There is a memory leak in the parent process of Celery's worker.
It is not a child process executing a task.
It happens suddenly every few days.
Unless you stop Celery, it consumes server memory in tens of hours.

This problem happens at least in Celery 4.1, and it also occurs in Celery 4.2.
Celery is running on Ubuntu 16 and brokers use RabbitMQ.