rcu: Shrink RCU based on number of CPUs

Currently, rcu_init_geometry() only reshapes RCU's combining trees
if the leaf fanout is changed at boot time.  This means that by
default, kernels compiled with (say) NR_CPUS=4096 will keep oversized
data structures, even when running on systems with (say) four CPUs.

This commit therefore checks to see if the maximum number of CPUs on
the actual running system (nr_cpu_ids) differs from NR_CPUS, and if so
reshapes the combining trees accordingly.

Reported-by: Mike Galbraith <efault@gmx.de>
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 8b9496fee235..b703989148e4 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -2821,7 +2821,8 @@ static void __init rcu_init_geometry(void)
         int rcu_capacity[MAX_RCU_LVLS + 1];
 
         /* If the compile-time values are accurate, just leave. */
-        if (rcu_fanout_leaf == CONFIG_RCU_FANOUT_LEAF)
+        if (rcu_fanout_leaf == CONFIG_RCU_FANOUT_LEAF &&
+            nr_cpu_ids == NR_CPUS)
                 return;
 
         /*