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

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Message-ID: <20160316080503.GS6344@twins.programming.kicks-ass.net>
Date:	Wed, 16 Mar 2016 09:05:03 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steve Muckle <steve.muckle@...aro.org>
Cc:	Michael Turquette <mturquette@...libre.com>, 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 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.

The problem with starting out as ABI is that its hard to take away
again.