mm, vmscan: prevent useless kswapd loops

On production we have noticed hard lockups on large machines running large jobs due to kswaps hoarding lru lock within isolate_lru_pages when sc->reclaim_idx is 0 which is a small zone. The lru was couple hundred GiBs and the condition (page_zonenum(page) > sc->reclaim_idx) in isolate_lru_pages was basically skipping GiBs of pages while holding the LRU spinlock with interrupt disabled.

On further inspection, it seems like there are two issues:

1) If the kswapd on the return from balance_pgdat() could not sleep (i.e. node is still unbalanced), the classzone_idx is unintentionally set to 0 and the whole reclaim cycle of kswapd will try to reclaim only the lowest and smallest zone while traversing the whole memory.

2) Fundamentally isolate_lru_pages() is really bad when the allocation has woken kswapd for a smaller zone on a very large machine running very large jobs. It can hoard the LRU spinlock while skipping over 100s of GiBs of pages.

This patch only fixes the (1). The (2) needs a more fundamental solution. To fix (1), in the kswapd context, if pgdat->kswapd_classzone_idx is invalid use the classzone_idx of the previous kswapd loop otherwise use the one the waker has requested.

Fixes e716f2eb24de ("mm, vmscan: prevent kswapd sleeping prematurely

due to mismatched classzone_idx")

Signed-off-by Shakeel Butt <[email protected]>

---
Changelog since v1:
- fixed the patch based on Yang Shi's comment.

  mm/vmscan.c | 27 +++++++++++++++------------
  1 file changed, 15 insertions(+), 12 deletions(-)

mm/vmscan.c +9 -11

index 9e3292ee5c7c..eacf87f07afe 100644
@@ -3760,19 +3760,18 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int classzone_idx)
  }
 
  /*
- * pgdat->kswapd_classzone_idx is the highest zone index that a recent
- * allocation request woke kswapd for. When kswapd has not woken recently,
- * the value is MAX_NR_ZONES which is not a valid index. This compares a
- * given classzone and returns it or the highest classzone index kswapd
- * was recently woke for.
+ * The pgdat->kswapd_classzone_idx is used to pass the highest zone index to be
+ * reclaimed by kswapd from the waker. If the value is MAX_NR_ZONES which is not
+ * a valid index then either kswapd runs for first time or kswapd couldn't sleep
+ * after previous reclaim attempt (node is still unbalanced). In that case
+ * return the zone index of the previous kswapd reclaim cycle.
   */
  static enum zone_type kswapd_classzone_idx(pg_data_t *pgdat,
-					   enum zone_type classzone_idx)
+					   enum zone_type prev_classzone_idx)
  {
  	if (pgdat->kswapd_classzone_idx == MAX_NR_ZONES)
-		return classzone_idx;
-
-	return max(pgdat->kswapd_classzone_idx, classzone_idx);
+		return prev_classzone_idx;
+	return pgdat->kswapd_classzone_idx;
  }
 
  static void kswapd_try_to_sleep(pg_data_t *pgdat, int alloc_order, int reclaim_order,

@@ -3908,7 +3907,7 @@ static int kswapd(void *p)
 
  		/* Read the new order and classzone_idx */
  		alloc_order = reclaim_order = pgdat->kswapd_order;
-		classzone_idx = kswapd_classzone_idx(pgdat, 0);
+		classzone_idx = kswapd_classzone_idx(pgdat, classzone_idx);
  		pgdat->kswapd_order = 0;
  		pgdat->kswapd_classzone_idx = MAX_NR_ZONES;
 

@@ -3961,8 +3960,12 @@ void wakeup_kswapd(struct zone *zone, gfp_t gfp_flags, int order,
  	if (!cpuset_zone_allowed(zone, gfp_flags))
  		return;
  	pgdat = zone->zone_pgdat;
-	pgdat->kswapd_classzone_idx = kswapd_classzone_idx(pgdat,

On production we have noticed hard lockups on large machines running large jobs due to kswaps hoarding lru lock within isolate_lru_pages when sc->reclaim_idx is 0 which is a small zone. The lru was couple hundred GiBs and the condition (page_zonenum(page) > sc->reclaim_idx) in isolate_lru_pages was basically skipping GiBs of pages while holding the LRU spinlock with interrupt disabled.

On further inspection, it seems like there are two issues:

1) If the kswapd on the return from balance_pgdat() could not sleep (i.e. node is still unbalanced), the classzone_idx is unintentionally set to 0 and the whole reclaim cycle of kswapd will try to reclaim only the lowest and smallest zone while traversing the whole memory.

2) Fundamentally isolate_lru_pages() is really bad when the allocation has woken kswapd for a smaller zone on a very large machine running very large jobs. It can hoard the LRU spinlock while skipping over 100s of GiBs of pages.

This patch only fixes the (1). The (2) needs a more fundamental solution. To fix (1), in the kswapd context, if pgdat->kswapd_classzone_idx is invalid use the classzone_idx of the previous kswapd loop otherwise use the one the waker has requested.

Fixes e716f2eb24de ("mm, vmscan: prevent kswapd sleeping prematurely

due to mismatched classzone_idx")

Signed-off-by Shakeel Butt <[email protected]>