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

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Message-ID: <20100329130418.2b5c068c@penta.localdomain>
Date:	Mon, 29 Mar 2010 13:04:18 -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 Mon, 29 Mar 2010 09:43:27 -0700
john stultz <johnstul@...ibm.com> wrote:

> > 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.  
> 
> 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().

Y

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