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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>
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cc5df65b03
commit
cca6f39319
2 changed files with 3 additions and 1 deletions
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@ -2675,7 +2675,7 @@ static void __init rcu_init_geometry(void)
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{
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int i;
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int j;
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int n = NR_CPUS;
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int n = nr_cpu_ids;
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int rcu_capacity[MAX_RCU_LVLS + 1];
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/* If the compile-time values are accurate, just leave. */
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@ -72,6 +72,8 @@ static void __init rcu_bootup_announce_oddness(void)
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#endif
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if (rcu_fanout_leaf != CONFIG_RCU_FANOUT_LEAF)
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printk(KERN_INFO "\tExperimental boot-time adjustment of leaf fanout to %d.\n", rcu_fanout_leaf);
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if (nr_cpu_ids != NR_CPUS)
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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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}
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#ifdef CONFIG_TREE_PREEMPT_RCU
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