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 a4c592b..0fdbc5e 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -2675,7 +2675,7 @@
{
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 70e0fd2..ef2b523 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -72,6 +72,8 @@
#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