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

[Ingo Molnar <mingo@elte.hu>]

* 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?

	Ingo