From: Paul E. McKenney Date: Wed, 9 May 2012 04:00:28 +0000 (-0700) Subject: rcu: Size rcu_node tree from nr_cpu_ids rather than NR_CPUS X-Git-Url: https://git.karo-electronics.de/?a=commitdiff_plain;h=cca6f3931920a7547d02e68adc2ca635bea5600c;p=linux-beck.git 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 --- diff --git a/kernel/rcutree.c b/kernel/rcutree.c index a4c592b66e10..0fdbc5e07302 100644 --- 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 70e0fd256cc6..ef2b5231afa4 100644 --- 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