[PATCH v2 5/5] mm/mglru: reschedule between memcg reclaim attempts

[Max Kellermann <max.kellermann@ionos.com>]

shrink_one() can return immediately for empty or protected memcgs, and
try_to_shrink_lruvec() can break out before reaching its
cond_resched().  A scan through many memcgs can repeatedly rotate
entries without giving other runnable tasks a scheduling opportunity.

Call cond_resched_tasks_rcu_qs() (as the existing shrink_lruvec() loop
does) after shrink_one(), while the memcg reference still pins the
cursor and before reacquiring the RCU read lock.  Also call it between
bins and nulls-marker retries, covering scans that skip every entry.

Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
---
 mm/vmscan.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 899f756bd15b..5dc5e698e243 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -5395,6 +5395,9 @@ static void shrink_many(struct pglist_data *pgdat, struct scan_control *sc)
 	gen = get_memcg_gen(READ_ONCE(pgdat->memcg_lru.seq));
 	bin = first_bin = get_random_u32_below(MEMCG_NR_BINS);
 restart:
+	/* Yield between bins/retries that contain only skipped entries. */
+	cond_resched_tasks_rcu_qs();
+
 	op = 0;
 	memcg = NULL;
 
@@ -5425,6 +5428,14 @@ static void shrink_many(struct pglist_data *pgdat, struct scan_control *sc)
 
 		op = shrink_one(lruvec, sc);
 
+		/*
+		 * Empty or protected memcgs can bypass the
+		 * rescheduling points in page reclaim.  The memcg
+		 * reference pins our cursor while we yield outside
+		 * the RCU read-side critical section.
+		 */
+		cond_resched_tasks_rcu_qs();
+
 		rcu_read_lock();
 
 		if (should_abort_scan(lruvec, sc))
-- 
2.53.0