Re: [PATCH v5 5/6] clone3: allow spawning processes into cgroups

[Peter Zijlstra <peterz@infradead.org>]

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.