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

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Date:   Sat, 24 Mar 2018 01:34:22 +0000
From:   Quentin Perret <quentin.perret@....com>
To:     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 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 ?

Thanks,
Quentin