[PATCH 1/1] mm: vmscan: Reduce throttling due to a failure to make progress

[Mel Gorman]

Mike Galbraith, Alexey Avramov and Darrick Wong all reported similar
problems due to reclaim throttling for excessive lengths of time.
In Alexey's case, a memory hog that should go OOM quickly stalls for
several minutes before stalling. In Mike and Darrick's cases, a small
memcg environment stalled excessively even though the system had enough
memory overall.

Commit 69392a403f49 ("mm/vmscan: throttle reclaim when no progress is being
made") introduced the problem although commit a19594ca4a8b ("mm/vmscan:
increase the timeout if page reclaim is not making progress") made it
worse. Systems at or near an OOM state that cannot be recovered must
reach OOM quickly and memcg should kill tasks if a memcg is near OOM.

To address this, only stall for the first zone in the zonelist, reduce
the timeout to 1 tick for VMSCAN_THROTTLE_NOPROGRESS and only stall if
the scan control nr_reclaimed is 0 and kswapd is still active.  If kswapd
has stopped reclaiming due to excessive failures, do not stall at all so
that OOM triggers relatively quickly.

Alexey's test case was the most straight forward

	for i in {1..3}; do tail /dev/zero; done

On vanilla 5.16-rc1, this test stalled and was reset after 10 minutes.
After the patch, the test gets killed after roughly 15 seconds which is
the same length of time taken in 5.15.

Link: https://lore.kernel.org/r/99e779783d6c7fce96448a3402061b9dc1b3b602.camel@xxxxxx
Link: https://lore.kernel.org/r/20211124011954.7cab9bb4@xxxxxxxxxxxxx
Link: https://lore.kernel.org/r/20211022144651.19914-1-mgorman@xxxxxxxxxxxxxxxxxxx

Fixes: 69392a403f49 ("mm/vmscan: throttle reclaim when no progress is being made")
Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Tested-by: Darrick J. Wong <djwong@xxxxxxxxxx>
---
 mm/vmscan.c | 21 ++++++++++++++++++---
 1 file changed, 18 insertions(+), 3 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index fb9584641ac7..176ddd28df21 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1057,7 +1057,17 @@ void reclaim_throttle(pg_data_t *pgdat, enum vmscan_throttle_state reason)
 
 		break;
 	case VMSCAN_THROTTLE_NOPROGRESS:
-		timeout = HZ/2;
+		timeout = 1;
+
+		/*
+		 * If kswapd is disabled, reschedule if necessary but do not
+		 * throttle as the system is likely near OOM.
+		 */
+		if (pgdat->kswapd_failures >= MAX_RECLAIM_RETRIES) {
+			cond_resched();
+			return;
+		}
+
 		break;
 	case VMSCAN_THROTTLE_ISOLATED:
 		timeout = HZ/50;
@@ -3395,7 +3405,7 @@ static void consider_reclaim_throttle(pg_data_t *pgdat, struct scan_control *sc)
 		return;
 
 	/* Throttle if making no progress at high prioities. */
-	if (sc->priority < DEF_PRIORITY - 2)
+	if (sc->priority < DEF_PRIORITY - 2 && !sc->nr_reclaimed)
 		reclaim_throttle(pgdat, VMSCAN_THROTTLE_NOPROGRESS);
 }
 
@@ -3415,6 +3425,7 @@ static void shrink_zones(struct zonelist *zonelist, struct scan_control *sc)
 	unsigned long nr_soft_scanned;
 	gfp_t orig_mask;
 	pg_data_t *last_pgdat = NULL;
+	pg_data_t *first_pgdat = NULL;
 
 	/*
 	 * If the number of buffer_heads in the machine exceeds the maximum
@@ -3478,14 +3489,18 @@ static void shrink_zones(struct zonelist *zonelist, struct scan_control *sc)
 			/* need some check for avoid more shrink_zone() */
 		}
 
+		if (!first_pgdat)
+			first_pgdat = zone->zone_pgdat;
+
 		/* See comment about same check for global reclaim above */
 		if (zone->zone_pgdat == last_pgdat)
 			continue;
 		last_pgdat = zone->zone_pgdat;
 		shrink_node(zone->zone_pgdat, sc);
-		consider_reclaim_throttle(zone->zone_pgdat, sc);
 	}
 
+	consider_reclaim_throttle(first_pgdat, sc);
+
 	/*
 	 * Restore to original mask to avoid the impact on the caller if we
 	 * promoted it to __GFP_HIGHMEM.
-- 
2.31.1