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

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Date:	Wed, 16 Mar 2016 15:05:06 -0700
From:	Michael Turquette <mturquette@...libre.com>
To:	Steve Muckle <steve.muckle@...aro.org>,
	"Juri Lelli" <Juri.Lelli@....com>,
	"Peter Zijlstra" <peterz@...radead.org>
Cc:	rjw@...ysocki.net, linux-kernel@...r.kernel.org,
	linux-pm@...r.kernel.org, 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 (2016-03-16 10:55:49)
> On 03/16/2016 03:02 AM, Juri Lelli wrote:
> > Hi,
> > 
> > On 16/03/16 09:05, Peter Zijlstra wrote:
> >> On Tue, Mar 15, 2016 at 08:36:57PM -0700, Steve Muckle wrote:
> >>>> 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.
> >>
> >> OK, so how about we start with it as a debug knob, and once we have
> >> experience and feel like it is indeed a useful runtime knob, we upgrade
> >> it to ABI.
> >>
> > 
> > I tend to agree here. To me the margin is something that we need to make
> > this thing work and to get acceptable performance out of the box. So we
> > can play with it while debugging, but I consider the schedtune slider as
> > the way to tune the system at runtime.
> 
> Could the default schedtune value not serve as the out of the box margin?

It can. Let's keep the kernel interfaces in patch #2 for changing the
margin/threshold, and schedtune can call these interfaces.

Regards,
Mike

> 
> Regardless I agree that a debug interface is the way to go for now while
> we figure things out.
>