linux-kernel - Re: [PATCH] ftrace based hard lockup detector

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Date:	Mon, 19 Jan 2009 14:04:42 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ftrace based hard lockup detector


* Steven Rostedt <rostedt@...dmis.org> wrote:

> On Sun, 18 Jan 2009, Frederic Weisbecker wrote:
> 
> > Like the NMI watchdog, this feature try to detect hard lockups by
> > lurking at the non-progress of the timer interrupts.
> > 
> > You can enable it at boot time by passing the ftrace_hardlockup parameter.
> > I plan to add a debugfs file to enable/disable at runtime.
> > 
> > When a hardlockup is detected, it will print a backtrace. Perhaps it
> > would be good to print the locks held from lockdep too?
> > 
> > It only support x86 for the moment, because a kind of generic timer interrupt
> > counter is needed on all archs to have it generic.
> > 
> > Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> 
> Hi Frederic,
> 
> This seems like a rewrite of the NMI lockup code. In my debugging, I 
> simply put ftrace_dump in the NMI lockup, which gives me a ftrace dump 
> as soon as NMI detects a lockup. I'm a bit confused at what this gives 
> us over that?

this is different from the NMI watchdog in a number of ways:

 - it works on all platforms and in all situations where the NMI watchdog 
   does not work.

 - in theory it can detect hard lockups in situations where the NMI 
   watchdog is disabled, such as suspend/resume or early bootup. 
   (especially early bootup lockups are nasty and the NMI watchdog is 
    enabled relatively late)

 - it could be extended to detect 'soft' lockups too - i.e. we could have 
   a one-stop facility to detect all kinds of "kernel does not seem to 
   progress" lockups.

But it's not as complete as the NMI watchdog: it relies on instrumented 
function calls rolling on and on during the lockup - that's not the case 
when we get a hard lockup due to a tight, infinite loop somewhere.

	Ingo
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