Commit cca6f393 authored May 08, 2012 by Paul E. McKenney

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>

parent cc5df65b

kernel/rcutree.c

+1 −1

Original line number	Diff line number	Diff line
		@@ -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

+2 −0

Original line number	Diff line number	Diff line
		@@ -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

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