Re: [Ksummit-discuss] [TECH(CORE?) TOPIC] Energy conservation bias interfaces

[Peter Zijlstra <peterz@infradead.org>]

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On Tue, May 06, 2014 at 02:54:03PM +0200, Rafael J. Wysocki wrote:
> Hi All,
> 
> During a recent discussion on linux-pm/LKML regarding the integration of the
> scheduler with cpuidle (http://marc.info/?t=139834240600003&r=1&w=4) it became
> apparent that the kernel might benefit from adding interfaces to let it know
> how far it should go with saving energy, possibly at the expense of performance.
> 
> First of all, it would be good to have a place where subsystems and device
> drivers can go and check what the current "energy conservation bias" is in
> case they need to make a decision between delivering more performance and
> using less energy.  Second, it would be good to provide user space with
> a means to tell the kernel whether it should care more about performance or
> energy.  Finally, it would be good to be able to adjust the overall "energy
> conservation bias" automatically in response to certain "power" events such
> as "battery is low/critical" etc.
> 
> It doesn't seem to be clear currently what level and scope of such interfaces
> is appropriate and where to place them.  Would a global knob be useful?  Or
> should they be per-subsystem, per-driver, per-task, per-cgroup etc?

per-task and per-cgroup doesn't seem to make sense to me; its the
hardware that consumes energy.

per-subsystem sounds right to me; I don't care which particular instance
of graphics cards I have, I want whichever one(s) I have to obey.

global doesn't make sense, like stated earlier I absolutely detest
automagic backlight dimming, whereas I don't particularly care about
compute speed at all.

So while I might want a energy conserving bias for say the CPU and GPU,
I most definitely don't want that to dim the screen.

> It also is not particularly clear what representation of "energy conservation
> bias" would be most useful.  Should that be a number or a set of well-defined
> discrete levels that can be given names (like "max performance", "high
> prerformance", "balanced" etc.)?  If a number, then what units to use and
> how many different values to take into account?

Yeah, fun.. we're not even sure how to make it do the 0,1 variants, and
now you want a sliding scale to make it do in-betweens ;-)

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