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: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index a4c592b66e10b7cee1fadf6ff93e42bd62d8f695..0fdbc5e0730234b124bd098c01a7eed2a717f76d 100644 (file)
--- a/kernel/rcutree.c
+++ b/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. */

diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index 70e0fd256cc6b824453cad62d1b1cfdb83df3f5c..ef2b5231afa41120d3c8a12f3fb8826717d7b26c 100644 (file)
--- a/kernel/rcutree_plugin.h
+++ b/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