Re: [PATCH 03/10] mm, page_alloc: Remove unnecessary recalculations for dirty zone balancing

[Michal Hocko]

On Wed 12-08-15 11:45:28, Mel Gorman wrote:
> File-backed pages that will be immediately are balanced between zones but
					    ^written to...

> it's unnecessarily expensive.

to do WHAT? I guess you meant checking gfp_mask resp. alloc_mask? I
doubt it would make a noticeable difference as this is a slow path
already but I agree it doesn't make sense to check it again.

> Move consider_zone_balanced into the alloc_context
> instead of checking bitmaps multiple times. The patch also gives the parameter
> a more meaningful name.
> 
> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> Acked-by: David Rientjes <rientjes@xxxxxxxxxx>
> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

Acked-by: Michal Hocko <mhocko@xxxxxxxx>

> ---
>  mm/internal.h   |  1 +
>  mm/page_alloc.c | 11 +++++++----
>  2 files changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/internal.h b/mm/internal.h
> index 36b23f1e2ca6..9331f802a067 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -129,6 +129,7 @@ struct alloc_context {
>  	int classzone_idx;
>  	int migratetype;
>  	enum zone_type high_zoneidx;
> +	bool spread_dirty_pages;
>  };
>  
>  /*
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 5e1f6f4370bc..94f2f6bdd6d5 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2297,8 +2297,6 @@ get_page_from_freelist(gfp_t gfp_mask, unsigned int order, int alloc_flags,
>  	struct zoneref *z;
>  	struct page *page = NULL;
>  	struct zone *zone;
> -	bool consider_zone_dirty = (alloc_flags & ALLOC_WMARK_LOW) &&
> -				(gfp_mask & __GFP_WRITE);
>  	int nr_fair_skipped = 0;
>  	bool zonelist_rescan;
>  
> @@ -2350,14 +2348,14 @@ get_page_from_freelist(gfp_t gfp_mask, unsigned int order, int alloc_flags,
>  		 *
>  		 * XXX: For now, allow allocations to potentially
>  		 * exceed the per-zone dirty limit in the slowpath
> -		 * (ALLOC_WMARK_LOW unset) before going into reclaim,
> +		 * (spread_dirty_pages unset) before going into reclaim,
>  		 * which is important when on a NUMA setup the allowed
>  		 * zones are together not big enough to reach the
>  		 * global limit.  The proper fix for these situations
>  		 * will require awareness of zones in the
>  		 * dirty-throttling and the flusher threads.
>  		 */
> -		if (consider_zone_dirty && !zone_dirty_ok(zone))
> +		if (ac->spread_dirty_pages && !zone_dirty_ok(zone))
>  			continue;
>  
>  		mark = zone->watermark[alloc_flags & ALLOC_WMARK_MASK];
> @@ -2997,6 +2995,10 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order,
>  
>  	/* We set it here, as __alloc_pages_slowpath might have changed it */
>  	ac.zonelist = zonelist;
> +
> +	/* Dirty zone balancing only done in the fast path */
> +	ac.spread_dirty_pages = (gfp_mask & __GFP_WRITE);
> +
>  	/* The preferred zone is used for statistics later */
>  	preferred_zoneref = first_zones_zonelist(ac.zonelist, ac.high_zoneidx,
>  				ac.nodemask, &ac.preferred_zone);
> @@ -3014,6 +3016,7 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order,
>  		 * complete.
>  		 */
>  		alloc_mask = memalloc_noio_flags(gfp_mask);
> +		ac.spread_dirty_pages = false;
>  
>  		page = __alloc_pages_slowpath(alloc_mask, order, &ac);
>  	}
> -- 
> 2.4.6
> 
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-- 
Michal Hocko
SUSE Labs
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