rcu: limit rcu_node leaf-level fanout

Some recent benchmarks have indicated possible lock contention on the
leaf-level rcu_node locks.  This commit therefore limits the number of
CPUs per leaf-level rcu_node structure to 16, in other words, there
can be at most 16 rcu_data structures fanning into a given rcu_node
structure.  Prior to this, the limit was 32 on 32-bit systems and 64 on
64-bit systems.

Note that the fanout of non-leaf rcu_node structures is unchanged.  The
organization of accesses to the rcu_node tree is such that references
to non-leaf rcu_node structures are much less frequent than to the
leaf structures.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 2 insertions, 1 deletions

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index c39ec5b4ae82..01c8ad33c510 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -1869,8 +1869,9 @@ static void __init rcu_init_levelspread(struct rcu_state *rsp)
 {
         int i;
 
-        for (i = NUM_RCU_LVLS - 1; i >= 0; i--)
+        for (i = NUM_RCU_LVLS - 1; i > 0; i--)
                 rsp->levelspread[i] = CONFIG_RCU_FANOUT;
+        rsp->levelspread[0] = RCU_FANOUT_LEAF;
 }
 #else /* #ifdef CONFIG_RCU_FANOUT_EXACT */
 static void __init rcu_init_levelspread(struct rcu_state *rsp)