[PATCH rcu 0/11] Add light-weight readers for SRCU

[Paul E. McKenney]

Hello!

This series provides light-weight readers for SRCU.  This lightness
is selected by the caller by using the new srcu_read_lock_lite() and
srcu_read_unlock_lite() flavors instead of the usual srcu_read_lock() and
srcu_read_unlock() flavors.  Although this passes significant rcutorture
testing, this should still be considered to be experimental.

There are a few restrictions:  (1) If srcu_read_lock_lite() is called
on a given srcu_struct structure, then no other flavor may be used on
that srcu_struct structure, before, during, or after.  (2) The _lite()
readers may only be invoked from regions of code where RCU is watching
(as in those regions in which rcu_is_watching() returns true).  (3)
There is no auto-expediting for srcu_struct structures that have
been passed to _lite() readers.  (4) SRCU grace periods for _lite()
srcu_struct structures invoke synchronize_rcu() at least twice, thus
having longer latencies than their non-_lite() counterparts.  (5) Even
with synchronize_srcu_expedited(), the resulting SRCU grace period
will invoke synchronize_rcu() at least twice, as opposed to invoking
the IPI-happy synchronize_rcu_expedited() function.  (6)  Just as with
srcu_read_lock() and srcu_read_unlock(), the srcu_read_lock_lite() and
srcu_read_unlock_lite() functions may not (repeat, *not*) be invoked
from NMI handlers (that is what the _nmisafe() interface are for).
Although one could imagine readers that were both _lite() and _nmisafe(),
one might also imagine that the read-modify-write atomic operations that
are needed by any NMI-safe SRCU read marker would make this unhelpful
from a performance perspective.

All that said, the patches in this series are as follows:

1.	Rename srcu_might_be_idle() to srcu_should_expedite().

2.	Introduce srcu_gp_is_expedited() helper function.

3.	Renaming in preparation for additional reader flavor.

4.	Bit manipulation changes for additional reader flavor.

5.	Standardize srcu_data pointers to "sdp" and similar.

6.	Convert srcu_data ->srcu_reader_flavor to bit field.

7.	Add srcu_read_lock_lite() and srcu_read_unlock_lite().

8.	rcutorture: Expand RCUTORTURE_RDR_MASK_[12] to eight bits.

9.	rcutorture: Add reader_flavor parameter for SRCU readers.

10.	rcutorture: Add srcu_read_lock_lite() support to
	rcutorture.reader_flavor.

11.	refscale: Add srcu_read_lock_lite() support using "srcu-lite".

						Thanx, Paul

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

 Documentation/admin-guide/kernel-parameters.txt   |    4 
 b/Documentation/admin-guide/kernel-parameters.txt |    8 +
 b/include/linux/srcu.h                            |   21 +-
 b/include/linux/srcutree.h                        |    2 
 b/kernel/rcu/rcutorture.c                         |   28 +--
 b/kernel/rcu/refscale.c                           |   54 +++++--
 b/kernel/rcu/srcutree.c                           |   16 +-
 include/linux/srcu.h                              |   86 +++++++++--
 include/linux/srcutree.h                          |    5 
 kernel/rcu/rcutorture.c                           |   37 +++-
 kernel/rcu/srcutree.c                             |  168 +++++++++++++++-------
 11 files changed, 308 insertions(+), 121 deletions(-)