On Tue, Jan 21, 2020 at 04:48:43PM +0100, Christian Brauner wrote:

This adds support for creating a process in a different cgroup than its
parent. Callers can limit and account processes and threads right from
the moment they are spawned:
- A service manager can directly spawn new services into dedicated
  cgroups.
- A process can be directly created in a frozen cgroup and will be
  frozen as well.
- The initial accounting jitter experienced by process supervisors and
  daemons is eliminated with this.
- Threaded applications or even thread implementations can choose to
  create a specific cgroup layout where each thread is spawned
  directly into a dedicated cgroup.

This feature is limited to the unified hierarchy. Callers need to pass
an directory file descriptor for the target cgroup. The caller can
choose to pass an O_PATH file descriptor. All usual migration
restrictions apply, i.e. there can be no processes in inner nodes. In
general, creating a process directly in a target cgroup adheres to all
migration restrictions.

AFAICT, he *big* win here is avoiding the write side of the
cgroup_threadgroup_rwsem. Or am I mis-reading the patch?

That global lock is what makes moving tasks/threads around super
expensive, avoiding that by use of this clone() variant wins the day.