task_prerun/task_postrun signals was not sent by celeryd. Thanks to Regent Blaise.

celery/worker/job.py

@@ -97,7 +97,7 @@ class WorkerTaskTrace(TaskTrace):
         """Execute, trace and store the result of the task."""
         self.loader.on_task_init(self.task_id, self.task)
         self.task.backend.process_cleanup()
-        return self._trace()
+        return super(WorkerTaskTrace, self).execute()
 
     def handle_success(self, retval, *args):
         """Handle successful execution."""