Messages in this thread | | | Date | Mon, 29 Jun 2009 16:21:59 +0800 | Subject | Re: [RFC patch] Use IPI_shortcut for lapic timer broadcast | From | Luming Yu <> |
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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 > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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