rcu: Control RCU_FANOUT_LEAF from boot-time parameter

Although making RCU_FANOUT_LEAF a kernel configuration parameter rather
than a fixed constant makes it easier for people to decrease cache-miss
overhead for large systems, it is of little help for people who must
run a single pre-built kernel binary.

This commit therefore allows the value of RCU_FANOUT_LEAF to be
increased (but not decreased!) via a boot-time parameter named
rcutree.rcu_fanout_leaf.

Reported-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 5 insertions, 0 deletions

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index a92c5ebf373e..12783fa833c3 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -2367,6 +2367,11 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
                         Set maximum number of finished RCU callbacks to process
                         in one batch.
 
+        rcutree.fanout_leaf=    [KNL,BOOT]
+                        Increase the number of CPUs assigned to each
+                        leaf rcu_node structure.  Useful for very large
+                        systems.
+
         rcutree.qhimark=        [KNL,BOOT]
                         Set threshold of queued
                         RCU callbacks over which batch limiting is disabled.