rcu: Merge preemptable-RCU functionality into hierarchical RCU

Create a kernel/rcutree_plugin.h file that contains definitions
for preemptable RCU (or, under the #else branch of the #ifdef,
empty definitions for the classic non-preemptable semantics).
These definitions fit into plugins defined in kernel/rcutree.c
for this purpose.

This variant of preemptable RCU uses a new algorithm whose
read-side expense is roughly that of classic hierarchical RCU
under CONFIG_PREEMPT. This new algorithm's update-side expense
is similar to that of classic hierarchical RCU, and, in absence
of read-side preemption or blocking, is exactly that of classic
hierarchical RCU.  Perhaps more important, this new algorithm
has a much simpler implementation, saving well over 1,000 lines
of code compared to mainline's implementation of preemptable
RCU, which will hopefully be retired in favor of this new
algorithm.

The simplifications are obtained by maintaining per-task
nesting state for running tasks, and using a simple
lock-protected algorithm to handle accounting when tasks block
within RCU read-side critical sections, making use of lessons
learned while creating numerous user-level RCU implementations
over the past 18 months.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: laijs@cn.fujitsu.com
Cc: dipankar@in.ibm.com
Cc: akpm@linux-foundation.org
Cc: mathieu.desnoyers@polymtl.ca
Cc: josht@linux.vnet.ibm.com
Cc: dvhltc@us.ibm.com
Cc: niv@us.ibm.com
Cc: peterz@infradead.org
Cc: rostedt@goodmis.org
LKML-Reference: <12509746134003-git-send-email->
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 16 insertions, 6 deletions

diff --git a/init/Kconfig b/init/Kconfig
index 25373cf32672..f88da2d1c1fb 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -335,11 +335,20 @@ config PREEMPT_RCU
           now-naive assumptions about each RCU read-side critical section
           remaining on a given CPU through its execution.
 
+config TREE_PREEMPT_RCU
+        bool "Preemptable tree-based hierarchical RCU"
+        depends on PREEMPT
+        help
+          This option selects the RCU implementation that is
+          designed for very large SMP systems with hundreds or
+          thousands of CPUs, but for which real-time response
+          is also required.
+
 endchoice
 
 config RCU_TRACE
         bool "Enable tracing for RCU"
-        depends on TREE_RCU || PREEMPT_RCU
+        depends on TREE_RCU || PREEMPT_RCU || TREE_PREEMPT_RCU
         help
           This option provides tracing in RCU which presents stats
           in debugfs for debugging RCU implementation.
@@ -351,7 +360,7 @@ config RCU_FANOUT
         int "Tree-based hierarchical RCU fanout value"
         range 2 64 if 64BIT
         range 2 32 if !64BIT
-        depends on TREE_RCU
+        depends on TREE_RCU || TREE_PREEMPT_RCU
         default 64 if 64BIT
         default 32 if !64BIT
         help
@@ -366,7 +375,7 @@ config RCU_FANOUT
 
 config RCU_FANOUT_EXACT
         bool "Disable tree-based hierarchical RCU auto-balancing"
-        depends on TREE_RCU
+        depends on TREE_RCU || TREE_PREEMPT_RCU
         default n
         help
           This option forces use of the exact RCU_FANOUT value specified,
@@ -379,11 +388,12 @@ config RCU_FANOUT_EXACT
           Say N if unsure.
 
 config TREE_RCU_TRACE
-        def_bool RCU_TRACE && TREE_RCU
+        def_bool RCU_TRACE && ( TREE_RCU || TREE_PREEMPT_RCU )
         select DEBUG_FS
         help
-          This option provides tracing for the TREE_RCU implementation,
-          permitting Makefile to trivially select kernel/rcutree_trace.c.
+          This option provides tracing for the TREE_RCU and
+          TREE_PREEMPT_RCU implementations, permitting Makefile to
+          trivially select kernel/rcutree_trace.c.
 
 config PREEMPT_RCU_TRACE
         def_bool RCU_TRACE && PREEMPT_RCU