1 files changed, 27 insertions, 0 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