[PATCH RFC v2 tip/core/rcu 0/4] Real-time elimination of RCU_SOFTIRQ

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

Hello!

This series is v2 of an early posting of code to add a boot-time option to
move RCU's softirq processing to per-CPU kthreads.  This is not done
by default for performance reasons, nor are these reasons theoretical.
In fact, earlier attempts to do just this were not met with silence.

The patches are as follows:

1.      Enable elimination of Tree-RCU softirq processing via a new
	rcutree.use_softirq kernel boot parameter.  This defaults to 1,
	so boot with "rcutree.use_softirq=0" to move RCU_SOFTIRQ work
	to the rcuc kthreads.  Courtesy of Sebastian Andrzej Siewior.

2.      Improve wakeup-safety checks in rcu_read_unlock_special(),
	thus allowing both rcuc kthreads and reasonably snappy
	expedited RCU grace periods.

3.	Only do rcu_read_unlock_special() wakeups if expedited.

4.	Allow rcu_read_unlock_special() to raise_softirq() if in_irq().

Changes since v1:

o	Inspired by feedback from Peter Zijlstra, avoid expensive wakeups
	unless there is a high probability that the current task or CPU
	is blocking an expedited grace period.

o	Account for the possibility of long-term scheduling-clock-free
	kernel sojourns in NO_HZ_FULL kernels.

o	As an exception to the above, allow use of raise_softirq_irqoff()
	when in an interrupt handler, in which case this is an extremely
	low-overhead option.

							Thanx, Paul

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

 Documentation/admin-guide/kernel-parameters.txt |    6 
 include/linux/sched.h                           |    2 
 kernel/rcu/tree.c                               |  138 +++++++++++++++++--
 kernel/rcu/tree.h                               |    2 
 kernel/rcu/tree_plugin.h                        |  167 +++++-------------------
 5 files changed, 172 insertions(+), 143 deletions(-)