On Mon, 20 Sep 2021 09:54:32 +0100 Mel Gorman wrote:

+static void
+reclaim_throttle(pg_data_t *pgdat, enum vmscan_throttle_state reason,
+							long timeout)
+{
+	wait_queue_head_t *wqh =3D &pgdat->reclaim_wait;
+	unsigned long start =3D jiffies;
+	long ret;
+	DEFINE_WAIT(wait);
+
+	atomic_inc(&pgdat->nr_reclaim_throttled);
+	WRITE_ONCE(pgdat->nr_reclaim_start,
+		 node_page_state(pgdat, NR_THROTTLED_WRITTEN));

Missing wakeup could happen if the current sleeper overwrites 
pgdat->nr_reclaim_start set by the existing sleeper.

	if (1 == atomic_inc_and_return(&pgdat->nr_reclaim_throttled))
		WRITE_ONCE(pgdat->nr_reclaim_start,
				node_page_state(pgdat, NR_THROTTLED_WRITTEN));

+
+	prepare_to_wait(wqh, &wait, TASK_INTERRUPTIBLE);
+	ret =3D schedule_timeout(timeout);
+	finish_wait(wqh, &wait);
+	atomic_dec(&pgdat->nr_reclaim_throttled);
+
+	trace_mm_vmscan_throttled(pgdat->node_id, jiffies_to_usecs(timeout),
+				jiffies_to_usecs(jiffies - start),
+				reason);
+}
+
+/*
+ * Account for pages written if tasks are throttled waiting on dirty
+ * pages to clean. If enough pages have been cleaned since throttling
+ * started then wakeup the throttled tasks.
+ */
+void __acct_reclaim_writeback(pg_data_t *pgdat, struct page *page)
+{
+	unsigned long nr_written;
+	int nr_throttled =3D atomic_read(&pgdat->nr_reclaim_throttled);
+
+	__inc_node_page_state(page, NR_THROTTLED_WRITTEN);
+	nr_written =3D node_page_state(pgdat, NR_THROTTLED_WRITTEN) -
+		READ_ONCE(pgdat->nr_reclaim_start);
+
+	if (nr_written > SWAP_CLUSTER_MAX * nr_throttled)
+		wake_up_interruptible_all(&pgdat->reclaim_wait);
+}