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From: Denis Vlasenko <vda@port.imtp.ilyichevsk.odessa.ua>
Reply-To: vda@port.imtp.ilyichevsk.odessa.ua
To: Robert de Bath <robert$@mayday.cix.co.uk>
Subject: Re: [PATCH *] use 64 bit jiffies
Date: Tue, 4 Feb 2003 10:41:31 +0200
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Cc: Matti Aarnio <matti.aarnio@zmailer.org>,
       Tim Schmielau <tim@physik3.uni-rostock.de>,
       lkml <linux-kernel@vger.kernel.org>
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On 4 February 2003 10:27, Robert de Bath wrote:
> On Tue, 4 Feb 2003, Denis Vlasenko wrote:
> > 		Jiffy Wrap Bugs
> >
> > There were reports of machines hanging on jiffy wrap.
> > This is typically a result of incorrect jiffy use in some driver.
> > Ask Tim - he is hunting those problems regularly, but he is
> > outnumbered by buggy driver authors. :(
> >
> > There is a better solution to ensure correct jiffy wrap handling in
> > *ALL* kernel code: make jiffy wrap in first five minutes of uptime.
> > Tim has a patch for such config option. This is almost right.
> > This MUST NOT be a config option, it MUST be mandatory in every
> > kernel. Driver writers would be bitten by their own bugs and will
> > fix it themself. Tim, what do you think?
>
> How about an option, either the jiffy timer wraps at 5 minutes or
> process 1 gets sent a SIGINT after 24 hours ? That way a driver
> with an MIA author can still be used even if it's buggy, just not
> for very long.

I prefer buggy driver be fixed ASAP. The first step is to know
that there *is* a (jiffywize-) buggy driver or something.

Jiffie wrap at 5 mins is done this way: jiffies is initialized to -5*60*HZ
instead of zero at boot. Accounting code is adjusted e.g. not to show 400+
days uptime (it subtracts -5*60*HZ from jiffies).

Jiffies wraps to zero in five minutes. Box should survive with no probs.
If it instead hangs, oops or otherwise feeling bad, you have a jiffy wrap
bug somewhere.

Today you need to wait 400 or so days before you can test what will happen.
Production servers' admins getting a bit nervous close to that date ;)

A nice printk "Timer code check..." at jiffies = -30*HZ
and "...timer code check passed" at jiffies = 30*HZ will let us have
good bug reports ("what was your last log message?") and would not
scare people. "A jiffie would wrap in 5 seconds!" is not that good -
please do not unnecessarily scare new users ;)
--
vda
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