Re: [PATCH tip/core/rcu 02/15] rcu: Size rcu_node tree from nr_cpu_ids rather than NR_CPUS

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

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.

							Thanx, Paul