linux-kernel - Re: [patch 1/4] Ignore stolen time in the softlockup watchdog

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Date:	Mon, 23 Apr 2007 23:58:20 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	virtualization@...ts.osdl.org, Prarit Bhargava <prarit@...hat.com>,
	Eric Dumazet <dada1@...mosbay.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	john stultz <johnstul@...ibm.com>,
	Zachary Amsden <zach@...are.com>,
	James Morris <jmorris@...ei.org>,
	Dan Hecht <dhecht@...are.com>,
	Paul Mackerras <paulus@...ba.org>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Chris Lalancette <clalance@...hat.com>,
	Rick Lindsley <ricklind@...ibm.com>
Subject: Re: [patch 1/4] Ignore stolen time in the softlockup watchdog

Andrew Morton wrote:
> On Tue, 27 Mar 2007 14:49:20 -0700 Jeremy Fitzhardinge <jeremy@...p.org> wrote:
>
>   
>> The softlockup watchdog is currently a nuisance in a virtual machine,
>> since the whole system could have the CPU stolen from it for a long
>> period of time.  While it would be unlikely for a guest domain to be
>> denied timer interrupts for over 10s, it could happen and any softlockup
>> message would be completely spurious.
>>
>> Earlier I proposed that sched_clock() return time in unstolen
>> nanoseconds, which is how Xen and VMI currently implement it.  If the
>> softlockup watchdog uses sched_clock() to measure time, it would
>> automatically ignore stolen time, and therefore only report when the
>> guest itself locked up.  When running native, sched_clock() returns
>> real-time nanoseconds, so the behaviour would be unchanged.
>>
>> Note that sched_clock() used this way is inherently per-cpu, so this
>> patch makes sure that the per-processor watchdog thread initialized
>> its own timestamp.
>>     
>
> This patch
> (ftp://ftp.kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.21-rc6/2.6.21-rc6-mm1/broken-out/ignore-stolen-time-in-the-softlockup-watchdog.patch)
> causes six failures in the locking self-tests, which I must say is rather
> clever of it.
>   

Interesting.  Which variation of sched_clock do you have in your tree at
the moment?

    J
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