Reduce latency impact of PREEMPT_RCU CPU offline

Hello!

This series reduces worst-case CPU-hotplug latency for PREEMPT_RCU by avoiding an O(N) list move, where N is the number of threads that were preempted recently while in an RCU read-side critical section. This situation is rare, occurring only when the last CPU corresponding to a given rcu_node structure goes offline, for example, when CPUs 1-14 are all offline and CPU 15 goes offline. In this case, all the tasks that were recently preempted while running on one of CPUs 0-15 will be moved to the root rcu_node structure. Because this could be a very large number of tasks, and because this moving is carried out with interrupts disabled, a very large latency spike can result. This series therefore reworks RCU's CPU-hotplug code and grace-period computation to remove the need for moving tasks.

Because there was an obscure bug in the task-move process, this change also fixes that bug by annihilating the function containing it. This bug manifests only when RCU priority boosting is enabled, and even then occurs only roughly once per hundred hours or so of focused rcutorture testing. It was made somewhat more probable by a recent change to rcutorture that registers 60,000 RCU callbacks in three batches of 20,000 each, with one jiffy between each batch.

The individual commits in this series are as follows:

1. Fix a comment: softirqs are disabled, not interrupts.

2. Apply ACCESS_ONCE to rcu_boost().

3. Change rcu_read_unlock_special()'s "empty" variable's name to
"empty_norm" to make room for a new "empty" that will track the emptiness of the full list rather than just that part that is blocking the current normal (non-expedited) grace period.

4. Abstract rcu_cleanup_dead_rnp() from rcu_cleanup_dead_cpu().
The new rcu_cleanup_dead_rnp() function clears ->qsmaskinit bits up the rcu_node tree to allow for the last CPU in a given leaf rcu_node structure going offline. This function will be called from rcu_read_unlock_special() when the last blocked task exits its outermost RCU read-side critical section.

5. Make rcu_read_unlock_special() to propagate ->qsmaskinit bit
clearing up the rcu_node tree once the last task has exited its RCU read-side critical section.

6. Stop migrating the list of blocked tasks to the root rcu_node
structure.

7. Stop holding the irq-disabled ->orphan_lock across the

	->qsmaskinit bit-clearing process, thus reducing the length
	of time that interrupts are disabled.

8. Make use of the new rcu_preempt_has_tasks() function.

9. Stop creating an rcub kthread for the root rcu_node structure,

	which no longer has tasks in its ->blkd_tasks list (unless it
	is also the sole leaf rcu_node structure).

10. Drop the code from force_qs_rnp() that attempts to awaken the
now-nonexistent root rcu_node structure's rcub kthread.

Thanx, Paul

------------------------------------------------------------------------

 b/kernel/rcu/tree.c        |  103 ++++++++++++++++-------------
 b/kernel/rcu/tree.h        |   13 ---
 b/kernel/rcu/tree_plugin.h |  159 +++++++++++----------------------------------
 3 files changed, 98 insertions(+), 177 deletions(-)