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(-)