RCU flavor-consolidation changes for v4.20/v5.0

Hello!

This series contains the RCU flavor consolidation, along with some initial cleanup work enabled by that consolidation (and some that became apparent while cleaning up):

1. Refactor rcu_{nmi,irq}_{enter,exit}(), saving a branch on the
idle entry/exit hotpaths, courtesy of Byungchul Park,

2. Defer reporting RCU-preempt quiescent states when disabled.
This is the key commit that consolidates the RCU-bh and RCU-sched flavors into RCU, however, the RCU-bh and RCU-sched flavors still exist independently as well at this point.

3. Test extended "rcu" read-side critical sections. This commit
causes rcutorture to test RCU's new-found ability to act as the combination of RCU, RCU-bh, and RCU-sched.

4. Allow processing deferred QSes for exiting RCU-preempt readers.
This is a optimization.

5. Remove now-unused ->b.exp_need_qs field from the rcu_special union.

6. Add warning to detect half-interrupts. Test the claim that
the Linux kernel no longer does half-interrupts.

7. Apply RCU-bh QSes to RCU-sched and RCU-preempt when safe, that is,
make the consolidated RCU inherit RCU-bh's denial-of-service avoidance mechanism.

8. Report expedited grace periods at context-switch time. This is
an optimization enabled by the RCU flavor consolidation.

9. Define RCU-bh update API in terms of RCU. This commit gets rid
of the RCU-bh update mechanism.

10. Update comments and help text to account for the removal of
the RCU-bh update mechanism.

11. Drop "wake" parameter from rcu_report_exp_rdp().

12. Fix typo in rcu_get_gp_kthreads_prio() header comment.

13. Define RCU-sched API in terms of RCU for Tree RCU PREEMPT builds.
Although this commit gets rid of the RCU-sched update mechanism from PREEMPT builds, it of course remains as the sole RCU flavor for !PREEMPT && !SMP builds.

14. Express Tiny RCU updates in terms of RCU rather than RCU-sched.
This will enable additional cleanups and code savings.

15. Remove RCU_STATE_INITIALIZER() in favor of just using an open-coded
initializer for the sole remaining rcu_state structure.

16. Eliminate rcu_state structure's ->call field, as it is now always
just call_rcu().

17. Remove rcu_state structure's ->rda field, as there is now only one
set of per-CPU rcu_data structures.

Thanx, Paul

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

 Documentation/RCU/Design/Requirements/Requirements.html |   50 -
 include/linux/rcupdate.h                                |   48 -
 include/linux/rcupdate_wait.h                           |    6 
 include/linux/rcutiny.h                                 |   61 +
 include/linux/rcutree.h                                 |   31 
 include/linux/sched.h                                   |    6 
 kernel/rcu/Kconfig                                      |   10 
 kernel/rcu/rcutorture.c                                 |    1 
 kernel/rcu/tiny.c                                       |  163 +--
 kernel/rcu/tree.c                                       |  655 +++++-----------
 kernel/rcu/tree.h                                       |   44 -
 kernel/rcu/tree_exp.h                                   |  256 +++---
 kernel/rcu/tree_plugin.h                                |  523 ++++++------
 kernel/rcu/update.c                                     |    2 
 kernel/softirq.c                                        |    3 
 15 files changed, 836 insertions(+), 1023 deletions(-)