linux-kernel - Re: [PATCH 3/7] mm: page_alloc: Use zone node IDs to approximate locality

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Date:	Mon, 16 Dec 2013 15:25:07 -0500
From:	Johannes Weiner <hannes@...xchg.org>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Hansen <dave.hansen@...el.com>,
	Rik van Riel <riel@...hat.com>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/7] mm: page_alloc: Use zone node IDs to approximate
 locality

On Fri, Dec 13, 2013 at 02:10:03PM +0000, Mel Gorman wrote:
> zone_local is using node_distance which is a more expensive call than
> necessary. On x86, it's another function call in the allocator fast path
> and increases cache footprint. This patch makes the assumption zones on a
> local node will share the same node ID. The necessary information should
> already be cache hot.
> 
> Signed-off-by: Mel Gorman <mgorman@...e.de>
> ---
>  mm/page_alloc.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 64020eb..fd9677e 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1816,7 +1816,7 @@ static void zlc_clear_zones_full(struct zonelist *zonelist)
>  
>  static bool zone_local(struct zone *local_zone, struct zone *zone)
>  {
> -	return node_distance(local_zone->node, zone->node) == LOCAL_DISTANCE;
> +	return zone_to_nid(zone) == numa_node_id();

Why numa_node_id()?  We pass in the preferred zone as @local_zone:

return zone_to_nid(local_zone) == zone_to_nid(zone)

Or even just compare the ->zone_pgdat pointers?
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