linux-kernel - Re: [PATCH v2 3/3] cpufreq: schedutil: map raw required frequency to driver frequency

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Message-ID: <20160530164833.GH9864@graphite.smuckle.net>
Date:	Mon, 30 May 2016 09:48:33 -0700
From:	Steve Muckle <steve.muckle@...aro.org>
To:	Wanpeng Li <kernellwp@...il.com>
Cc:	Steve Muckle <steve.muckle@...aro.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux PM list <linux-pm@...r.kernel.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Juri Lelli <Juri.Lelli@....com>,
	Patrick Bellasi <patrick.bellasi@....com>,
	Michael Turquette <mturquette@...libre.com>
Subject: Re: [PATCH v2 3/3] cpufreq: schedutil: map raw required frequency to
 driver frequency

On Fri, May 27, 2016 at 01:41:02PM +0800, Wanpeng Li wrote:
> 2016-05-26 10:53 GMT+08:00 Steve Muckle <steve.muckle@...aro.org>:
> > The slow-path frequency transition path is relatively expensive as it
> > requires waking up a thread to do work. Should support be added for
> > remote CPU cpufreq updates that is also expensive since it requires an
> > IPI. These activities should be avoided if they are not necessary.
> >
> > To that end, calculate the actual driver-supported frequency required by
> > the new utilization value in schedutil by using the recently added
> > cpufreq_driver_resolve_freq callback. If it is the same as the
> > previously requested driver frequency then there is no need to continue
> > with the update assuming the cpu frequency limits have not changed. This
> > will have additional benefits should the semantics of the rate limit be
> > changed to apply solely to frequency transitions rather than to
> > frequency calculations in schedutil.
> 
> sugov_should_update_freq() still be called before get_nex_freq() after
> the patch applied, so rate limit still apply to both frequency
> transitions and frequency calculations, right?

Yes this series does not change the semantics of the rate limit. It
only includes the changes required for resolving raw target frequencies
to driver-supported frequencies so redundant operations can be avoided.

FWIW the approach I took to change the rate_limit semantics [0] just
moves where last_freq_update_time is set. I was going to return to that
once these changes are complete.

[0]: https://lkml.org/lkml/2016/5/9/857

thanks,
Steve