Date: Wed, 16 Mar 2016 10:02:57 +0000
From: Juri Lelli <Juri.Lelli@arm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Steve Muckle <steve.muckle@linaro.org>,
        Michael Turquette <mturquette@baylibre.com>, rjw@rjwysocki.net,
        linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org,
        morten.rasmussen@arm.com, dietmar.eggemann@arm.com,
        vincent.guittot@linaro.org,
        Michael Turquette <mturquette+renesas@baylibre.com>,
        Patrick Bellasi <patrick.bellasi@arm.com>
Subject: Re: [PATCH 4/8] cpufreq/schedutil: sysfs capacity margin tunable
Message-ID: <20160316100257.GC18212@e106622-lin>
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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.

Best,

- Juri