workqueue: don't use WQ_HIGHPRI for unbound workqueues

Unbound wqs aren't concurrency-managed and try to execute work items
as soon as possible.  This is currently achieved by implicitly setting
%WQ_HIGHPRI on all unbound workqueues; however, WQ_HIGHPRI
implementation is about to be restructured and this usage won't be
valid anymore.

Add an explicit chain-wakeup path for unbound workqueues in
process_one_work() instead of piggy backing on %WQ_HIGHPRI.

Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/workqueue.c

@@ -580,6 +580,10 @@ static bool __need_more_worker(struct global_cwq *gcwq)
 /*
  * Need to wake up a worker?  Called from anything but currently
  * running workers.
+ *
+ * Note that, because unbound workers never contribute to nr_running, this
+ * function will always return %true for unbound gcwq as long as the
+ * worklist isn't empty.
  */
 static bool need_more_worker(struct global_cwq *gcwq)
 {
@@ -1867,6 +1871,13 @@ __acquires(&gcwq->lock)
 	if (unlikely(cpu_intensive))
 		worker_set_flags(worker, WORKER_CPU_INTENSIVE, true);
 
+	/*
+	 * Unbound gcwq isn't concurrency managed and work items should be
+	 * executed ASAP.  Wake up another worker if necessary.
+	 */
+	if ((worker->flags & WORKER_UNBOUND) && need_more_worker(gcwq))
+		wake_up_worker(gcwq);
+
 	spin_unlock_irq(&gcwq->lock);
 
 	work_clear_pending(work);
@@ -2984,13 +2995,6 @@ struct workqueue_struct *__alloc_workqueue_key(const char *fmt,
 	if (flags & WQ_MEM_RECLAIM)
 		flags |= WQ_RESCUER;
 
-	/*
-	 * Unbound workqueues aren't concurrency managed and should be
-	 * dispatched to workers immediately.
-	 */
-	if (flags & WQ_UNBOUND)
-		flags |= WQ_HIGHPRI;
-
 	max_active = max_active ?: WQ_DFL_ACTIVE;
 	max_active = wq_clamp_max_active(max_active, flags, wq->name);