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

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Date:	Fri, 16 May 2008 09:20:58 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Justin Madru <jdm64@...ab.com>, Ingo Molnar <mingo@...e.hu>,
	David Airlie <airlied@...ux.ie>,
	Sylvain Meyer <sylvain.meyer@...ldonline.fr>,
	Antonino Daplas <adaplas@...il.com>,
	Roman Zippel <zippel@...ux-m68k.org>, Greg KH <gregkh@...e.de>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [regression bisected] HR-timers bug >=2.6.25

On Fri, 2008-05-16 at 09:15 +0200, Thomas Gleixner wrote:
> On Thu, 15 May 2008, Justin Madru wrote:
> > I've reported this problem previously:
> > http://lkml.org/lkml/2008/3/12/290
> > http://bugzilla.kernel.org/show_bug.cgi?id=10235
> > https://bugs.freedesktop.org/show_bug.cgi?id=15602
> > 
> > This bug is still in mainline as of 2.6.26-rc1. (affected versions 2.6.25-rc0
> > - 2.6.26-rc1)
> > 
> > Ever since 2.6.25-rc3 (the first rc I tested), my screen would blank out when
> > mode switching from the Ubuntu uplash to the gdm login. Normally after the
> > boot splash, the screen goes black and "refreshes" to select the correct
> > resolution. But when using a kernel newer than 2.6.24 the screen doesn't come
> > back on - it stays blank. This doesn't happen every time, but a good majority
> > of the time. The workaround is to disable the splash, and that fixes the
> > problem almost completely.
> > 
> > I did a git bisect. Rebooting 3 times if the problem didn't occur just to make
> > sure.
> > The git bisect reports that it's timing related - which is what Jesse Barnes
> > said it could be.
> 
> Well, it's a timing problem related to splash which is exposed by the
> scheduler timing changes. This is neither a hrtimer nor a scheduler
> bug. Something in the splash / video driver switchover relies on some
> obscure timing which is nowhere guaranteed.

@Justin:

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.

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