Date: Thu, 17 Mar 2016 09:40:46 +0000
From: Juri Lelli <Juri.Lelli@arm.com>
To: Steve Muckle <steve.muckle@linaro.org>
Cc: Peter Zijlstra <peterz@infradead.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: <20160317094046.GF18212@e106622-lin>
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On 16/03/16 10:55, Steve Muckle wrote:
> 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?
> 

I'm not sure I understand you here. For me schedtune should be disabled
by default, so I'd say that it doesn't introduce any additional margin
by default. But we still need a margin to make the governor work without
schedtune in the mix.

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

Looks good to me.

Best,

- Juri