3 files changed, 31 insertions, 8 deletions

diff --git a/init/Kconfig b/init/Kconfig
index 85c6870ed476..6d18ef8071b5 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -458,6 +458,33 @@ config RCU_FANOUT
           Select a specific number if testing RCU itself.
           Take the default if unsure.
 
+config RCU_FANOUT_LEAF
+        int "Tree-based hierarchical RCU leaf-level fanout value"
+        range 2 RCU_FANOUT if 64BIT
+        range 2 RCU_FANOUT if !64BIT
+        depends on TREE_RCU || TREE_PREEMPT_RCU
+        default 16
+        help
+          This option controls the leaf-level fanout of hierarchical
+          implementations of RCU, and allows trading off cache misses
+          against lock contention.  Systems that synchronize their
+          scheduling-clock interrupts for energy-efficiency reasons will
+          want the default because the smaller leaf-level fanout keeps
+          lock contention levels acceptably low.  Very large systems
+          (hundreds or thousands of CPUs) will instead want to set this
+          value to the maximum value possible in order to reduce the
+          number of cache misses incurred during RCU's grace-period
+          initialization.  These systems tend to run CPU-bound, and thus
+          are not helped by synchronized interrupts, and thus tend to
+          skew them, which reduces lock contention enough that large
+          leaf-level fanouts work well.
+
+          Select a specific number if testing RCU itself.
+
+          Select the maximum permissible value for large systems.
+
+          Take the default if unsure.
+
 config RCU_FANOUT_EXACT
         bool "Disable tree-based hierarchical RCU auto-balancing"
         depends on TREE_RCU || TREE_PREEMPT_RCU
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 1050d6d3922c..780acf8e15e9 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -2418,7 +2418,7 @@ static void __init rcu_init_levelspread(struct rcu_state *rsp)
 
         for (i = NUM_RCU_LVLS - 1; i > 0; i--)
                 rsp->levelspread[i] = CONFIG_RCU_FANOUT;
-        rsp->levelspread[0] = RCU_FANOUT_LEAF;
+        rsp->levelspread[0] = CONFIG_RCU_FANOUT_LEAF;
 }
 #else /* #ifdef CONFIG_RCU_FANOUT_EXACT */
 static void __init rcu_init_levelspread(struct rcu_state *rsp)
diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index cdd1be0a4072..a905c200405c 100644
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -29,18 +29,14 @@
 #include <linux/seqlock.h>
 
 /*
- * Define shape of hierarchy based on NR_CPUS and CONFIG_RCU_FANOUT.
+ * Define shape of hierarchy based on NR_CPUS, CONFIG_RCU_FANOUT, and
+ * CONFIG_RCU_FANOUT_LEAF.
  * In theory, it should be possible to add more levels straightforwardly.
  * In practice, this did work well going from three levels to four.
  * Of course, your mileage may vary.
  */
 #define MAX_RCU_LVLS 4
-#if CONFIG_RCU_FANOUT > 16
-#define RCU_FANOUT_LEAF       16
-#else /* #if CONFIG_RCU_FANOUT > 16 */
-#define RCU_FANOUT_LEAF       (CONFIG_RCU_FANOUT)
-#endif /* #else #if CONFIG_RCU_FANOUT > 16 */
-#define RCU_FANOUT_1          (RCU_FANOUT_LEAF)
+#define RCU_FANOUT_1          (CONFIG_RCU_FANOUT_LEAF)
 #define RCU_FANOUT_2          (RCU_FANOUT_1 * CONFIG_RCU_FANOUT)
 #define RCU_FANOUT_3          (RCU_FANOUT_2 * CONFIG_RCU_FANOUT)
 #define RCU_FANOUT_4          (RCU_FANOUT_3 * CONFIG_RCU_FANOUT)