linux-kernel - Re: [Bug #11989] Suspend failure on NForce4-based boards due to chanes in stop

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Message-ID: <20081112165118.GA30743@osiris.boeblingen.de.ibm.com>
Date:	Wed, 12 Nov 2008 17:51:18 +0100
From:	Heiko Carstens <heiko.carstens@...ibm.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Ingo Molnar <mingo@...e.hu>, "Rafael J. Wysocki" <rjw@...k.pl>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Kernel Testers List <kernel-testers@...r.kernel.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Vegard Nossum <vegard.nossum@...il.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Oleg Nesterov <oleg@...hat.com>,
	Dmitry Adamushko <dmitry.adamushko@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Steven Rostedt <srostedt@...hat.com>, manfred@...orfullife.com
Subject: Re: [Bug #11989] Suspend failure on NForce4-based boards due to
	chanes in stop_machine

On Wed, Nov 12, 2008 at 08:03:49AM -0800, Paul E. McKenney wrote:
> On Wed, Nov 12, 2008 at 10:05:08AM +0100, Heiko Carstens wrote:
> > On Tue, Nov 11, 2008 at 09:34:51AM -0800, Paul E. McKenney wrote:
> > > > Were you using preemptible RCU earlier as well?  Raphael was using
> > > > classic RCU.  Don't get me wrong, all problems need fixing, just trying
> > > > to make sure I understand where the problems are occurring.
> > 
> > Indeed, my fault. I just try to reproduce a cpu hotplug bug with classic RCU
> > and cpu hotplug stress test, but no luck so far.
> 
> OK, then my next step will be to send Rafael an updated version of
> my hierarchical RCU, which is more robust than classic RCU against
> online/offline stress tests.  On the machines I have access to, anyway.  ;-)
> 
> Then I will look at preemptable RCU, which undoubtably needs some of the
> same help that I have been giving to hierarchical RCU.  Manfred thus
> wins the clairvoyance award!

Well, I tried all day long to reproduce a cpu hotplug/stop_machine hang
with classic RCU and a kernel configuration that is as close as possible
to Raphael's configuration, but it just continues to work without a bug.

One of the machines is a virtual machine with 8 virtual cpus mapped on
two real cpus. The real cpus are again shared with other guests. So I end
up with cpu steal times of 50-90%. That should have revealed races in the
stop_machine code, considering that thousands of cpu hotplug operations
happened.

I let these test machines running over night. Maybe something happens...
but at a first glance it looks more like the reworked stop_machine code
triggers a different bug that already is present. Hmmm...
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