Re: [PATCH] sched/fair: Rearrange select_task_rq_fair() to optimize it

From: Joel Fernandes <joel.opensrc@gmail.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Valentin Schneider <valentin.schneider@arm.com>,
	Viresh Kumar <viresh.kumar@linaro.org>,
	Ingo Molnar <mingo@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Daniel Lezcano <daniel.lezcano@linaro.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Quentin Perret <quentin.perret@arm.com>,
	c@hirez.programming.kicks-ass.net,
	Joel Fernandes <joelaf@google.com>
Subject: Re: [PATCH] sched/fair: Rearrange select_task_rq_fair() to optimize it
Date: Tue, 24 Apr 2018 08:46:08 -0700	[thread overview]
Message-ID: <CAEi0qN=qnej22F31jM2CMajee2h+ie0+AptFscwa5L8Arp8q3g@mail.gmail.com> (raw)
In-Reply-To: <20180424123523.GF4064@hirez.programming.kicks-ass.net>

On Tue, Apr 24, 2018 at 5:35 AM, Peter Zijlstra <peterz@infradead.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 */
}

thanks,

- Joel