core: don't use the unified hierarchy for the systemd cgroup yet

[martinpitt]

Too many things don't get along with the unified hierarchy yet:

• Cannot start a container with systemd 232 opencontainers/runc#1175
• Can not run any docker container (fresh install after reboot) moby/moby#28109
• lxc-start: segfault lxc/lxc#1280

So revert the default to the legacy hierarchy for now. Developers of the above
software can opt into the unified hierarchy with
"systemd.legacy_systemd_cgroup_controller=0".

[martinpitt]

@htejun, WDYT?

[keszybz]

Implementation-wise this looks correct.

[htejun]

Looks good to me. Thanks!

[keszybz]

Like I said on semaphore, I think it'd be nice to provide a ./configure switch for this, similar to the one for the KillUserProcesses default. But anyway, we can merge this, and add the switch later.

[poettering]

Can we please not play games of toggling this all the time. Please fix this properly, add the configure switch, and leave this enabled by default.

I am pretty sure this should stay on by default, and it should be on distros to turn it off, but we should make it easy for them by providing said configure switch plus an explanation in NEWS. But playing ping-pong and turning it off and on every second version is just wrong...

@martinpitt any chance you can prep a proper fix for this that adds such a switch and turns it on again? thanks!

[poettering]

i created a blocker issue in #4669 now, so that this is fixed properly before the 233 release

[htejun]

Just posted #4670 which puts cgroup v2 systemd hierarchy on /sys/fs/cgroup/systemd-cgroup2 (any idea for a better name?) while maintaining the "name=systemd" hierarchy on /sys/fs/cgroup/systemd in parallel. This should avoid issues with most tools. For the ones which fail to parse if there's an entry for the v2 hierarchy in /proc/$PID/cgroup, I have no idea yet.

[martinpitt]

Please fix this properly, add the configure switch, and leave this enabled by default.

I didn't because IMHO a configure switch is the worst way; a runtime switch is a much lower barrier for both affected users for which it breaks, and developers of lxc, docker, and friends to try out the unified hiearchy without rebuilding. Also, it's another doubling of combinatorial explosion of which we don't test the other half.

I'd actually prefer reverting the revert and let unified cgroups be always on in upstream releases, and only revert it downstream. But if you still want that even after #4670, I'll add a configure option.

[poettering]

I didn't because IMHO a configure switch is the worst way; a runtime switch is a much lower barrier for both affected users for which it breaks, and developers of lxc, docker, and friends to try out the unified hiearchy without rebuilding.

A compile time switch doesn't prohibt a kernel cmdline switch. This was discussed before, there should be a compile-time default selectable via a configure switch, and then there should be a kernel cmdline switch that overrides that if set.