linux-kernel - Re: [PATCH] hangcheck-timer is broken on x86

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Message-ID: <20100326175201.3dbac6ce@penta.localdomain>
Date:	Fri, 26 Mar 2010 17:52:01 -0400
From:	Yury Polyanskiy <ypolyans@...nceton.edu>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Joel Becker <joel.becker@...cle.com>, linux-kernel@...r.kernel.org,
	john stultz <johnstul@...ibm.com>,
	Jan Glauber <jan.glauber@...ibm.com>
Subject: Re: [PATCH] hangcheck-timer is broken on x86

On Fri, 26 Mar 2010 14:24:23 -0700
Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Tue, 23 Mar 2010 23:36:11 -0400
> Yury Polyanskiy <ypolyans@...nceton.edu> wrote:
> 
> > The drivers/char/hangcheck-timer.c is doubly broken. First, the
> > following line overflows unsigned long:
> > # define TIMER_FREQ (HZ*loops_per_jiffy)
> > 
> > Second, and more importantly, loops_per_jiffy has little to do with the conversion from the
> > the time scale of get_cycles() (aka rdtsc) to the time scale of jiffies.
> 
> It's a bit odd to have a driver be this broken on x86_32 for five years
> without anyone noticing.  What are the user-visible effects of these
> shortcomings?

When the overflown value of TIMER_FREQ is abnormally low, it spams the
syslog with KERN_CRIT messages "Hangcheck: hangcheck value past margin!"
But whether it happens or not depends on HZ and lpj in a complex way.
People have hit it occasionally as far as google search can tell. 

> 
> Also, please do send us a Signed-off-by: for this patch, as explained
> in Documentation/SubmittingPatches, thanks.
> 

Sorry.

Signed-off-by: Yury Polyanskiy <polyanskiy@...il.com>

Thank you Andrew!

Yury

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