workqueue: unfold start_worker() into create_worker()

Simply unfold the code of start_worker() into create_worker() and
remove the original start_worker() and create_and_start_worker().

The only trade-off is the introduced overhead that the pool->lock
is released and regrabbed after the newly worker is started.
The overhead is acceptible since the manager is slow path.

And because this new locking behavior, the newly created worker
may grab the lock earlier than the manager and go to process
work items. In this case, the recheck need_to_create_worker() may be
true as expected and the manager goes to restart which is the
correct behavior.

tj: Minor updates to description and comments.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/workqueue.c

@@ -1540,7 +1540,7 @@ static void worker_enter_idle(struct worker *worker)
 			 (worker->hentry.next || worker->hentry.pprev)))
 		return;
 
-	/* can't use worker_set_flags(), also called from start_worker() */
+	/* can't use worker_set_flags(), also called from create_worker() */
 	worker->flags |= WORKER_IDLE;
 	pool->nr_idle++;
 	worker->last_active = jiffies;
@@ -1661,8 +1661,7 @@ static void worker_detach_from_pool(struct worker *worker,
  * create_worker - create a new workqueue worker
  * @pool: pool the new worker will belong to
  *
- * Create a new worker which is attached to @pool.  The new worker must be
- * started by start_worker().
+ * Create and start a new worker which is attached to @pool.
  *
  * CONTEXT:
  * Might sleep.  Does GFP_KERNEL allocations.
@@ -1707,6 +1706,13 @@ static struct worker *create_worker(struct worker_pool *pool)
 	/* successful, attach the worker to the pool */
 	worker_attach_to_pool(worker, pool);
 
+	/* start the newly created worker */
+	spin_lock_irq(&pool->lock);
+	worker->pool->nr_workers++;
+	worker_enter_idle(worker);
+	wake_up_process(worker->task);
+	spin_unlock_irq(&pool->lock);
+
 	return worker;
 
 fail:
@@ -1716,44 +1722,6 @@ fail:
 	return NULL;
 }
 
-/**
- * start_worker - start a newly created worker
- * @worker: worker to start
- *
- * Make the pool aware of @worker and start it.
- *
- * CONTEXT:
- * spin_lock_irq(pool->lock).
- */
-static void start_worker(struct worker *worker)
-{
-	worker->pool->nr_workers++;
-	worker_enter_idle(worker);
-	wake_up_process(worker->task);
-}
-
-/**
- * create_and_start_worker - create and start a worker for a pool
- * @pool: the target pool
- *
- * Grab the managership of @pool and create and start a new worker for it.
- *
- * Return: 0 on success. A negative error code otherwise.
- */
-static int create_and_start_worker(struct worker_pool *pool)
-{
-	struct worker *worker;
-
-	worker = create_worker(pool);
-	if (worker) {
-		spin_lock_irq(&pool->lock);
-		start_worker(worker);
-		spin_unlock_irq(&pool->lock);
-	}
-
-	return worker ? 0 : -ENOMEM;
-}
-
 /**
  * destroy_worker - destroy a workqueue worker
  * @worker: worker to be destroyed
@@ -1892,19 +1860,7 @@ restart:
 	mod_timer(&pool->mayday_timer, jiffies + MAYDAY_INITIAL_TIMEOUT);
 
 	while (true) {
-		struct worker *worker;
-
-		worker = create_worker(pool);
-		if (worker) {
-			del_timer_sync(&pool->mayday_timer);
-			spin_lock_irq(&pool->lock);
-			start_worker(worker);
-			if (WARN_ON_ONCE(need_to_create_worker(pool)))
-				goto restart;
-			return true;
-		}
-
-		if (!need_to_create_worker(pool))
+		if (create_worker(pool) || !need_to_create_worker(pool))
 			break;
 
 		schedule_timeout_interruptible(CREATE_COOLDOWN);
@@ -1915,6 +1871,11 @@ restart:
 
 	del_timer_sync(&pool->mayday_timer);
 	spin_lock_irq(&pool->lock);
+	/*
+	 * This is necessary even after a new worker was just successfully
+	 * created as @pool->lock was dropped and the new worker might have
+	 * already become busy.
+	 */
 	if (need_to_create_worker(pool))
 		goto restart;
 	return true;
@@ -3537,7 +3498,7 @@ static struct worker_pool *get_unbound_pool(const struct workqueue_attrs *attrs)
 		goto fail;
 
 	/* create and start the initial worker */
-	if (create_and_start_worker(pool) < 0)
+	if (!create_worker(pool))
 		goto fail;
 
 	/* install */
@@ -4611,7 +4572,7 @@ static int workqueue_cpu_up_callback(struct notifier_block *nfb,
 		for_each_cpu_worker_pool(pool, cpu) {
 			if (pool->nr_workers)
 				continue;
-			if (create_and_start_worker(pool) < 0)
+			if (!create_worker(pool))
 				return NOTIFY_BAD;
 		}
 		break;
@@ -4911,7 +4872,7 @@ static int __init init_workqueues(void)
 
 		for_each_cpu_worker_pool(pool, cpu) {
 			pool->flags &= ~POOL_DISASSOCIATED;
-			BUG_ON(create_and_start_worker(pool) < 0);
+			BUG_ON(!create_worker(pool));
 		}
 	}