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

[Rafael J. Wysocki]

On Wed, Mar 16, 2016 at 9:05 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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.
>
> The problem with starting out as ABI is that its hard to take away
> again.

Agreed, plus it is quite hard to get ABI right from the outset.  Even
if we decide on a sysfs knob, it still is unclear what exactly should
be represented by it in what units etc.