rcu: Size rcu_node tree from nr_cpu_ids rather than NR_CPUS

The rcu_node tree array is sized based on compile-time constants,
including NR_CPUS.  Although this approach has worked well in the past,
the recent trend by many distros to define NR_CPUS=4096 results in
excessive grace-period-initialization latencies.

This commit therefore substitutes the run-time computed nr_cpu_ids for
the compile-time NR_CPUS when building the tree.  This can result in
much of the compile-time-allocated rcu_node array being unused.  If
this is a major problem, you are in a specialized situation anyway,
so you can manually adjust the NR_CPUS, RCU_FANOUT, and RCU_FANOUT_LEAF
kernel config parameters.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcutree.c

@@ -2675,7 +2675,7 @@ static void __init rcu_init_geometry(void)
 {
 	int i;
 	int j;
-	int n = NR_CPUS;
+	int n = nr_cpu_ids;
 	int rcu_capacity[MAX_RCU_LVLS + 1];
 
 	/* If the compile-time values are accurate, just leave. */

kernel/rcutree_plugin.h

@@ -72,6 +72,8 @@ static void __init rcu_bootup_announce_oddness(void)
 #endif
 	if (rcu_fanout_leaf != CONFIG_RCU_FANOUT_LEAF)
 		printk(KERN_INFO "\tExperimental boot-time adjustment of leaf fanout to %d.\n", rcu_fanout_leaf);
+	if (nr_cpu_ids != NR_CPUS)
+		printk(KERN_INFO "\tRCU restricting CPUs from NR_CPUS=%d to nr_cpu_ids=%d.\n", NR_CPUS, nr_cpu_ids);
 }
 
 #ifdef CONFIG_TREE_PREEMPT_RCU