Re: [PATCH 21/25] Use allocation flags as an index to the zone watermark

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

> ALLOC_WMARK_MIN, ALLOC_WMARK_LOW and ALLOC_WMARK_HIGH determin whether
> pages_min, pages_low or pages_high is used as the zone watermark when
> allocating the pages. Two branches in the allocator hotpath determine which
> watermark to use. This patch uses the flags as an array index and places
> the three watermarks in a union with an array so it can be offset. This
> means the flags can be used as an array index and reduces the branches
> taken.
> 
> Signed-off-by: Mel Gorman <mel@csn.ul.ie>
> Reviewed-by: Christoph Lameter <cl@linux-foundation.org>
> ---
>  include/linux/mmzone.h |    8 +++++++-
>  mm/page_alloc.c        |   18 ++++++++----------
>  2 files changed, 15 insertions(+), 11 deletions(-)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index f82bdba..c1fa208 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -275,7 +275,13 @@ struct zone_reclaim_stat {
>  
>  struct zone {
>  	/* Fields commonly accessed by the page allocator */
> -	unsigned long		pages_min, pages_low, pages_high;
> +	union {
> +		struct {
> +			unsigned long	pages_min, pages_low, pages_high;
> +		};
> +		unsigned long pages_mark[3];
> +	};
> +

hmmm... I don't like union hack. 
Why can't we change all caller to use page_mark?




>  	/*
>  	 * We don't know if the memory that we're going to allocate will be freeable
>  	 * or/and it will be released eventually, so to avoid totally wasting several
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 376d848..e61867e 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1157,10 +1157,13 @@ failed:
>  	return NULL;
>  }
>  
> -#define ALLOC_NO_WATERMARKS	0x01 /* don't check watermarks at all */
> -#define ALLOC_WMARK_MIN		0x02 /* use pages_min watermark */
> -#define ALLOC_WMARK_LOW		0x04 /* use pages_low watermark */
> -#define ALLOC_WMARK_HIGH	0x08 /* use pages_high watermark */
> +/* The WMARK bits are used as an index zone->pages_mark */
> +#define ALLOC_WMARK_MIN		0x00 /* use pages_min watermark */
> +#define ALLOC_WMARK_LOW		0x01 /* use pages_low watermark */
> +#define ALLOC_WMARK_HIGH	0x02 /* use pages_high watermark */
> +#define ALLOC_NO_WATERMARKS	0x08 /* don't check watermarks at all */
> +#define ALLOC_WMARK_MASK	0x07 /* Mask to get the watermark bits */

the mask only use two bit. but mask definition is three bit (0x07), why?


> +
>  #define ALLOC_HARDER		0x10 /* try to alloc harder */
>  #define ALLOC_HIGH		0x20 /* __GFP_HIGH set */
>  #ifdef CONFIG_CPUSETS
> @@ -1463,12 +1466,7 @@ zonelist_scan:
>  
>  		if (!(alloc_flags & ALLOC_NO_WATERMARKS)) {
>  			unsigned long mark;
> -			if (alloc_flags & ALLOC_WMARK_MIN)
> -				mark = zone->pages_min;
> -			else if (alloc_flags & ALLOC_WMARK_LOW)
> -				mark = zone->pages_low;
> -			else
> -				mark = zone->pages_high;
> +			mark = zone->pages_mark[alloc_flags & ALLOC_WMARK_MASK];
>  			if (!zone_watermark_ok(zone, order, mark,
>  				    classzone_idx, alloc_flags)) {
>  				if (!zone_reclaim_mode ||
> -- 
> 1.5.6.5
> 



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