rcu: Make SRCU optional by using CONFIG_SRCU

SRCU is not necessary to be compiled by default in all cases. For tinification
efforts not compiling SRCU unless necessary is desirable.

The current patch tries to make compiling SRCU optional by introducing a new
Kconfig option CONFIG_SRCU which is selected when any of the components making
use of SRCU are selected.

If we do not select CONFIG_SRCU, srcu.o will not be compiled at all.

   text    data     bss     dec     hex filename
   2007       0       0    2007     7d7 kernel/rcu/srcu.o

Size of arch/powerpc/boot/zImage changes from

   text    data     bss     dec     hex filename
 831552   64180   23944  919676   e087c arch/powerpc/boot/zImage : before
 829504   64180   23952  917636   e0084 arch/powerpc/boot/zImage : after

so the savings are about ~2000 bytes.

Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
CC: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
CC: Josh Triplett <josh@joshtriplett.org>
CC: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: resolve conflict due to removal of arch/ia64/kvm/Kconfig. ]

1 files changed, 9 insertions, 0 deletions

diff --git a/init/Kconfig b/init/Kconfig
index 9afb971497f4..f085969ba340 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -501,9 +501,17 @@ config TINY_RCU
 
 endchoice
 
+config SRCU
+        bool
+        help
+          This option selects the sleepable version of RCU. This version
+          permits arbitrary sleeping or blocking within RCU read-side critical
+          sections.
+
 config TASKS_RCU
         bool "Task_based RCU implementation using voluntary context switch"
         default n
+        select SRCU
         help
           This option enables a task-based RCU implementation that uses
           only voluntary context switch (not preemption!), idle, and
@@ -1595,6 +1603,7 @@ config PERF_EVENTS
         depends on HAVE_PERF_EVENTS
         select ANON_INODES
         select IRQ_WORK
+        select SRCU
         help
           Enable kernel support for various performance events provided
           by software and hardware.