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

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Date:	Sat, 27 Mar 2010 18:51:01 -0400
From:	Yury Polyanskiy <ypolyans@...nceton.edu>
To:	Yury Polyanskiy <ypolyans@...nceton.edu>,
	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 6:03 PM, Joel Becker <Joel.Becker@...cle.com> wrote:
> On Fri, Mar 26, 2010 at 10:02:59PM -0400, Yury Polyanskiy wrote:
>> Joel, just realized there is a slight mistake in what I said before.
>> getrawmonotonic() is a refined jiffies (and actually resolves to
>> get_cycles() on my system in the end). Thus it doesn't count while in
>> suspend. However, jiffies-based timers (aka timer-wheel) are also
>> stopped while in suspend. So getrawmonotonic() is the right call to
>> check the precision of the jiffies-based timer (i.e. you dont need to
>> make a correction by calling monotonic_to_bootbased()).
>
>        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).

>        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()).


Yury
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