drbd: reduce lock contention in drbd_worker

The worker may now dequeue work items in batches.
This should reduce lock contention during busy periods.
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

drivers/block/drbd/drbd_worker.c

@@ -2026,8 +2026,6 @@ static void wait_for_work(struct drbd_connection *connection, struct list_head *
 		prepare_to_wait(&connection->sender_work.q_wait, &wait, TASK_INTERRUPTIBLE);
 		spin_lock_irq(&connection->resource->req_lock);
 		spin_lock(&connection->sender_work.q_lock);	/* FIXME get rid of this one? */
-		/* dequeue single item only,
-		 * we still use drbd_queue_work_front() in some places */
 		if (!list_empty(&connection->sender_work.q))
 			list_splice_tail_init(&connection->sender_work.q, work_list);
 		spin_unlock(&connection->sender_work.q_lock);	/* FIXME get rid of this one? */
@@ -2114,7 +2112,7 @@ int drbd_worker(struct drbd_thread *thi)
 		if (get_t_state(thi) != RUNNING)
 			break;
 
-		while (!list_empty(&work_list)) {
+		if (!list_empty(&work_list)) {
 			w = list_first_entry(&work_list, struct drbd_work, list);
 			list_del_init(&w->list);
 			update_worker_timing_details(connection, w->cb);
@@ -2130,13 +2128,13 @@ int drbd_worker(struct drbd_thread *thi)
 			update_worker_timing_details(connection, do_unqueued_work);
 			do_unqueued_work(connection);
 		}
-		while (!list_empty(&work_list)) {
+		if (!list_empty(&work_list)) {
 			w = list_first_entry(&work_list, struct drbd_work, list);
 			list_del_init(&w->list);
 			update_worker_timing_details(connection, w->cb);
 			w->cb(w, 1);
-		}
-		dequeue_work_batch(&connection->sender_work, &work_list);
+		} else
+			dequeue_work_batch(&connection->sender_work, &work_list);
 	} while (!list_empty(&work_list) || test_bit(DEVICE_WORK_PENDING, &connection->flags));
 
 	rcu_read_lock();