[PATCH tip/core/rcu 15/40] rcu: Default RCU_FANOUT_LEAF to 16 unless explicitly changed

[Paul E. McKenney]

If the RCU_EXPERT Kconfig option is not set (the default), then the
RCU_FANOUT_LEAF Kconfig option will not be defined, which will cause
the leaf-level rcu_node tree fanout to default to 32 on 32-bit systems
and 64 on 64-bit systems.  This can result in excessive lock contention.
This commit therefore changes the computation of the leaf-level rcu_node
tree fanout so that the result will be 16 unless an explicit Kconfig or
kernel-boot setting says otherwise.

Reported-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
---
 kernel/rcu/tree.h | 6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index d2f276fc2edc..376c01e539c7 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -52,11 +52,7 @@
 #ifdef CONFIG_RCU_FANOUT_LEAF
 #define RCU_FANOUT_LEAF CONFIG_RCU_FANOUT_LEAF
 #else /* #ifdef CONFIG_RCU_FANOUT_LEAF */
-# ifdef CONFIG_64BIT
-# define RCU_FANOUT_LEAF 64
-# else
-# define RCU_FANOUT_LEAF 32
-# endif
+#define RCU_FANOUT_LEAF 16
 #endif /* #else #ifdef CONFIG_RCU_FANOUT_LEAF */
 
 #define RCU_FANOUT_1	      (RCU_FANOUT_LEAF)
-- 
2.5.2