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

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Message-ID: <20090629081615.GB571@elte.hu>
Date:	Mon, 29 Jun 2009 10:16:15 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Luming Yu <luming.yu@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, suresh.b.siddha@...el.com,
	venkatesh.pallipadi@...el.com,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [RFC patch] Use IPI_shortcut for lapic timer broadcast


* Luming Yu <luming.yu@...il.com> wrote:

> On Mon, Jun 29, 2009 at 3:20 PM, Ingo Molnar<mingo@...e.hu> wrote:
> >
> > * Luming Yu <luming.yu@...il.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
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