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

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Date:   Fri, 23 Mar 2018 23:06:11 -0700
From:   Joel Fernandes <joel.opensrc@...il.com>
To:     Quentin Perret <quentin.perret@....com>,
        Joel Fernandes <joelaf@...gle.com>
CC:     Morten Rasmussen <morten.rasmussen@....com>,
        Patrick Bellasi <patrick.bellasi@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thara Gopinath <thara.gopinath@...aro.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Chris Redpath <chris.redpath@....com>,
        Valentin Schneider <valentin.schneider@....com>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Todd Kjos <tkjos@...gle.com>
Subject: Re: [RFC PATCH 5/6] sched/fair: Select an energy-efficient CPU on task wake-up



On March 23, 2018 6:34:22 PM PDT, Quentin Perret <quentin.perret@....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@....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


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