Re: mm: 5.16 regression: reclaim_throttle leads to stall in near-OOM conditions

[Mel Gorman]

On Wed, Nov 24, 2021 at 07:54:49PM +0900, Alexey Avramov wrote:
> > it does eventually get killed OOM
> 
> However, a full minute freeze can be a great evil in many situations - 
> during such a freeze, the system is completely unresponsive. 
> 
> So my next question is: How reasonable is the value MAX_RECLAIM_RETRIES?
> Is it also get "out of thin air"?
> 

The value is out of thin air but adjusting it may reintroduce issues
with kswapd running at 100% CPU.

> And would it make sense to have buttons to adjust the timeouts?

I don't think we should introduce a tunable for something like this,
it'll be impossible to use properly but can you test this?

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 07db03883062..aa72c0f39dcc 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1058,6 +1058,14 @@ void reclaim_throttle(pg_data_t *pgdat, enum vmscan_throttle_state reason)
 		break;
 	case VMSCAN_THROTTLE_NOPROGRESS:
 		timeout = HZ/2;
+
+		/*
+		 * If kswapd is disabled, use the minimum timeout as the
+		 * system may be at or near OOM.
+		 */
+		if (pgdat->kswapd_failures >= MAX_RECLAIM_RETRIES)
+			timeout = 1;
+
 		break;
 	case VMSCAN_THROTTLE_ISOLATED:
 		timeout = HZ/50;
@@ -3395,7 +3403,7 @@ 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 +3423,7 @@ 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 +3487,18 @@ 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.