linux-kernel - Re: [RFC PATCH v2 4/6] sched/fair: Introduce an energy estimation helper function

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Date:   Tue, 10 Apr 2018 14:56:41 +0100
From:   Quentin Perret <quentin.perret@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Dietmar Eggemann <dietmar.eggemann@....com>,
        linux-kernel@...r.kernel.org,
        Thara Gopinath <thara.gopinath@...aro.org>,
        linux-pm@...r.kernel.org,
        Morten Rasmussen <morten.rasmussen@....com>,
        Chris Redpath <chris.redpath@....com>,
        Patrick Bellasi <patrick.bellasi@....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>,
        Joel Fernandes <joelaf@...gle.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Steve Muckle <smuckle@...gle.com>,
        Eduardo Valentin <edubezval@...il.com>
Subject: Re: [RFC PATCH v2 4/6] sched/fair: Introduce an energy estimation
 helper function

On Tuesday 10 Apr 2018 at 14:51:05 (+0200), Peter Zijlstra wrote:
> On Fri, Apr 06, 2018 at 04:36:05PM +0100, Dietmar Eggemann wrote:
> > +static inline
> > +struct capacity_state *find_cap_state(int cpu, unsigned long util)
> > +{
> > +	struct sched_energy_model *em = *per_cpu_ptr(energy_model, cpu);
> > +	struct capacity_state *cs = NULL;
> > +	int i;
> > +
> > +	util += util >> 2;
> > +
> > +	for (i = 0; i < em->nr_cap_states; i++) {
> > +		cs = &em->cap_states[i];
> > +		if (cs->cap >= util)
> > +			break;
> > +	}
> > +
> > +	return cs;
> > +}
> 
> So in the last thread there was some discussion about this; in
> particular on how this related to schedutil and if we should tie it into
> that.
> 
> I think for starters tying it to schedutil is not a bad idea; ideally
> people _should_ migrate towards using that.
> 
> Also; I think it makes sense to better integrate cpufreq and the
> energy-model values like what rjw already suggested, such that maybe we
> can have cpufreq_driver_resolve_freq() return a structure containing the
> relevant information for the selected frequency.

I guess if we want to do that in the wake-up path, we would also need to
add a new parameter to it to make sure we don't actually call into
cpufreq_driver->resolve_freq() ...

But then, we could sort of rely on cpufreq_schedutil.c::get_next_freq()
to replace find_cap_state() ... Is this what you had in mind ?

> 
> But implementing the frequency selection thing in multiple places like
> now sounds like a very bad idea to me.

Understood. Making sure we share the same code everywhere might have
consequences though. I guess we'll have to either accept the cost of
function calls in the wake-up path, or to accept to inline those
functions for ex. Or maybe you had something else in mind ?

Anyways, that's probably another good discussion topic for OSPM
next week :-)

Thanks,
Quentin