workqueue: Process extra works in rescuer on memory pressure

Make the rescuer process more work on the last pwq when there are no
more to rescue for the whole workqueue to help the regular workers in
case it is a temporary memory pressure relief and to reduce relapse.

Signed-off-by: Lai Jiangshan <[email protected]>
Signed-off-by: Tejun Heo <[email protected]>

Author: Lai Jiangshan

kernel/workqueue.c

@@ -3461,10 +3461,37 @@ static bool assign_rescuer_work(struct pool_workqueue *pwq, struct worker *rescu
 	struct work_struct *cursor = &pwq->mayday_cursor;
 	struct work_struct *work, *n;
 
-	/* need rescue? */
-	if (!pwq->nr_active || !need_to_create_worker(pool))
+	/* have work items to rescue? */
+	if (!pwq->nr_active)
 		return false;
 
+	/* need rescue? */
+	if (!need_to_create_worker(pool)) {
+		/*
+		 * The pool has idle workers and doesn't need the rescuer, so it
+		 * could simply return false here.
+		 *
+		 * However, the memory pressure might not be fully relieved.
+		 * In PERCPU pool with concurrency enabled, having idle workers
+		 * does not necessarily mean memory pressure is gone; it may
+		 * simply mean regular workers have woken up, completed their
+		 * work, and gone idle again due to concurrency limits.
+		 *
+		 * In this case, those working workers may later sleep again,
+		 * the pool may run out of idle workers, and it will have to
+		 * allocate new ones and wait for the timer to send mayday,
+		 * causing unnecessary delay - especially if memory pressure
+		 * was never resolved throughout.
+		 *
+		 * Do more work if memory pressure is still on to reduce
+		 * relapse, using (pool->flags & POOL_MANAGER_ACTIVE), though
+		 * not precisely, unless there are other PWQs needing help.
+		 */
+		if (!(pool->flags & POOL_MANAGER_ACTIVE) ||
+		    !list_empty(&pwq->wq->maydays))
+			return false;
+	}
+
 	/* search from the start or cursor if available */
 	if (list_empty(&cursor->entry))
 		work = list_first_entry(&pool->worklist, struct work_struct, entry);