Re: [RFC PATCH 5/6] sched/fair: Select an energy-efficient CPU on task wake-up

From: Joel Fernandes <joel.opensrc@gmail.com>
To: Quentin Perret <quentin.perret@arm.com>,Joel Fernandes
	<joelaf@google.com>
Cc: Morten Rasmussen <morten.rasmussen@arm.com>,Patrick Bellasi
	<patrick.bellasi@arm.com>,Dietmar Eggemann
	<dietmar.eggemann@arm.com>,LKML
	<linux-kernel@vger.kernel.org>,Peter Zijlstra
	<peterz@infradead.org>,Thara Gopinath
	<thara.gopinath@linaro.org>,Linux PM
	<linux-pm@vger.kernel.org>,Chris Redpath
	<chris.redpath@arm.com>,Valentin Schneider
	<valentin.schneider@arm.com>,"Rafael J . Wysocki"
	<rjw@rjwysocki.net>,Greg Kroah-Hartman
	<gregkh@linuxfoundation.org>,Vincent Guittot
	<vincent.guittot@linaro.org>,Viresh Kumar
	<viresh.kumar@linaro.org>,Todd Kjos <tkjos@google.com>
Subject: Re: [RFC PATCH 5/6] sched/fair: Select an energy-efficient CPU on task wake-up
Date: Fri, 23 Mar 2018 23:06:11 -0700	[thread overview]
Message-ID: <BE9CF57A-877A-4F2D-A64E-9693C54FF058@gmail.com> (raw)
In-Reply-To: <20180324013421.GB1317@queper01-VirtualBox>

On March 23, 2018 6:34:22 PM PDT, Quentin Perret <quentin.perret@arm.com> wrote:
>On Friday 23 Mar 2018 at 18:13:56 (-0700), Joel Fernandes wrote:
>> Hi Morten,
>> 
>> On Fri, Mar 23, 2018 at 8:47 AM, Morten Rasmussen
>> <morten.rasmussen@arm.com> wrote:
>> > On Thu, Mar 22, 2018 at 01:10:22PM -0700, Joel Fernandes wrote:
>
>[...]
>
>> > You mean if SD_BALANCE_WAKE isn't set on sched_domains?
>> 
>> Yes.
>> 
>> > The current code seems to rely on that flag to be set to work
>correctly.
>> > Otherwise, the loop might bail out on !want_affine and we end up
>doing
>> > the find_energy_efficient_cpu() on the lowest level sched_domain
>even if
>> > there is higher level one which isn't over-utilized.
>> >
>> > However, SD_BALANCE_WAKE should be set if SD_ASYM_CPUCAPACITY is
>set so
>> > sd == NULL shouldn't be possible? This only holds as long as we
>only
>> > want EAS for asymmetric systems.
>> 
>> Yes, I see you had topology code that set SD_BALANCE_WAKE for ASYM.
>It
>> makes sense to me then, thanks for the clarification.
>> 
>> Still I feel it is a bit tedious/confusing when reading code to draw
>> the conclusion about why sd is checked first before doing
>> find_energy_efficient_cpu (and that sd will != NULL for ASYM
>systems).
>> If energy_sd is set, then we can just proceed with EAS without
>> checking that sd != NULL. This function in mainline is already pretty
>> confusing as it is :-(
>
>Right I see your point. The code is correct as is, but I agree that
>having
>a code structured as
>
>	if (energy_sd) {
>		new_cpu = find_energy_efficient_cpu(energy_sd, p, prev_cpu);
>	} else if (!sd) {
>		...
>
>might be easier to understand and functionally equivalent. What do you
>think ?

Yeah definitely. Go for it.

- Joel

-- 
Sent from my Android device with K-9 Mail. Please excuse my brevity.