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Subject[PATCH tip/core/rcu 03/22] rcu: Size rcu_node tree from nr_cpu_ids rather than NR_CPUS
From: "Paul E. McKenney" <>

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 <>
kernel/rcutree.c | 2 +-
kernel/rcutree_plugin.h | 2 ++
2 files changed, 3 insertions(+), 1 deletions(-)

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 260c1a3..f06a655 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -2672,7 +2672,7 @@ 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 61aab6f..67b3b97 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -72,6 +72,8 @@ static void __init rcu_bootup_announce_oddness(void)
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);


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