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docs: make clear that if you use threaded cgroups you need to do that two levels down from your delegated cgroup

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Prompted by: #22486
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Lennart Poettering 2022-02-14 18:05:31 +01:00 committed by Luca Boccassi
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docs/CGROUP_DELEGATION.md
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@ -266,6 +266,15 @@ tree by the time it notifies the service manager about start-up readiness, so
that the service's main cgroup is definitely an inner node by the time the
service manager might start `ExecStartPost=`.)
(Also note, if you intend to use "threaded" cgroups — as added in Linux 4.14 —,
then you should do that *two* levels down from the main service cgroup your
turned delegation on for. Why that? You need one level so that systemd can
properly create the `.control` subgroup, as described above. But that one
cannot be threaded, since that would mean `.control` has to be threaded too —
this is a requirement of threaded cgroups: either a cgroup and all its siblings
are threaded or none –, but systemd expects it to be a regular cgroup. Thus you
have to nest a second cgroup beneath it which then can be threaded.)
## Three Scenarios
Let's say you write a container manager, and you wonder what to do regarding