linux-kernel - Re: rcu self-detected stall messages on OMAP3, 4 boards

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Date:	Sat, 22 Sep 2012 22:20:19 +0000 (UTC)
From:	Paul Walmsley <paul@...an.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
cc:	"Bruce, Becky" <bbruce@...com>,
	"Paul E. McKenney" <paul.mckenney@...aro.org>,
	"<linux-kernel@...r.kernel.org>" <linux-kernel@...r.kernel.org>,
	"<linux-omap@...r.kernel.org>" <linux-omap@...r.kernel.org>,
	"<linux-arm-kernel@...ts.infradead.org>" 
	<linux-arm-kernel@...ts.infradead.org>,
	"Hilman, Kevin" <khilman@...com>,
	"Shilimkar, Santosh" <santosh.shilimkar@...com>,
	"Hunter, Jon" <jon-hunter@...com>,
	"<snijsure@...d-net.com>" <snijsure@...d-net.com>
Subject: Re: rcu self-detected stall messages on OMAP3, 4 boards

Hi Paul

On Sat, 22 Sep 2012, Paul E. McKenney wrote:

> Strangely enough, I believe that I have inadvertently fixed this in
> my -rcu tree:
> 
> git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git rcu/next
> 
> Nevertheless, if you get a chance to try it, I would be interested to
> hear if my guess is correct.

Yes, good news: the stall warnings go away with that branch.

> The trick is that a kthread drives the grace period in -rcu, regardless 
> of whether or not there are callbacks.

This is "rcu: Move quiescent-state forcing into kthread" ?

Added some debugging into rcu_gp_kthread() after that commit and can 
confirm that the quiescent-state forcing loop does start a few times when 
there are zero callbacks pending (modulo any races in my measurement 
code).

> However, the backport would not be something that -stable would be happy
> with, so I will be putting together a fix for mainline.  This thing
> has been in the kernel since about 2004, not sure why you didn't hit
> it earlier.

One other data point in that regard - noticed the warnings don't appear 
when the board is booted with:

commit 4fa3b6cb1bc8c14b81b4c8ffdfd3f2500a7e9367
Author: Paul E. McKenney <paul.mckenney@...aro.org>
Date:   Tue Jun 5 15:53:53 2012 -0700

    rcu: Fix qlen_lazy breakage

...


- Paul
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