linux-kernel - Re: [PATCH 4/8] cpufreq/schedutil: sysfs capacity margin tunable

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Date:	Tue, 15 Mar 2016 20:36:57 -0700
From:	Steve Muckle <steve.muckle@...aro.org>
To:	Michael Turquette <mturquette@...libre.com>,
	Peter Zijlstra <peterz@...radead.org>
Cc:	rjw@...ysocki.net, linux-kernel@...r.kernel.org,
	linux-pm@...r.kernel.org, Juri.Lelli@....com,
	morten.rasmussen@....com, dietmar.eggemann@....com,
	vincent.guittot@...aro.org,
	Michael Turquette <mturquette+renesas@...libre.com>,
	Patrick Bellasi <patrick.bellasi@....com>
Subject: Re: [PATCH 4/8] cpufreq/schedutil: sysfs capacity margin tunable

On 03/15/2016 03:37 PM, Michael Turquette wrote:
>>>> Yuck sysfs.. I would really rather we did not expose this per default.
>>>> > > > And certainly not in this weird form.
>>> > > 
>>> > > I'm happy to change capacity_margin to up_threshold and use a
>>> > > percentage.
>>> > > 
>>> > > The sysfs approach has two benefits. First, it is aligned with cpufreq
>>> > > user expectations. Second, there has been rough consensus that this
>>> > > value should be tunable and sysfs gets us there quickly and painlessly.
>>> > > We're already exporting rate_limit_us for schedutil via sysfs. Is there
>>> > > a better way interface you can recommend?
>> > 
>> > It really depends on how tunable you want this to be. Do we always want
>> > this to be a tunable, or just now while we're playing about with the
>> > whole thing?
>
> I had considered this myself, and I really think that Steve and Juri
> should chime in as they have spent more time tuning and running the
> numbers.
> 
> I'm inclined to think that a debug version would be good enough, as I
> don't imagine this value being changed at run-time by some userspace
> daemon or something.
> 
> Then again, maybe this knob will be part of the mythical
> power-vs-performance slider?

Patrick Bellasi's schedtune series [0] (which I think is the referenced
mythical slider) aims to provide a more sophisticated interface for
tuning scheduler-driven frequency selection. In addition to a global
boost value it includes a cgroup controller as well for per-task tuning.

I would definitely expect the margin/boost value to be modified at
runtime, for example if the battery is running low, or the user wants
100% performance for a while, or the userspace framework wants to
temporarily tailor the performance level for a particular set of tasks, etc.

[0] http://article.gmane.org/gmane.linux.kernel/2022959