v2 Add callback-free CPUs

Hello!

RCU callback execution can add significant OS jitter and also can
degrade scheduling latency.  This commit therefore adds the ability
for selected CPUs ("rcu_nocbs=" boot parameter) to have their callbacks
offloaded to kthreads, inspired by Joe Korty's and Jim Houston's JRCU.
If the "rcu_nocb_poll" boot parameter is also specified, these kthreads
will do polling, removing the need for the offloaded CPUs to do wakeups.
At least one CPU must be doing normal callback processing: currently CPU
0 cannot be selected as a no-CBs CPU.  In addition, attempts to offline
the last normal-CBs CPU will fail.

Changes since v1 (https://lkml.org/lkml/2012/9/5/572):

1.	Contains fixes for a few problems located by Fengguang Wu's
	kbuild test robot.
2.	Counters are now atomic_long_t rather than atomic_t, as
	suggested by Peter Zijlstra.
3.	The rcu_nocbs= kernel boot parameter is now documented.
4.	Fixed a bug introduced by commit c96ea7cf (Avoid spurious
	RCU CPU stall warnings) that resulted in boot-time NULL-pointer
	dereferences (reported by Paul Gortmaker).
4.	Account for normal and offloaded callbacks separately, so that
	offloading is represented in debugfs output.

The patches in this series are as follows:

1.	Offload RCU callbacks based on boot-time kernel parameter.
2.	Account for normal and offloaded callbacks separately, so that
	offloading is represented in debugfs output.

							Thanx, Paul

 b/Documentation/kernel-parameters.txt |    5 
 b/include/trace/events/rcu.h          |    1 
 b/init/Kconfig                        |   19 +
 b/kernel/rcutree.c                    |   63 ++++-
 b/kernel/rcutree.h                    |   48 ++++
 b/kernel/rcutree_plugin.h             |  399 +++++++++++++++++++++++++++++++++-
 b/kernel/rcutree_trace.c              |   12 -
 7 files changed, 528 insertions(+), 19 deletions(-)

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