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    SubjectRe: [PATCH RFC/TEST] sched: make sync affine wakeups work
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    Hi Rik, Mike

    On Fri, May 2, 2014 at 12:00 PM, Rik van Riel <riel@redhat.com> wrote:
    > On 05/02/2014 02:13 AM, Mike Galbraith wrote:
    >> On Fri, 2014-05-02 at 00:42 -0400, Rik van Riel wrote:
    >>
    >>> Whether or not this is the right thing to do remains to be seen,
    >>> but it does allow us to verify whether or not the wake_affine
    >>> strategy of always doing affine wakeups and only disabling them
    >>> in a specific circumstance is sound, or needs rethinking...
    >>
    >> Yes, it needs rethinking.
    >>
    >> I know why you want to try this, yes, select_idle_sibling() is very much
    >> a two faced little bitch.
    >
    > My biggest problem with select_idle_sibling and wake_affine in
    > general is that it will override NUMA placement, even when
    > processes only wake each other up infrequently...

    As far as my understanding goes, the logic in select_task_rq_fair()
    does wake_affine() or calls select_idle_sibling() only at those
    levels of sched domains where the flag SD_WAKE_AFFINE is set.
    This flag is not set at the numa domain and hence they will not be
    balancing across numa nodes. So I don't understand how
    *these functions* are affecting NUMA placements.

    The wake_affine() and select_idle_sibling() will shuttle tasks
    within a NUMA node as far as I can see.i.e. if the cpu that the task
    previously ran on and the waker cpu belong to the same node.
    Else they are not called.

    If the prev_cpu and the waker cpu are on different NUMA nodes
    then naturally the tasks will get shuttled across NUMA nodes but
    the culprits are the find_idlest* functions.
    They do a top-down search for the idlest group and cpu, starting
    at the NUMA domain *attached to the waker and not the prev_cpu*.
    This means that the task will end up on a different NUMA node.
    Looks to me that the problem lies here and not in the wake_affine()
    and select_idle_siblings().

    Regards
    Preeti U Murthy

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