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    SubjectRe: [PATCHv3 0/5] coupled cpuidle state support
    On Tue, May 1, 2012 at 3:43 AM, Lorenzo Pieralisi
    <> wrote:
    > Hi Colin,
    > On Mon, Apr 30, 2012 at 10:37:30PM +0100, Colin Cross wrote:

    >> On Tegra3, the deepest individual cpu state for cpus 1-3 is OFF, the
    >> same state the cpu would go into as the first step of a transition to
    >> a deeper power state (cpus 0-3 OFF).  It would be more optimal in that
    >> case to bypass the SMP cross call, and leave the cpu in OFF, but that
    >> would require some way of disabling all wakeups for the secondary cpus
    >> and then verifying that they didn't start waking up just before the
    >> wakeups were disabled.  I have just started considering this
    >> optimization, but I don't see anything in the existing code that would
    >> prevent adding it later.
    > I agree it is certainly an optimization that can be added later if benchmarks
    > show it is needed (but again it is heavily platform dependent, ie technology
    > dependent).
    > On a side note, disabling (or move to the primary) wake-ups for "secondaries"
    > on platforms where every core is in a different power domain is still needed
    > to avoid having a situation where a CPU can independently get out of idle, ie
    > abort idle, after hitting the coupled barrier.
    > Still do not know if for those platforms coupled C-states should be used, but
    > it is much better to have a choice there IMHO.

    Yes, that is the primary need for the coupled_cpuidle_parallel_barrier
    function - secondary cpus need to disable their wakeup sources, then
    check that a wakeup was not already pending and abort if necessary.

    > I have also started thinking about a cluster or multi-CPU "next-event" that
    > could avoid triggering heavy operations like L2 cleaning (ie cluster shutdown)
    > if a timer is about to expire on a given CPU (as you know CPUs get in and out
    > of idle independently so the governor decision at the point the coupled state
    > barrier is hit might be stale).

    It would be possible to re-check the governor to decide the next state
    (maybe only if the previous decision is out of date by more than the
    target_residency?), but I left that as an additional optimization.

    > I reckon the coupled C-state concept can prove to be an effective one for
    > some platforms, currently benchmarking it.
    >> A simple measurement using the tracing may show that it is
    >> unnecessary.  If the wakeup time for CPU1 to go from OFF to active is
    >> small there might be no need to optimize out the extra wakeup.
    > Indeed, it is all about resetting the CPU and getting it started, with
    > inclusive L2 the power cost of shutting down a CPU and resuming it should be
    > low (and timing very fast) for most platforms.

    The limiting factor may be the amount of time spent in ROM/Trustzone
    code when bringing a cpu back online.
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