Re: [regression bisected] HR-timers bug >=2.6.25

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Fri, 2008-05-16 at 10:26 -0700, Justin Madru wrote:
> Peter Zijlstra wrote:
> > The kernel doesn't hang, right? My reading of the description tells me
> > the splash just doesn't work.
> >
> > If it does hang the kernel, an NMI trace collected over serial or
> > netconsole (regular console being out of the question since its
> > graphical stuff :/) would be most helpful.
> 
> I've seen it blank out in 2 different ways.
> 
> 1) Usplash completely finishes. Then screen goes blank for the usual mode change/screen refreshing,
> doing several refreshes of a blank screen. But, the screen then stays black.
> I do still hear the gdm sound, and can switch to a console.
> Pressing ctrl+alt+f1 makes the screen "refresh" again, but stay blank/black. I can still login but blindly.
> 
> 2) More rarely. Usplash hasn't finished (but near the end). The usplash screen fades out to black.
> It's like a screen burn in, or after image; the screen slowly fades out to black.
> After this the computer is _seemingly_unresponsive_ - Only alt+sysrq+b seems to work. (I have to hard reset)
> After I reboot the backlight is at the lowest level.
> 
> Anyways, I think I can figure out how to set up a netconsole, but
> is there something more to setup a NMI trace? I found Documentation/networking/netconsole.txt
> But, it doesn't say anything about a NMI trace.

those can be obtained by adding:

  nmi_watchdog=[12]

to the kernel boot parameters - it depends a bit on the hardware which
of the two choices works best, just start with 1 and if that doesn't
work try 2.

This enabled the NMI watchdog and that will print a backtrace when it
times out after 30 or so seconds.