[PATCH 10/14] mm: only count actual rotations as LRU reclaim cost

[Johannes Weiner <hannes@cmpxchg.org>]

When shrinking the active file list we rotate referenced pages only
when they're in an executable mapping. The others get deactivated.
When it comes to balancing scan pressure, though, we count all
referenced pages as rotated, even the deactivated ones. Yet they do
not carry the same cost to the system: the deactivated page *might*
refault later on, but the deactivation is tangible progress toward
freeing pages; rotations on the other hand cost time and effort
without getting any closer to freeing memory.

Don't treat both events as equal. The following patch will hook up LRU
balancing to cache and anon refaults, which are a much more concrete
cost signal for reclaiming one list over the other. Thus, remove the
maybe-IO cost bias from page references, and only note the CPU cost
for actual rotations that prevent the pages from getting reclaimed.

v2: readable changelog (Michal Hocko)

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Minchan Kim <minchan@kernel.org>
Acked-by: Michal Hocko <mhocko@suse.com>
---
 mm/vmscan.c | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 6ff63906a288..2c3fb8dd1159 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2054,7 +2054,6 @@ static void shrink_active_list(unsigned long nr_to_scan,
 
 		if (page_referenced(page, 0, sc->target_mem_cgroup,
 				    &vm_flags)) {
-			nr_rotated += hpage_nr_pages(page);
 			/*
 			 * Identify referenced, file-backed active pages and
 			 * give them one more trip around the active list. So
@@ -2065,6 +2064,7 @@ static void shrink_active_list(unsigned long nr_to_scan,
 			 * so we ignore them here.
 			 */
 			if ((vm_flags & VM_EXEC) && page_is_file_lru(page)) {
+				nr_rotated += hpage_nr_pages(page);
 				list_add(&page->lru, &l_active);
 				continue;
 			}
@@ -2080,10 +2080,8 @@ static void shrink_active_list(unsigned long nr_to_scan,
 	 */
 	spin_lock_irq(&pgdat->lru_lock);
 	/*
-	 * Count referenced pages from currently used mappings as rotated,
-	 * even though only some of them are actually re-activated.  This
-	 * helps balance scan pressure between file and anonymous pages in
-	 * get_scan_count.
+	 * Rotating pages costs CPU without actually
+	 * progressing toward the reclaim goal.
 	 */
 	lru_note_cost(lruvec, file, nr_rotated);
 
-- 
2.26.2