linux-kernel - Re: [patch 1/2] Ignore stolen time in the softlockup watchdog

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Date:	Tue, 27 Mar 2007 00:12:53 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Eric Dumazet <dada1@...mosbay.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	virtualization@...ts.osdl.org, Ingo Molnar <mingo@...e.hu>,
	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>,
	Prarit Bhargava <prarit@...hat.com>,
	Chris Lalancette <clalance@...hat.com>,
	Rick Lindsley <ricklind@...ibm.com>
Subject: Re: [patch 1/2] Ignore stolen time in the softlockup watchdog

Eric Dumazet wrote:
> Jeremy Fitzhardinge a écrit :
>
>> +static DEFINE_PER_CPU(unsigned long long, touch_timestamp);
>
> ...
>
>>  void touch_softlockup_watchdog(void)
>>  {
>> -    __raw_get_cpu_var(touch_timestamp) = jiffies;
>> +    __raw_get_cpu_var(touch_timestamp) = sched_clock();
>>  }
>
> Not very clear if this is safe on 32bit, since this is not anymore
> atomic.

Hm, good point.  Don't think it matters very much.  These values are
per-cpu, and if an interrupt happens between the word updates and the
intermediate values causes a timeout, then it was pretty marginal
anyway.  I guess the worst case is if the low-word gets written first,
and it goes from a high value to low, then it could be sampled as if
time had gone back by up to ~4 seconds.

I'll give it another look.

    J
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