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Date: Tue, 6 May 2014 15:51:25 +0100
From: Morten Rasmussen <morten.rasmussen@arm.com>
To: Peter Zijlstra <peterz@infradead.org>
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	<20140506134909.GM11096@twins.programming.kicks-ass.net>
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Cc: Len Brown <len.brown@intel.com>,
	"ksummit-discuss@lists.linuxfoundation.org"
	<ksummit-discuss@lists.linuxfoundation.org>,
	Daniel Lezcano <daniel.lezcano@linaro.org>,
	Amit Kucheria <amit.kucheria@linaro.org>, Ingo Molnar <mingo@kernel.org>
Subject: Re: [Ksummit-discuss] [TECH(CORE?) TOPIC] Energy conservation bias
	interfaces
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On Tue, May 06, 2014 at 02:49:09PM +0100, Peter Zijlstra wrote:
> 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.

True. But performance requirements are associated with tasks or groups
of tasks. We also need an interface to get input from userspace to tell
us when it is acceptable to potentially loose performance to save
energy. IIUC, that is Rafael's second point above.

Morten