airflow backfill 失败但是没有日志

运行airflow backfill时一直失败，但是看不到log，仔细观察运行backfill 命令后的输出发现有如下的错误

ERROR - Executor reports task instance <TaskInstance: example_trigger_controller_dag.test_trigger_dagrun 2017-07-10 00:00:00 [queued]> finished (failed) although the task says its queued. Was the task killed externally?

这是由于airflow默认将dag pickle后发送给worker运行导致的，使用-x参数可以避免pickle操作，问题得到解决。