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

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Message-ID: <462E6CB7.9070403@goop.org>
Date:	Tue, 24 Apr 2007 13:46:47 -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>, Andi Kleen <ak@...e.de>
Subject: Re: [patch 1/4] Ignore stolen time in the softlockup watchdog

Andrew Morton wrote:
> On Tue, 24 Apr 2007 13:00:49 -0700 Jeremy Fitzhardinge <jeremy@...p.org> wrote:
>
>   
>> Andrew Morton wrote:
>>     
>>> Well, it _is_ mysterious.
>>>
>>> Did you try to locate the code which failed?  I got lost in macros and
>>> include files, and gave up very very easily.  Stop hiding, Ingo.
>>>   
>>>       
>> OK, I've managed to reproduce it.  Removing the local_irq_save/restore
>> from sched_clock() makes it go away, as I'd expect (otherwise it would
>> really be magic).
>>     
>
> erm, why do you expect that?  A local_irq_save()/local_irq_restore() pair
> shouldn't be affecting anything?
>   

Well, yes.  I have no idea why it causes a problem.  But other than
that, sched_clock does absolutely nothing which would affect lockdep state.

>>  But given that it never seems to touch the softlockup
>> during testing, I have no idea what difference it makes...
>>     
>
> To what softlockup are you referring, and what does that have to do with
> anything?

You dropped this patch, "Ignore stolen time in the softlockup watchdog"
because its presence triggers the lock tester errors.  The only thing
this patch does is use sched_clock() rather than jiffies to measure
lockup time.  It therefore appears, for some reason, that using
sched_clock() in the softlockup code is making the lock-test fail. 
Since the lock test doesn't explicitly do any softlockup stuff, the
connection must be implicit via sched_lock - but how, I can't imagine.

Since sched_clock() itself looks perfectly OK, and the softlockup
watchdog seems fine too, I can only conclude its a bug in the lock
testing stuff.  But I don't know what.

    J

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