linux-kernel - Re: [PATCH 0/3] coupled cpuidle state support

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Message-ID: <CAMbhsRTbVqV6p0py9NGZrpFPf2yvp-B0dr+o-m7qbdja_-Lfzw@mail.gmail.com>
Date:	Fri, 20 Jan 2012 12:40:03 -0800
From:	Colin Cross <ccross@...roid.com>
To:	Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-pm@...ts.linux-foundation.org,
	Len Brown <len.brown@...el.com>, Kevin Hilman <khilman@...com>,
	Santosh Shilimkar <santosh.shilimkar@...com>,
	Amit Kucheria <amit.kucheria@...aro.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Trinabh Gupta <g.trinabh@...il.com>,
	Deepthi Dharwar <deepthi@...ux.vnet.ibm.com>,
	linux-omap@...r.kernel.org, linux-tegra@...r.kernel.org
Subject: Re: [PATCH 0/3] coupled cpuidle state support

On Fri, Jan 20, 2012 at 12:46 AM, Daniel Lezcano
<daniel.lezcano@...aro.org> wrote:
> Hi Colin,
>
> this patchset could be interesting to resolve in a generic way the cpu
> dependencies.
> What is the status of this patchset ?

I can't do much with it right now, because I don't have any devices
that can do SMP idle with a v3.2 kernel.  I've started working on an
updated version that avoids the spinlock, but it might be a while
before I can test and post it.  I'm mostly looking for feedback on the
approach taken in this patch, and whether it will be useful for other
SoCs besides Tegra and OMAP4.

> Did you have the opportunity to measure the power consumption with and
> without this patchset ?

Power consumption will be very dependent on the specific SoC in
question.  The most important factors are the power savings of the
independent cpuidle state (normally WFI) vs. the hotplug state
(normally 1 cpu in OFF), and the workload being tested.

On a very idle system, these patches result in the same total power as
hotplugging one cpu and letting the other idle normally.  On a 25%
busy system, you might see a slight increase in power, as the best
independent cpuidle state might be WFI, vs 1 cpu in OFF mode in
hotplug.  On OMAP4, that difference is small, on the order of 10 mW.
Once you hit the threshold where a hotplug governor would have
hotplugged in the second cpu (lets say 40%), the savings from these
patches are enormous, as you can hit the lowest power state up to 60%
of the time, where the hotplug solution would never be going below WFI
on both cpus.  On OMAP4, that can be well over 100 mW.
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