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    SubjectRe: [RFC PATCH v2 1/4] cpuidle: (powerpc) Add cpu_idle_wait() to allow switching of idle routines
    On 11/29/2011 02:05 AM, Benjamin Herrenschmidt wrote:

    > On Mon, 2011-11-28 at 16:32 +0530, Deepthi Dharwar wrote:
    >>> Additionally, I'm a bit worried (but maybe we already discussed that a
    >>> while back, I don't know) but cpu_idle_wait() has "wait" in the name,
    >>> which makes me think it might need to actually -wait- for all cpus to
    >>> have come out of the function.
    >> cpu_idle_wait is used to ensure that all the CPUs discard old idle
    >> handler and update to new one. Required while changing idle
    >> handler on SMP systems.
    >>> Now your implementation doesn't provide that guarantee. It might be
    >>> fine, I don't know, but if it is, you'd better document it well in the
    >>> comments surrounding the code, because as it is, all you do is shoot an
    >>> interrupt which will cause the target CPU to eventually come out of idle
    >>> some time in the future.
    >> I was hoping that sending an explicit reschedule to the cpus would
    >> do the trick but sure we can add some documentation around the code.
    > Well, the question is what guarantee do you expect. Sending a reschedule
    > IPI will take the other CPUs out of the actual sleep mode, but it will
    > be some time from there back to getting out of the handler function
    > (first back out of hypervisor etc...).
    > The code as you implemented it doesn't wait for that to happen. It might
    > be fine ... or not. I don't know what semantics you are after precisely.
    > Cheers,
    > Ben.

    Yes, this could be problematic as there is small window for the
    race condition to occur . Otherwise we need to manually schedule
    it by running a kernel thread but this would definitely have a
    overhead and would be an overkill.


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