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    SubjectRe: [PATCH 0/3] coupled cpuidle state support
    On 12/21/2011 10:55 AM, Colin Cross wrote:
    > On Wed, Dec 21, 2011 at 1:44 AM, Arjan van de Ven <arjan@linux.intel.com> wrote:
    >> On 12/21/2011 10:40 AM, Colin Cross wrote:
    >>
    >>>> this smells fundamentally racey to me; you can get an interrupt one
    >>>> cycle after you think you're done, but before the last guy enters WFI...
    >>>>
    >>>> how do you solve that issue ?
    >>>
    >>> All the cpus have interrupts off when they increment the counter, so
    >>> they cannot receive an interrupt. If an interrupt is pending on one
    >>> of those cpus, it will be handled later when WFI aborts due to the
    >>> pending interrupt.
    >>
    >> ... but this leads to cases where you're aborting before other cpus are
    >> entering..... so your "last guy in" doesn't really work, since while cpu
    >> 0 thinks it's the last guy, cpu 1 is already on the way out/out
    >> already... (heck it might already be going back to sleep if your idle
    >> code can run fast, like in the size of a cache miss)
    >
    > Once a cpu has incremented the counter, it has no way out unless either
    > 1: another cpu (that hasn't incremented the counter yet) receives an
    > interrupt, aborts idle, and clears its idle flag
    > or
    > 2: all cpus enter the ready counter, and call the cpuidle driver's
    > enter function.

    .. or it enters WFI, and a physical device sends it an interrupt,
    at which point it exits.


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