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    SubjectRe: [RFC patch] Use IPI_shortcut for lapic timer broadcast
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    On Mon, Jun 29, 2009 at 4:16 PM, Ingo Molnar<mingo@elte.hu> wrote:
    >
    > * Luming Yu <luming.yu@gmail.com> wrote:
    >
    >> On Mon, Jun 29, 2009 at 3:20 PM, Ingo Molnar<mingo@elte.hu> wrote:
    >> >
    >> > * Luming Yu <luming.yu@gmail.com> wrote:
    >> >
    >> >> Hello,
    >> >>
    >> >> We need to use IPI shortcut to send lapic timer broadcast
    >> >> to avoid the latency of sending IPI one bye one on systems with many
    >> >> logical processors when NO_HZ is disabled.
    >> >> Without this patch,I have seen upstream kernel with RHEL 5 kernel
    >> >> config boot hang .
    >> >
    >> > hm, that might be a valid optimization - but why does the lack of
    >> > this optimization result in a hang?
    >>
    >> It is hang caused by kernel code for work around lapic-timer-stop
    >> issue. With HZ=1000, and a lot of cpus (eg. 64 logical cpus), cpu
    >> 0 will be busy working on send TIMER IPI instead of making
    >> progress in boot (right after deep-C-state has been used).
    >
    > that's a bit weird. With HZ=1000 we have 1000 usecs between each
    > timer tick. Assuming a CPU sends to a lot of CPUs (64 logical CPUs)
    > that means that each IPI takes more than ~15 microseconds to
    > process. On what hardware/platform can this happen realistically?

    https://bugzilla.redhat.com/show_bug.cgi?id=499271

    Someone has measured that it needs 50-100us latency to send one IPI

    >
    >        Ingo
    >
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