workqueue: Make wq_adjust_max_active() round-robin pwqs while activating

wq_adjust_max_active() needs to activate work items after max_active is
increased. Previously, it did that by visiting each pwq once activating all
that could be activated. While this makes sense with per-pwq nr_active,
nr_active will be shared across multiple pwqs for unbound wqs. Then, we'd
want to round-robin through pwqs to be fairer.

In preparation, this patch makes wq_adjust_max_active() round-robin pwqs
while activating. While the activation ordering changes, this shouldn't
cause user-noticeable behavior changes.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <jiangshanlai@gmail.com>

kernel/workqueue.c

@@ -4710,7 +4710,7 @@ static int init_rescuer(struct workqueue_struct *wq)
  */
 static void wq_adjust_max_active(struct workqueue_struct *wq)
 {
-	struct pool_workqueue *pwq;
+	bool activated;
 
 	lockdep_assert_held(&wq->mutex);
 
@@ -4730,19 +4730,26 @@ static void wq_adjust_max_active(struct workqueue_struct *wq)
 	 */
 	WRITE_ONCE(wq->max_active, wq->saved_max_active);
 
+	/*
+	 * Round-robin through pwq's activating the first inactive work item
+	 * until max_active is filled.
+	 */
+	do {
+		struct pool_workqueue *pwq;
+
+		activated = false;
 		for_each_pwq(pwq, wq) {
 			unsigned long flags;
 
-		/* this function can be called during early boot w/ irq disabled */
+			/* can be called during early boot w/ irq disabled */
 			raw_spin_lock_irqsave(&pwq->pool->lock, flags);
-
-		while (pwq_activate_first_inactive(pwq))
-			;
-
+			if (pwq_activate_first_inactive(pwq)) {
+				activated = true;
 				kick_pool(pwq->pool);
-
+			}
 			raw_spin_unlock_irqrestore(&pwq->pool->lock, flags);
 		}
+	} while (activated);
 }
 
 __printf(1, 4)