docs: clarify controller mount logic in hybrid cgroups mode

Fixes: #10107

diff --git a/docs/CGROUP_DELEGATION.md b/docs/CGROUP_DELEGATION.md
index 63d9d41b1093e4bda265c99f482ed2590b29ef18..c63b75e58b39e5604cf1c9bedf67d74568e25571 100644 (file)
--- a/docs/CGROUP_DELEGATION.md
+++ b/docs/CGROUP_DELEGATION.md
@@ -91,10 +91,14 @@ hierarchy for managing purposes as `/sys/fs/cgroup/systemd/`.
 
 3. **Hybrid** — this is a hybrid between the unified and legacy mode. It's set
 up mostly like legacy, except that there's also an additional hierarchy
-`/sys/fs/cgroup/unified/` that contains the cgroupsv2 hierarchy. In this mode
-compatibility with cgroupsv1 is retained while some cgroupsv2 features are
-available too. This mode is a stopgap. Don't bother with this too much unless
-you have too much free time.
+`/sys/fs/cgroup/unified/` that contains the cgroupsv2 hierarchy. (Note that in
+this mode the unified hierarchy won't have controllers attached, the
+controllers are all mounted as separate hierarchies as in legacy mode,
+i.e. `/sys/fs/cgroup/unified/` is purely and exclusively about core cgroupsv2
+functionality and not about resource management.) In this mode compatibility
+with cgroupsv1 is retained while some cgroupsv2 features are available
+too. This mode is a stopgap. Don't bother with this too much unless you have
+too much free time.
 
 To say this clearly, legacy and hybrid modes have no future. If you develop
 software today and don't focus on the unified mode, then you are writing