diff --git a/swh/scheduler/celery_backend/config.py b/swh/scheduler/celery_backend/config.py
--- a/swh/scheduler/celery_backend/config.py
+++ b/swh/scheduler/celery_backend/config.py
@@ -108,7 +108,7 @@
                     and obj != Task  # Don't register the abstract class itself
             ):
                 class_name = '%s.%s' % (module_name, name)
-                instance.app.register_task_class(class_name, obj)
+                register_task_class(instance.app, class_name, obj)
 
     for task_name in instance.app.tasks:
         if task_name.startswith('swh.'):
@@ -127,58 +127,59 @@
         return {'queue': name}
 
 
-class CustomCelery(Celery):
-    def get_queue_stats(self, queue_name):
-        """Get the statistics regarding a queue on the broker.
-
-        Arguments:
-          queue_name: name of the queue to check
-
-        Returns a dictionary raw from the RabbitMQ management API;
-        or `None` if the current configuration does not use RabbitMQ.
-
-        Interesting keys:
-         - consumers (number of consumers for the queue)
-         - messages (number of messages in queue)
-         - messages_unacknowledged (number of messages currently being
-           processed)
-
-        Documentation: https://www.rabbitmq.com/management.html#http-api
-        """
-
-        conn_info = self.connection().info()
-        if conn_info['transport'] == 'memory':
-            # We're running in a test environment, without RabbitMQ.
-            return None
-        url = 'http://{hostname}:{port}/api/queues/{vhost}/{queue}'.format(
-            hostname=conn_info['hostname'],
-            port=conn_info['port'] + 10000,
-            vhost=urllib.parse.quote(conn_info['virtual_host'], safe=''),
-            queue=urllib.parse.quote(queue_name, safe=''),
-        )
-        credentials = (conn_info['userid'], conn_info['password'])
-        r = requests.get(url, auth=credentials)
-        if r.status_code == 404:
-            return {}
-        if r.status_code != 200:
-            raise ValueError('Got error %s when reading queue stats: %s' % (
-                r.status_code, r.json()))
-        return r.json()
-
-    def get_queue_length(self, queue_name):
-        """Shortcut to get a queue's length"""
-        stats = self.get_queue_stats(queue_name)
-        if stats:
-            return stats.get('messages')
-
-    def register_task_class(self, name, cls):
-        """Register a class-based task under the given name"""
-        if name in self.tasks:
-            return
-
-        task_instance = cls()
-        task_instance.name = name
-        self.register_task(task_instance)
+def get_queue_stats(app, queue_name):
+    """Get the statistics regarding a queue on the broker.
+
+    Arguments:
+      queue_name: name of the queue to check
+
+    Returns a dictionary raw from the RabbitMQ management API;
+    or `None` if the current configuration does not use RabbitMQ.
+
+    Interesting keys:
+     - consumers (number of consumers for the queue)
+     - messages (number of messages in queue)
+     - messages_unacknowledged (number of messages currently being
+       processed)
+
+    Documentation: https://www.rabbitmq.com/management.html#http-api
+    """
+
+    conn_info = app.connection().info()
+    if conn_info['transport'] == 'memory':
+        # We're running in a test environment, without RabbitMQ.
+        return None
+    url = 'http://{hostname}:{port}/api/queues/{vhost}/{queue}'.format(
+        hostname=conn_info['hostname'],
+        port=conn_info['port'] + 10000,
+        vhost=urllib.parse.quote(conn_info['virtual_host'], safe=''),
+        queue=urllib.parse.quote(queue_name, safe=''),
+    )
+    credentials = (conn_info['userid'], conn_info['password'])
+    r = requests.get(url, auth=credentials)
+    if r.status_code == 404:
+        return {}
+    if r.status_code != 200:
+        raise ValueError('Got error %s when reading queue stats: %s' % (
+            r.status_code, r.json()))
+    return r.json()
+
+
+def get_queue_length(app, queue_name):
+    """Shortcut to get a queue's length"""
+    stats = get_queue_stats(app, queue_name)
+    if stats:
+        return stats.get('messages')
+
+
+def register_task_class(app, name, cls):
+    """Register a class-based task under the given name"""
+    if name in app.tasks:
+        return
+
+    task_instance = cls()
+    task_instance.name = name
+    app.register_task(task_instance)
 
 
 INSTANCE_NAME = os.environ.get(CONFIG_NAME_ENVVAR)
@@ -196,11 +197,7 @@
 for queue in CONFIG['task_queues']:
     CELERY_QUEUES.append(Queue(queue, Exchange(queue), routing_key=queue))
 
-# Instantiate the Celery app
-app = CustomCelery()
-app.conf.update(
-    # The broker
-    broker_url=CONFIG['task_broker'],
+DEFAULT_CONFIG = dict(
     # Timezone configuration: all in UTC
     enable_utc=True,
     timezone='UTC',
@@ -254,4 +251,11 @@
     worker_send_task_events=True,
     # Do not send useless task_sent events
     task_send_sent_event=False,
-)
+    )
+
+# Instantiate the Celery app
+app = Celery(broker=CONFIG['task_broker'])
+app.add_defaults(DEFAULT_CONFIG)
+
+# XXX for BW compat
+Celery.get_queue_length = get_queue_length
diff --git a/swh/scheduler/tests/scheduler_testing.py b/swh/scheduler/tests/scheduler_testing.py
--- a/swh/scheduler/tests/scheduler_testing.py
+++ b/swh/scheduler/tests/scheduler_testing.py
@@ -4,7 +4,7 @@
 
 from celery.result import AsyncResult
 from celery.contrib.testing.worker import start_worker
-import celery.contrib.testing.tasks  # noqa           
+import celery.contrib.testing.tasks  # noqa
 import pytest
 
 from swh.core.tests.db_testing import DbTestFixture, DB_DUMP_TYPES
@@ -12,7 +12,7 @@
 
 from swh.scheduler import get_scheduler
 from swh.scheduler.celery_backend.runner import run_ready_tasks
-from swh.scheduler.celery_backend.config import app
+from swh.scheduler.celery_backend.config import app, register_task_class
 from swh.scheduler.tests.celery_testing import CeleryTestFixture
 
 from . import SQL_DIR
@@ -42,7 +42,7 @@
         }
         self.scheduler.create_task_type(task_type)
         if task_class:
-            app.register_task_class(backend_name, task_class)
+            register_task_class(app, backend_name, task_class)
 
     def run_ready_tasks(self):
         """Runs the scheduler and a Celery worker, then blocks until