Hello.
On Tue, Oct 29, 2019 at 11:25:30AM +0100, Jiri Slaby
what happens if I boot older kernels if we switch to pure v2 in systemd? That is usual practice during bughunting. It depends on how old kernel you boot.
(The recent systemd should handle it gracefully, i.e. as you go more deeper in history, available controllers start disappearing, then (v4.5) cgroup2 isn't available anymore, systemd would fall back to v1 and that's again close to the current state from the general debugging perspective.)
So is this build-time or boot-time switch? The default behavior of systemd is a build-time switch.
It can be overriden with a boot-time option though (the option is interpretted by systemd which then interacts with the kernel accordingly). Michal