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

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Message-ID: <20100327233630.GB1810@mail.oracle.com>
Date:	Sat, 27 Mar 2010 16:36:30 -0700
From:	Joel Becker <Joel.Becker@...cle.com>
To:	Yury Polyanskiy <ypolyans@...nceton.edu>
Cc:	linux-kernel@...r.kernel.org, Andrew Morton <akpm@...l.org>,
	john stultz <johnstul@...ibm.com>,
	Jan Glauber <jan.glauber@...ibm.com>
Subject: Re: [PATCH] hangcheck-timer is broken on x86

On Sat, Mar 27, 2010 at 06:51:01PM -0400, Yury Polyanskiy wrote:
> >        It's OK to tell hangcheck-timer users that suspend is not
> > allowed.  After all, you're running something that you don't want to see
> > hang.
> 
> Joel, what I am saying is exactly the opposite: it is totally ok to
> suspend-resume with hangcheck-timer (jiffies are stopped and so is
> getrawmonotonic() when system suspended).

	Nope.  The point of hangcheck-timer is that it reboots should
the system not be running for a certain amountof time.  If
suspend-resume is allowed, a system can resume after days and think it
wasn't more than a second.  hangcheck-timer will not know to reboot.

> >        Is there a clock in the system that is a true wallclock?  I'm
> > guessing, since getrawmonotonic() is get_cycles() based, that it doesn't
> > provide accurate time in the face of cpufreq changes.  Is that true?
> 
> Of course, getrawmonotonic accounts for cpufreq changes (see
> arch/x86/kernel/tsc.c:time_cpufreq_notifier()).

	Excellent!  That's a definite improvement over raw get_cycles().

Joel

-- 

Life's Little Instruction Book #182

	"Be romantic."

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