man: add a note about session autogrouping

When cpu controller is disabled, thing would often still behave as if
it was. And since the cpu controller can be enabled "magically" e.g. by
starting user@1000, add a note for users to be careful. Autogrouping
is described well in the man page, incl. how to enable or disable it,
so it should be enough to refer to that.

diff --git a/man/systemd.resource-control.xml b/man/systemd.resource-control.xml
index 1cfe1ed52a9cbb03c407e65869e010b3416263f7..45b006be5749f2c44c8dd0b397e4284f0b8df2ca 100644 (file)
--- a/man/systemd.resource-control.xml
+++ b/man/systemd.resource-control.xml
@@ -225,6 +225,13 @@ CPUWeight=20   DisableControllers=cpu              /          \
           <varname>CPUWeight=</varname> applies to normal runtime of the system, and if the former is not set also to
           the startup and shutdown phases. Using <varname>StartupCPUWeight=</varname> allows prioritizing specific services at
           boot-up and shutdown differently than during normal runtime.</para>
+
+          <para>In addition to the resource allocation performed by the <option>cpu</option> controller, the
+          kernel may automatically divide resources based on session-id grouping, see "The autogroup feature"
+          in <citerefentry
+          project='man-pages'><refentrytitle>sched</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
+          The effect of this feature is similar to the <option>cpu</option> controller with no explicit
+          configuration, so users should be careful to not mistake one for the other.</para>
         </listitem>
       </varlistentry>