Another daemon for managing control groups

Without having written any prototype code, but only looked at the problem architecturally, I agree that pushing cgroup management out of PID 1 will likely require *more* code in PID 1 than simply doing the cgroups management directly in systemd in the first place. But even so you don't need to drag dbus-daemon into the picture. cgmanager could use the same trick as systemd does, and initially only listen on a private socket for root-only direct libdbus communication, and then register itself on the dbus-daemon system bus whenever it becomes available.

That way cgmanager can start as PID 3 (after only systemd and journald), and the only early boot special handling in PID 1 would be to first start journald and cgmanager and then add them to their respective cgroups after the fact, but before continuing with any other units. Obviously that adds a bit of serialisation before going of and doing everything else in parallel, so will likely increase boot time by a second or two, but it's not like getting it to work is rocket science.

The trickier question is how to deal with cgmanager dying unexpectedly. PID 1 enjoys a privileged position and the kernel makes sure it never ever dies for any reason whatsoever, but PID 3 has no such protection. If systemd depends on it to do regular service management, it can't just restart cgmanager like it does any other service. Instead it would need special recovery code for that rare-but-inevitable case, code that likely won't get much testing, a perfect recipe for eventual disaster...