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

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Date:	Mon, 29 Mar 2010 17:08:28 -0400
From:	Yury Polyanskiy <ypolyans@...nceton.edu>
To:	john stultz <johnstul@...ibm.com>
Cc:	Joel Becker <Joel.Becker@...cle.com>, 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

>> > What I'm saying is that if you're using getrawmonotonic() to detect
>> > hangs, you might miss them, as getrawmonotonic may wrap (and thus stop
>> > continually increasing) if the timer interrupt is delayed. This does not
>> > apply to systems using the TSC clocksource, but does apply to systems
>> > using the acpi_pm.
>>
>> 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.
>

Oh, I see. You mean that getrawmonotonic() wouldn't work under
abnormal conditions. I understand now, sorry for the confusion. You
are correct, of course.

I personally don't like the idea of relying on read_persistent_clock()
not only because of hwclock and ntp. In fact, my core interest in
hangcheck-timer is to set a very low margin (1 to 3 jiffies for
example) so that I would get a log message upon any kernel slow down
or a tick-miss (as a hardware integrity check). I don't think
read_persistent_clock() is precise enough for this purpose, is it?

Also, hooking to ntp update code complicates an otherwise simple
driver. I propose to simply check on non-S390 if the clock source
resolves to something other than TSC and dump a warning message on
driver load (something like "Hangcheck: kernel using clocksource %s,
which is not reliable for hang detection").

What do you think about it?

Thanks,
Yury
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