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: <20120615214726.GT31184@leaf>
Date:	Fri, 15 Jun 2012 14:47:26 -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 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?

- Josh Triplett
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