The Linux control-group mechanism was designed to make it easy to assign
processes to groups or move them around; it is a simple matter of writing a
process ID to the appropriate cgroup.procs file in the
control-group filesystem hierarchy. That only works for processes that
actually exist, though. Adding the ability to place a new process into a
control group at birth is the subject of this
patch set from Christian Brauner.

Source: LWN.net – [$] Cloning into a control group