On 20 January 2012 21:40, Colin Cross [off-list ref] wrote:

On Fri, Jan 20, 2012 at 12:46 AM, Daniel Lezcano
[off-list ref] wrote:

quoted

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,

In your patch, you put in safe state (WFI for most of platform) the
cpus that become idle and these cpus are woken up each time a new cpu
of the cluster becomes idle. Then, the cluster state is chosen and the
cpus enter the selected C-state. On ux500, we are using another
behavior for synchronizing  the cpus. The cpus are prepared to enter
the c-state that has been chosen by the governor and the last cpu,
that enters idle, chooses the final cluster state (according to cpus'
C-state). The main advantage of this solution is that you don't need
to wake other cpus to enter the C-state of a cluster. This can be
quite worth full when tasks mainly run on one cpu. Have you also think
about such behavior when developing the coupled cpuidle driver ? It
could be interesting to add such behavior.

Regards,
Vincent

quoted

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.
_______________________________________________
linux-pm mailing list
linux-pm at lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/linux-pm