Re: [PATCH] mm: get rid of unnecessary pageblock scanning insetup_zone_migrate_reserve

[Mel Gorman]

On Wed, Oct 30, 2013 at 04:19:07PM -0400, KOSAKI Motohiro wrote:
> >@@ -3926,11 +3929,11 @@ static void setup_zone_migrate_reserve(struct zone *zone)
> >  	/*
> >  	 * Reserve blocks are generally in place to help high-order atomic
> >  	 * allocations that are short-lived. A min_free_kbytes value that
> >-	 * would result in more than 2 reserve blocks for atomic allocations
> >-	 * is assumed to be in place to help anti-fragmentation for the
> >-	 * future allocation of hugepages at runtime.
> >+	 * would result in more than MAX_MIGRATE_RESERVE_BLOCKS reserve blocks
> >+	 * for atomic allocations is assumed to be in place to help
> >+	 * anti-fragmentation for the future allocation of hugepages at runtime.
> >  	 */
> >-	reserve = min(2, reserve);
> >+	reserve = min(MAX_MIGRATE_RESERVE_BLOCKS, reserve);
> >
> >  	for (pfn = start_pfn; pfn < end_pfn; pfn += pageblock_nr_pages) {
> >  		if (!pfn_valid(pfn))
> >@@ -3956,6 +3959,7 @@ static void setup_zone_migrate_reserve(struct zone *zone)
> >  			/* If this block is reserved, account for it */
> >  			if (block_migratetype == MIGRATE_RESERVE) {
> >  				reserve--;
> >+				found++;
> >  				continue;
> >  			}
> >
> >@@ -3970,6 +3974,10 @@ static void setup_zone_migrate_reserve(struct zone *zone)
> >  			}
> >  		}
> >
> >+		/* If all possible reserve blocks have been found, we're done */
> >+		if (found >= MAX_MIGRATE_RESERVE_BLOCKS)
> >+			break;
> >+
> >  		/*
> >  		 * If the reserve is met and this is a previous reserved block,
> >  		 * take it back
> 
> Nit. I would like to add following hunk. This is just nit because moving
> reserve pageblock is extreme rare.
> 
> 		if (block_migratetype == MIGRATE_RESERVE) {
> +                       found++;
> 			set_pageblock_migratetype(page, MIGRATE_MOVABLE);
> 			move_freepages_block(zone, page, MIGRATE_MOVABLE);
> 		}

I don't really see the advantage but if you think it is necessary then I
do not object either.

-- 
Mel Gorman
SUSE Labs
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