RCU simplification and RT needs

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

Hello!

At Linus's request, I am simplifying the Linux-kernel RCU implementation,
which includes removing code that implements features and options that
are no longer needed.  This is not a half-hearted effort.  In fact,
I expect that my submission to the next merge window will be a net
removal of more than 2500 lines of code.

But wait, there is more!  ;-)

Although the following two features are not being axed in v4.13, they
will be in v4.14 unless someone makes a convincing case for them:

1.	The ability to build a CONFIG_RCU_NOCB_CPUS=y kernel without
	also specifying CONFIG_NO_HZ_FULL.

	Unless someone speaks for this configuration option,
	CONFIG_RCU_NOCB_CPUS will be slaved off of CONFIG_NO_HZ_FULL,
	and the rcu_nocbs= boot parameter will be dropped.  (RCU would
	instead use the nohz_full= boot parameter to determine which
	CPUs get their callbacks offloaded.)

2.	The ability to specify polling for callback-offloaded CPUs.  This
	means that the rcu_nocb_poll= boot parameter will be dropped,
	and the CPU doing call_rcu() would do explicit wakeups, when
	needed, to get the corresponding rcuo kthread on the job.

	I have no evidence that anyone has ever used this option, other
	than me running the occasional rcutorture test.

So, anyone need either of these?  If not, out they go!  ;-)

							Thanx, Paul