linux-kernel - Re: [PATCH tip/core/rcu 02/15] rcu: Size rcu_node tree from nr_cpu_ids rather than NR

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Message-ID: <20120616203106.GA12293@leaf>
Date:	Sat, 16 Jun 2012 13:31:06 -0700
From:	Josh Triplett <josh@...htriplett.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, eric.dumazet@...il.com, darren@...art.com,
	fweisbec@...il.com, patches@...aro.org
Subject: Re: [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 07:51:54AM -0700, Paul E. McKenney wrote:
> On Sat, Jun 16, 2012 at 07:44:53AM -0700, Paul E. McKenney wrote:
> > 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@...ux.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@...ux.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!
> 
> Except that the compiler doesn't like the run-time initialization of
> a static const variable.
> 
> Can't have everything, I guess.  ;-)

Ah, right, nr_cpu_ids is not a static const.

In that case, just waiting until you think this definition won't churn
anymore and substituting nr_cpu_ids for n everywhere seems like the
right solution.

- Josh Triplett
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