quoted

What is it actually trying to do, and why?  It counts up the number of
contiguous empty lists and then frees that number of pages from the
first-encountered non-empty list and then advances onto the next list?

What's the point in that?  What relationship does the number of
contiguous empty lists have with the number of pages to free from one
list?

It at least recovers some of the otherwise wasted effort of looking at
an empty list, by flushing more pages once it encounters a non-empty
list.  After all, freeing to_free pages is the goal.

That breaks the round-robin fashion, though.  If list-1 has pages,
list-2 is empty and list-3 has pages, it will repeatedly free one page
from list-1 and two pages from list-3.

My initial response to Namhyung's patch was to write up a version that
used a bitmap for all lists.  It starts with all lists set and clears
their respective bit once the list is empty, so it would never
consider them again.  But it looked a bit over-engineered for 3 lists
and the resulting object code was bigger than what we have now.
Though, it would be more readable.  Attached for reference (untested
and all).

	Hannes

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 60e58b0..c77ab28 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c

@@ -590,8 +590,7 @@ static inline int free_pages_check(struct page *page)
 static void free_pcppages_bulk(struct zone *zone, int count,
 					struct per_cpu_pages *pcp)
 {
-	int migratetype = 0;
-	int batch_free = 0;
+	unsigned long listmap = (1 << MIGRATE_PCPTYPES) - 1;
 	int to_free = count;
 
 	spin_lock(&zone->lock);

@@ -599,31 +598,29 @@ static void free_pcppages_bulk(struct zone *zone, int count,
 	zone->pages_scanned = 0;
 
 	while (to_free) {
-		struct page *page;
-		struct list_head *list;
-
+		int migratetype;
 		/*
-		 * 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
+		 * Remove pages from lists in a round-robin fashion.
+		 * Empty lists are excluded from subsequent rounds.
 		 */
-		do {
-			batch_free++;
-			if (++migratetype == MIGRATE_PCPTYPES)
-				migratetype = 0;
-			list = &pcp->lists[migratetype];
-		} while (list_empty(list));
+		for_each_set_bit (migratetype, &listmap, MIGRATE_PCPTYPES) {
+			struct list_head *list;
+			struct page *page;
 
-		do {
+			list = &pcp->lists[migratetype];
+			if (list_empty(list)) {
+				listmap &= ~(1 << migratetype);
+				continue;
+			}
+			if (!to_free--)
+				break;
 			page = list_entry(list->prev, struct page, lru);
 			/* must delete as __free_one_page list manipulates */
 			list_del(&page->lru);
 			/* MIGRATE_MOVABLE list may include MIGRATE_RESERVEs */
 			__free_one_page(page, zone, 0, page_private(page));
 			trace_mm_page_pcpu_drain(page, 0, page_private(page));
-		} while (--to_free && --batch_free && !list_empty(list));
+		}
 	}
 	__mod_zone_page_state(zone, NR_FREE_PAGES, count);
 	spin_unlock(&zone->lock);