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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 Sat, Jun 16, 2012 at 02:17:33AM -0700, Josh Triplett wrote:
    > On Fri, Jun 15, 2012 at 11:38:48PM -0700, Paul E. McKenney wrote:
    > > On Fri, Jun 15, 2012 at 10:17:12PM -0700, Josh Triplett wrote:
    > > > On Fri, Jun 15, 2012 at 05:37:14PM -0700, Paul E. McKenney wrote:
    > > > > On Fri, Jun 15, 2012 at 02:47:26PM -0700, Josh Triplett wrote:
    > > > > > 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?
    > > > >
    > > > > Ah, that explains why. This prevented me from forgetting the random
    > > > > NR_CPUS.
    > > >
    > > > Does that mean it can go away now that you've written the patches?
    > >
    > > If I don't have to change from nr_cpu_ids to yet another thing over
    > > the next while, then it might be worth changing.
    >
    > That sounds like an argument for a #define or a static const, rather
    > than a local variable. :)

    OK, static const it is!

    Thanx, Paul



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