rcu: Convert CONFIG_RCU_FANOUT_EXACT to boot parameter

The CONFIG_RCU_FANOUT_EXACT Kconfig parameter is used primarily (and
perhaps only) by rcutorture to verify that RCU works correctly in specific
rcu_node combining-tree configurations.  It therefore does not make
much sense have this as a question to people attempting to configure
their kernels.  So this commit creates an rcutree.rcu_fanout_exact=
boot parameter that rcutorture can use, and eliminates the original
CONFIG_RCU_FANOUT_EXACT Kconfig parameter.

Reported-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>

1 files changed, 6 insertions, 0 deletions

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 10a4fb80c033..f5582dcdf80d 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -3010,6 +3010,12 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
                         the rcu_node combining tree.  This only has effect
                         when CONFIG_RCU_TORTURE_TEST_SLOW_PREINIT is set.
 
+        rcutree.rcu_fanout_exact= [KNL]
+                        Disable autobalancing of the rcu_node combining
+                        tree.  This is used by rcutorture, and might
+                        possibly be useful for architectures having high
+                        cache-to-cache transfer latencies.
+
         rcutree.rcu_fanout_leaf= [KNL]
                         Increase the number of CPUs assigned to each
                         leaf rcu_node structure.  Useful for very large