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

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Date:	Wed, 25 Jan 2012 15:04:18 +0100
From:	Daniel Lezcano <daniel.lezcano@...e.fr>
To:	Colin Cross <ccross@...roid.com>
CC:	Daniel Lezcano <daniel.lezcano@...aro.org>,
	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 01/20/2012 09:40 PM, Colin Cross wrote:
> 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.

Hi Colin,

I will be happy to test your patchset. Do you have a pointer to a more 
recent kernel ?


>> 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.

Interesting.


Thanks

   -- Daniel

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