Chris Down
e16647c39d
Up until now, the behaviour in systemd has (mostly) been to silently ignore failures to action unit directives that refer to an unavailble controller. The addition of AssertControlGroupController and its conditional counterpart allow explicit specification of the desired behaviour when such a situation occurs. As for how this can happen, it is possible that a particular controller is not available in the cgroup hierarchy. One possible reason for this is that, in the running kernel, the controller simply doesn't exist -- for example, the CPU controller in cgroup v2 has only recently been merged and was out of tree until then. Another possibility is that the controller exists, but has been forcibly disabled by `cgroup_disable=` on the kernel command line. In future this will also support whatever comes out of issue #7624, `DefaultXAccounting=never`, or similar.
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