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

From: Peter Zijlstra <peterz@infradead.org>
To: Quentin Perret <quentin.perret@arm.com>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>,
	linux-kernel@vger.kernel.org,
	Thara Gopinath <thara.gopinath@linaro.org>,
	linux-pm@vger.kernel.org,
	Morten Rasmussen <morten.rasmussen@arm.com>,
	Chris Redpath <chris.redpath@arm.com>,
	Patrick Bellasi <patrick.bellasi@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>, Joel Fernandes <joelaf@google.com>,
	Juri Lelli <juri.lelli@redhat.com>,
	Steve Muckle <smuckle@google.com>,
	Eduardo Valentin <edubezval@gmail.com>
Subject: Re: [RFC PATCH v2 4/6] sched/fair: Introduce an energy estimation helper function
Date: Tue, 10 Apr 2018 16:08:53 +0200	[thread overview]
Message-ID: <20180410140853.GG4082@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20180410135640.GB3866@e108498-lin.cambridge.arm.com>

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.