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

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Date:	Sat, 22 Sep 2012 18:42:08 +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>,
	fweisbec@...il.com
Subject: Re: rcu self-detected stall messages on OMAP3, 4 boards

On Fri, 21 Sep 2012, Paul E. McKenney wrote:

> Could you please point me to a recipe for creating a minimal userspace?
> Just in case it is the userspac erather than the architecture/hardware
> that makes the difference.

Tony's suggestion is pretty good.  Note that there may also be differences 
in kernel timers -- either between x86 and ARM architectures, or loaded 
device drivers -- that may confound the problem.

> 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.
> 
> Did I get that right, or am I missing a combination?

That's correct.

> Indeed, rcu_idle_gp_timer_func() is a bit strange in that it is 
> cancelled upon exit from idle, and therefore should (almost) never 
> actually execute. Its sole purpose is to wake up the CPU.  ;-)

Right.  Just curious, what would wake up the kernel from idle to handle a 
grace period expiration when CONFIG_RCU_FAST_NO_HZ=n?  On a very idle 
system, the time between timer ticks could potentially be several tens of 
seconds.


- Paul
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