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

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Message-ID: <20100329101106.3678a312@penta.localdomain>
Date:	Mon, 29 Mar 2010 10:11:06 -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

On Sun, 28 Mar 2010 18:00:36 -0700
john stultz <johnstul@...ibm.com> wrote:

> > 1) Does getrawmonotonic() satisfy hangcheck-timer?  What I mean is, will
> > it always return the wallclock nanoseconds even in the face of CPU speed
> > changes, suspend, udelay, or any other suspension of kernel operation?
> > Yes, I know this is a tougher standard than rdtsc(), but this is what
> > hangcheck-timer wants.  rdtsc() at least satisfied udelay and PCI hangs.
> 
> getrawmonotonic() can be stalled and will wrap on some hardware (acpi pm
> timer wraps every 5 seconds).
> 

I am not sure which archs do you mean. But in any case,
getrawmonotonic() is not just a wrap around a call to rdtsc() (or acpi
pm timer read). It is based on the clock->raw_time, which is updated
every timer interrupt by the update_wall_time(). So even if underlying
timer wraps, it doesn't lead to getrawmonotonic() returning 0 sec.


Y

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