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

[Paul Walmsley <paul@pwsan.com>]

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