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    SubjectRe: [RFC][PATCH 0/8] sched,idle: need resched polling rework
    On Fri, Apr 11, 2014 at 6:42 AM, Peter Zijlstra <peterz@infradead.org> wrote:
    > A while ago both Mike and Andy complained that we still get pointless wakeup
    > IPIs, we had a few patches back and forth but eventually more or less agreed
    > and then nothing... :-)
    >
    > So here's a number of patches that implement something near what we left off
    > with.
    >
    > Its only been compile/boot tested on x86_64, I've no actually looked at the IPI
    > numbers yet.
    >

    Looks generally good and it should be a nice cleanup.

    That being said, I think that this addresses once one of the two major
    issues. While the race you're fixing is more interesting, I think its
    impact is dwarfed by the fact that ttwu_queue_remote completely
    ignores polling. (NB: I haven't actually tested this patch set, but I
    did try to instrument this stuff awhile ago.)

    To fix this, presumably the wake-from-idle path needs a
    sched_ttwu_pending call, and ttwu_queue_remote could use resched_task.
    sched_ttwu_pending could benefit from a straightforward optimization:
    it doesn't need rq->lock if llist is empty.

    If you're not planning on trying to fix that, I can try to write up a
    patch in the next day or two.

    Even with all of this fixed, what happens when ttwu_queue_remote is
    called with a task that has lower priority than whatever is currently
    running on the targeted cpu? I think the result is an IPI that serves
    very little purpose other than avoiding taking a spinlock in the
    waking thread. This may be a bad tradeoff. I doubt that this matters
    for my particular workload, though.

    --Andy


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