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

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Message-ID: <3877989d0906290104q11a767b6qcc1acc4c2d1feba6@mail.gmail.com>
Date:	Mon, 29 Jun 2009 16:04:35 +0800
From:	Luming Yu <luming.yu@...il.com>
To:	Ingo Molnar <mingo@...e.hu>
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

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).




>
>        Ingo
>
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