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

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Message-ID: <20160316220349.11016.67079@quark.deferred.io>
Date:	Wed, 16 Mar 2016 15:03:49 -0700
From:	Michael Turquette <mturquette@...libre.com>
To:	Steve Muckle <steve.muckle@...aro.org>,
	"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

Quoting Steve Muckle (2016-03-15 20:36:57)
> 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.

/me spends 15 seconds looking schedtune

> 
> 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.

Right, and it looks like schedtune is a kernel solution, not userspace
solution. The following three interfaces from patch #2 could be used by
schedtune:

unsigned long cpufreq_get_cfs_capacity_margin(void);
void cpufreq_set_cfs_capacity_margin(unsigned long margin);
void cpufreq_reset_cfs_capacity_margin(void);

Then we can let schedtune worry about the userspace abi.

So I'll keep the basic idea of this patch, but explore making it
debuggy, instead of sysfsy.

Regards,
Mike

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