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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>
This commit is contained in:
Paul E. McKenney 2012-05-08 21:00:28 -07:00
parent cc5df65b03
commit cca6f39319
2 changed files with 3 additions and 1 deletions
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2
kernel/rcutree.c
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@ -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. */

2
kernel/rcutree_plugin.h
View File

@ -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