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    SubjectRe: Does smp_reschedule_interrupt really reschedule?

    * Steven Rostedt <> wrote:

    > As the comment says, do nothing since all the work is automatically
    > done at the return from interrupt. But is it? Doesn't the
    > need_resched need to be set? Here's what I'm seeing with Ingo's
    > kernel. I capture the time in sched.c when the
    > smp_send_reschedule_allbutself is called, and also a capture of the
    > time when the schedule actually takes place. I'm finding differences
    > up to 2 tenths of a second. That's TENTHS! I added the following
    > patch:

    it's all a bit tricky. The short story is that i think both vanilla and
    -RT kernels are fine.

    Here is how smp_send_reschedule() is used:

    CPU#0 CPU#1

    --- IPI --->
    entry.S's need_resched check

    _but_, this is intentionally racy: if CPU#1 happens to reschedule before
    the IPI reaches CPU#1 (an IPI can take 10 usecs easily so the window is
    not small), then need_resched might be cleared before the IPI hits. In
    that case you wont get a reschedule after the IPI hits, because it was
    done before!

    so the correct thing to measure is what the -RT kernel's wakeup-latency
    timing feature does: the time from setting need_resched, to the point
    the task starts to run. The feature works on SMP too - and it doesnt
    show any large latencies.

    are you seeing actual process delays? If not then i think those large
    latencies are just the result of the wrong assumptions in your
    measurement code.

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      Last update: 2005-05-13 20:31    [W:0.021 / U:6.012 seconds]
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