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

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Message-ID: <20180410140853.GG4082@hirez.programming.kicks-ass.net>
Date:   Tue, 10 Apr 2018 16:08:53 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Quentin Perret <quentin.perret@....com>
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 Tue, Apr 10, 2018 at 02:56:41PM +0100, Quentin Perret wrote:
> > 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 ?

Yes, something along those lines; we could also of course factor
get_next_freq() into two parts.

> > 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 :-)

Yes, that wants a wee bit of discussion. Ideally we'd have a shared data
structure we can iterate instead of a chain of indirect calls.