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SubjectRe: [PATCH tip/core/rcu 02/15] rcu: Size rcu_node tree from nr_cpu_ids rather than NR_CPUS
On Fri, Jun 15, 2012 at 02:05:57PM -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
>
> 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>
> ---
> 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 a151184..9098910 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;

Same question as before: why have this as a variable when it never
changes?

- Josh Triplett


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