Airflow tasks get stuck at "queued" status and never gets running : <p>I'm using Airflow v1.8.1 and run all components (worker, web, flower, scheduler) on kubernetes &amp; Docker.
I use Celery Executor with Redis and my tasks are looks like:</p>

<pre><code>(start) -&gt; (do_work_for_product1)
     ├  -&gt; (do_work_for_product2)
     ├  -&gt; (do_work_for_product3)
     ├  …
</code></pre>

<p>So the <code>start</code> task has multiple downstreams. 
And I setup concurrency related configuration as below:</p>

<pre><code>parallelism = 3
dag_concurrency = 3
max_active_runs = 1
</code></pre>

<p>Then when I run this DAG manually (not sure if it never happens on a scheduled task) , some downstreams get executed, but others stuck at "queued" status.</p>

<p>If I clear the task from Admin UI, it gets executed.
There is no worker log (after processing some first downstreams, it just doesn't output any log). </p>

<p>Web server's log (not sure <code>worker exiting</code> is related)</p>

<pre><code>/usr/local/lib/python2.7/dist-packages/flask/exthook.py:71: ExtDeprecationWarning: Importing flask.ext.cache is deprecated, use flask_cache instead.
  .format(x=modname), ExtDeprecationWarning
[2017-08-24 04:20:56,496] [51] {models.py:168} INFO - Filling up the DagBag from /usr/local/airflow_dags
[2017-08-24 04:20:57 +0000] [27] [INFO] Handling signal: ttou
[2017-08-24 04:20:57 +0000] [37] [INFO] Worker exiting (pid: 37)
</code></pre>

<p>There is no error log on scheduler, too. And a number of tasks get stuck is changing whenever I try this.</p>

<p>Because I also use Docker I'm wondering if this is related:
<a href="https://github.com/puckel/docker-airflow/issues/94" rel="noreferrer">https://github.com/puckel/docker-airflow/issues/94</a>
But so far, no clue.</p>

<p>Has anyone faced with a similar issue or have some idea what I can investigate for this issue...?</p>
