linux-kernel - Re: [PATCH 20/35] Use a pre-calculated value for num_online

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Date:	Mon, 16 Mar 2009 22:42:34 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Mel Gorman <mel@....ul.ie>
Cc:	Linux Memory Management List <linux-mm@...ck.org>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Nick Piggin <npiggin@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Lin Ming <ming.m.lin@...el.com>,
	Zhang Yanmin <yanmin_zhang@...ux.intel.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 20/35] Use a pre-calculated value for num_online_nodes()

On Monday 16 March 2009 20:46:15 Mel Gorman wrote:
> num_online_nodes() is called by the page allocator to decide whether the
> zonelist needs to be filtered based on cpusets or the zonelist cache.
> This is actually a heavy function and touches a number of cache lines.
> This patch stores the number of online nodes at boot time and when
> nodes get onlined and offlined.
>
> Signed-off-by: Mel Gorman <mel@....ul.ie>
> ---
>  include/linux/nodemask.h |   16 ++++++++++++++--
>  mm/page_alloc.c          |    6 ++++--
>  2 files changed, 18 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/nodemask.h b/include/linux/nodemask.h
> index 848025c..4749e30 100644
> --- a/include/linux/nodemask.h
> +++ b/include/linux/nodemask.h
> @@ -449,13 +449,25 @@ static inline int num_node_state(enum node_states
> state) node;					\
>  })
>
> +/* Recorded value for num_online_nodes() */
> +extern int static_num_online_nodes;

__read_mostly, please. Check this for any other place you've added
global cachelines that are referenced by the allocator.

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