RCU ->gp_seq conversion for v4.19

Hello!

This series moves from the two-variable ->gpnum and ->completed to a single-variable ->gp_seq approach. The key point is that ->gpnum and ->completed are either identical or ->gpnum is only one greater, which means that we are using a full unsigned long were a single bit would do. This series therefore applies the pre-existing grace-period sequence-number code used by expedited grace periods, _rcu_barrier(), and SRCU. This change also has the beneficial effect of allowing the grace-period state to be sampled with a single load, which will be used to improve the grace-period start code. The individual commits are as follows:

1. Introduce grace-period sequence numbers (->gp_seq) to normal RCU.

2-17. Make core RCU code use ->gp_seq instead of ->gpnum and ->completed.

18. Move from ->need_future_gp[] to ->gp_seq_needed, which allows
simpler grace-period-already-requested checks.

19. Use the new ->gp_seq_needed to make rcu_nocb_wait_gp() check if
a given grace has already been requested, thus reducing overhead for no-CBs CPUs.

20-26. Move RCU tracepoints to ->gp_seq.

27. Remove ->gpnum and ->completed.

28. Make simple callback acceleration refer to rdp->gp_seq_needed.

29-31. Update documentation to reflect ->gp_seq.

32. Don't funnel-lock above the leaf rcu_node structure if a grace period

	is in progress, thus saving an upper-level rcu_node ->lock
	acquisition.

33. Produce the last CleanupMore trace only if the corresponding
grace-period request was quite recent, courtesy of Joel Fernandes.

34. Fix cpustart tracepoint ->gp_seq number, courtesy of Joel Fernandes.

35. Make rcu_start_this_gp() check for grace period already started.

36. Correctly handle grace-period sequence (->gp_seq) wrap in
rcutorture.

37. Regularize resetting of rcu_data wrap indicator.

38-39. Rename grace-period-request variables and parameters, courtesy
of Joel Fernandes.

40. Check current node rather than leaf node for requested grace
period already being in progress.

Thanx, Paul

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 Documentation/RCU/Design/Data-Structures/Data-Structures.html          |  118 -
 Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.html |   22 
 Documentation/RCU/Design/Memory-Ordering/TreeRCU-gp-cleanup.svg        |  123 -
 Documentation/RCU/Design/Memory-Ordering/TreeRCU-gp-init-1.svg         |   16 
 Documentation/RCU/Design/Memory-Ordering/TreeRCU-gp-init-3.svg         |   56 
 Documentation/RCU/Design/Memory-Ordering/TreeRCU-gp.svg                |  237 +--
 Documentation/RCU/Design/Memory-Ordering/TreeRCU-qs.svg                |   12 
 Documentation/RCU/stallwarn.txt                                        |   24 
 include/trace/events/rcu.h                                             |  114 -
 kernel/rcu/rcu.h                                                       |   70 -
 kernel/rcu/rcuperf.c                                                   |   44 
 kernel/rcu/rcutorture.c                                                |   93 -
 kernel/rcu/srcutree.c                                                  |    5 
 kernel/rcu/tree.c                                                      |  678 ++++------
 kernel/rcu/tree.h                                                      |   40 
 kernel/rcu/tree_plugin.h                                               |   54 
 16 files changed, 832 insertions(+), 874 deletions(-)