either of these (I think); but it would be
>>> > good to see this in combination with that proposed EAS change.
>>> >
>>>
>>> True, I would've said the call to find_energy_efficient_cpu() ([1]) could
>>> simply be added to the if (sd) {} case, but...
>>
>> I think the proposal was to put it before the for_each_domain() loop
>> entirely, however...
>>
>>> > I think you (valentin) wanted to side-step the entire domain loop in
>>> > that case or something.
>>> >
>>>
>>> ...this would change more things. Admittedly I've been sort of out of the loop
>>> (no pun intended) lately, but this doesn't ring a bell. That might have been
>>> the other frenchie (Quentin) :)
>>
>> It does indeed appear I confused the two of you, it was Quentin playing
>> with that.
>>
>> In any case, if there not going to be conflicts here, this all looks
>> good.
>
> Both Viresh's and Valentin's patch looks lovely to me too. I couldn't
> spot anything wrong with them either. One suggestion I was thinking
> off is can we add better comments to this code (atleast label fast
> path vs slow path) ?
>
> Also, annotate the conditions for the fast/slow path with
> likely/unlikely since fast path is the common case? so like:
>
> if (unlikely(sd)) {
>   /* Fast path, common case */
>   ...
> } else if (...) {
>   /* Slow path */
> }

Aargh, I messed that up, I meant:

if (unlikely(sd)) {
   /* Slow path */
   ...
} else if (...) {
   /* Fast path */
}


thanks, :-)

- Joel