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    SubjectRe: [linux-pm] [PATCH 0/3] coupled cpuidle state support
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    On 20 January 2012 21:40, Colin Cross <ccross@android.com> wrote:
    > On Fri, Jan 20, 2012 at 12:46 AM, Daniel Lezcano
    > <daniel.lezcano@linaro.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,

    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

    >> 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@lists.linux-foundation.org
    > https://lists.linuxfoundation.org/mailman/listinfo/linux-pm
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