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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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