[$] Cleaning up after dying control groups

Control groups are a useful mechanism for managing resource usage in the
system, but what happens when the control groups themselves become a
resource problem? In a plenary session at the 2019 Linux Storage,
Filesystem, and Memory-Management Summit, Roman Gushchin described problems
he has been facing with deleted control groups that take their time before
actually going away. Some of these problems have been fixed, but the issue
has not been truly resolved.