core: translate between IO and BlockIO settings to ease transition

Due to the substantial interface changes in cgroup unified hierarchy, new IO
settings are introduced.  Currently, IO settings apply only to unified
hierarchy and BlockIO to legacy.  While the transition is necessary, it's
painful for users to have to provide configs for both.  This patch implements
translation from one config set to another for configs which make sense.

* The translation takes place during application of the configs.  Users won't
  see IO or BlockIO settings appearing without being explicitly created.

* The translation takes place only if there is no config for the matching
  cgroup hierarchy type at all.

While this doesn't provide comprehensive compatibility, it should considerably
ease transition to the new IO settings which are a superset of BlockIO
settings.

v2:

- Update test-cgroup-mask.c so that it accounts for the fact that
  CGROUP_MASK_IO and CGROUP_MASK_BLKIO move together.  Also, test/parent.slice
  now sets IOWeight instead of BlockIOWeight.

1 files changed, 25 insertions, 0 deletions

diff --git a/man/systemd.resource-control.xml b/man/systemd.resource-control.xml
index 8a95e1196b..066f2cc19b 100644
--- a/man/systemd.resource-control.xml
+++ b/man/systemd.resource-control.xml
@@ -99,6 +99,31 @@
   </refsect1>
 
   <refsect1>
+    <title>Unified and Legacy Control Group Hierarchies</title>
+
+    <para>Unified control group hierarchy is the new version of kernel control group interface. Depending on the
+    resource type, there are differences in resource control capabilities.  Also, because of interface changes, some
+    resource types have a separate set of options on the unified hierarchy.</para>
+
+    <para>
+      <variablelist>
+        <varlistentry>
+          <term><option>IO</option></term>
+          <listitem>
+            <para><varname>IO</varname> prefixed settings are superset of and replace <varname>BlockIO</varname>
+            prefixed ones. On unified hierarchy, IO resource control also applies to buffered writes.</para>
+          </listitem>
+        </varlistentry>
+      </variablelist>
+    </para>
+
+    <para>To ease the transition, there is best-effort translation between the two versions of settings. If all
+    settings of a unit for a given resource type are for the other hierarchy type, the settings are translated and
+    applied. If there are any valid settings for the hierarchy in use, all translations are disabled for the resource
+    type. Mixing the two types of settings on a unit can lead to confusing results.</para>
+  </refsect1>
+
+  <refsect1>
     <title>Options</title>
 
     <para>Units of the types listed above can have settings