From mboxrd@z Thu Jan  1 00:00:00 1970
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Subject: Re: rcu self-detected stall messages on OMAP3, 4 boards
Date: Sat, 22 Sep 2012 09:00:31 -0700
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To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Tony Lindgren <tony@atomide.com>, Paul Walmsley <paul@pwsan.com>, "Bruce, Becky" <bbruce@ti.com>, "Paul E. McKenney" <paul.mckenney@linaro.org>, "<linux-kernel@vger.kernel.org>" <linux-kernel@vger.kernel.org>, "<linux-omap@vger.kernel.org>" <linux-omap@vger.kernel.org>, "<linux-arm-kernel@lists.infradead.org>" <linux-arm-kernel@lists.infradead.org>, "Hilman, Kevin" <khilman@ti.com>, "Shilimkar, Santosh" <santosh.shilimkar@ti.com>, "Hunter, Jon" <jon-hunter@ti.com>, "<snijsure@grid-net.com>" <snijsure@grid-net.com>
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On Sat, Sep 22, 2012 at 05:45:12PM +0200, Frederic Weisbecker wrote:
> 2012/9/22 Paul E. McKenney <paulmck@linux.vnet.ibm.com>:
> > On Fri, Sep 21, 2012 at 01:31:49PM -0700, Tony Lindgren wrote:
> >> * Paul E. McKenney <paulmck@linux.vnet.ibm.com> [120921 12:58]:
> >> >
> >> > Just to make sure I understand the combinations:
> >> >
> >> > o   All stalls have happened when running a minimal userspace.
> >> > o   CONFIG_NO_HZ=n suppresses the stalls.
> >> > o   CONFIG_RCU_FAST_NO_HZ (which depends on CONFIG_NO_HZ=y) has
> >> >     no observable effect on the stalls.
> >>
> >> The reason why you may need minimal userspace is to cut down
> >> the number of timers waking up the system with NO_HZ.
> >> Booting with init=/bin/sh might also do the trick for that.
> >
> > Good point!  This does make for a very quiet system, but does not
> > reproduce the problem under kvm, even after waiting for four minutes.
> > I will leave it for more time, but it looks like I really might need to
> > ask Linaro for remote access to a Panda.
> 
> I have one. I'm currently installing Ubuntu on it and I'll try to
> manage to build
> a kernel and reproduce the issue.
> 
> I'll give more news soon.

Thank you!

My bet is that you have to have a userspace that is so small that it
registers only a few (but at least one!) RCU callback at boot time,
then never registers any callbacks ever again.  I have coded up a
crude test case, using Tony Lindgren's suggestion of "init=/bin/sh",
but I appear to have inadvertently fixed this bug in current -rcu
(git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git,
branch rcu/next).

But I have been wrong a few times already on this particular bug...

							Thanx, Paul