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

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Date:	Mon, 29 Mar 2010 12:53:20 -0700
From:	Joel Becker <Joel.Becker@...cle.com>
To:	john stultz <johnstul@...ibm.com>
Cc:	Yury Polyanskiy <ypolyans@...nceton.edu>,
	linux-kernel@...r.kernel.org, Andrew Morton <akpm@...l.org>,
	Jan Glauber <jan.glauber@...ibm.com>
Subject: Re: [PATCH] hangcheck-timer is broken on x86

On Mon, Mar 29, 2010 at 11:44:51AM -0700, john stultz wrote:
> > But if timer interrupt is delayed by more than acpi_pm wrap-around
> > time, then the update_wall_time() is also screwed. Since it is not, we
> > can rely on getrawmonotonic().
> 
> Right, if the box hangs for longer then the clocksource can count for,
> the timekeeping subsystem will be off by some multiple of that length.
> 
> And That's exactly why I'm advising against using
> gettimeofday/getrawmonotonic or any other software managed sense of time
> for the hangcheck timer, as you won't be able to correctly detect hangs.
> 
> I'm also suggesting using something like read_persistent_clock() is
> better, because there is no OS/software management involved (other then
> the minor syncing issue I mentioned before) so if the system hangs for a
> long period of time, then returns, you'll still be able to detect the
> hang.
> 
> But maybe what folks are using the hangcheck timer for is shifting, so
> its possible that I'm not quite understanding what you're trying to do
> here.

	The people who use hangcheck-timer for the reasons I originally
wrote it absolutely want any hang, including long ones, detected.

Joel

-- 

"For every complex problem there exists a solution that is brief,
     concise, and totally wrong."
                                        -Unknown

Joel Becker
Principal Software Developer
Oracle
E-mail: joel.becker@...cle.com
Phone: (650) 506-8127
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