page-allocator: maintain rolling count of pages to free from the PCP

When round-robin freeing pages from the PCP lists, empty lists may be
encountered.  In the event one of the lists has more pages than another,
there may be numerous checks for list_empty() which is undesirable.  This
patch maintains a count of pages to free which is incremented when empty
lists are encountered.  The intention is that more pages will then be
freed from fuller lists than the empty ones reducing the number of empty
list checks in the free path.

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Mel Gorman <mel@csn.ul.ie>
Cc: Nick Piggin <npiggin@suse.de>
Cc: Christoph Lameter <cl@linux-foundation.org>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
Cc: Pekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_alloc.c

@@ -525,32 +525,38 @@ static void free_pcppages_bulk(struct zone *zone, int count,
 					struct per_cpu_pages *pcp)
 {
 	int migratetype = 0;
+	int batch_free = 0;
 
 	spin_lock(&zone->lock);
 	zone_clear_flag(zone, ZONE_ALL_UNRECLAIMABLE);
 	zone->pages_scanned = 0;
 
 	__mod_zone_page_state(zone, NR_FREE_PAGES, count);
-	while (count--) {
+	while (count) {
 		struct page *page;
 		struct list_head *list;
 
 		/*
-		 * Remove pages from lists in a round-robin fashion. This spinning
-		 * around potentially empty lists is bloody awful, alternatives that
-		 * don't suck are welcome
+		 * Remove pages from lists in a round-robin fashion. A
+		 * batch_free count is maintained that is incremented when an
+		 * empty list is encountered.  This is so more pages are freed
+		 * off fuller lists instead of spinning excessively around empty
+		 * lists
 		 */
 		do {
+			batch_free++;
 			if (++migratetype == MIGRATE_PCPTYPES)
 				migratetype = 0;
 			list = &pcp->lists[migratetype];
 		} while (list_empty(list));
 
+		do {
 			page = list_entry(list->prev, struct page, lru);
-		/* have to delete it as __free_one_page list manipulates */
+			/* must delete as __free_one_page list manipulates */
 			list_del(&page->lru);
-		trace_mm_page_pcpu_drain(page, 0, migratetype);
 			__free_one_page(page, zone, 0, migratetype);
+			trace_mm_page_pcpu_drain(page, 0, migratetype);
+		} while (--count && --batch_free && !list_empty(list));
 	}
 	spin_unlock(&zone->lock);
 }