Re: [RFC patch] Use IPI_shortcut for lapic timer broadcast

[Luming Yu]

On Mon, Jun 29, 2009 at 4:30 PM, Ingo Molnar<mingo@xxxxxxx> wrote:
>
> * Luming Yu <luming.yu@xxxxxxxxx> wrote:
>
>> On Mon, Jun 29, 2009 at 4:16 PM, Ingo Molnar<mingo@xxxxxxx> wrote:
>> >
>> > * Luming Yu <luming.yu@xxxxxxxxx> wrote:
>> >
>> >> On Mon, Jun 29, 2009 at 3:20 PM, Ingo Molnar<mingo@xxxxxxx> wrote:
>> >> >
>> >> > * Luming Yu <luming.yu@xxxxxxxxx> 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
>
> Ugh. What platform is it that takes this much time to pass an IPI?
>
> IPIs are the lifeline of process messaging under Linux. TLB flushes
> in threaded apps rely on it (heavily), the scheduler relies on it
> for wakeups (heavily) and a lot of other code relies on IPIs as
> well.
>
> Even a Pentium-5 100 MHz dual box was able to do cross-CPU IPIs
> within 10-20 microseconds more than a decade ago - so 50-100 usecs
> latency on a modern platform is totally out of this planet and will
> hurt Linux performance big time. And the worst thing about it is
> that none of the usual performance metrics will really show _why_
> performance is tanking ...
>


Please note this is deep-C-state related.
C state does add extra latency.. but I don't know how much...
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