vircgroup: fix cgroups v2 controllers detection

When creating new group for cgroups v2 the we cannot check
cgroups.controllers for that cgroup because the directory is created
later.  In that case we should check cgroups.subtree_control of parent
group to get list of controllers enabled for child cgroups.

In order to achieve that we will prefer the parent group if it exists,
the current group will be used only for root group.
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>
Acked-by: NPeter Krempa <pkrempa@redhat.com>

src/util/vircgroup.c

@@ -407,7 +407,7 @@ virCgroupDetect(virCgroupPtr group,
 
     for (i = 0; i < VIR_CGROUP_BACKEND_TYPE_LAST; i++) {
         if (group->backends[i]) {
-            int rc = group->backends[i]->detectControllers(group, controllers);
+            int rc = group->backends[i]->detectControllers(group, controllers, parent);
             if (rc < 0)
                 return -1;
             controllersAvailable |= rc;

src/util/vircgroupbackend.h

@@ -98,7 +98,8 @@ typedef char *
 
 typedef int
 (*virCgroupDetectControllersCB)(virCgroupPtr group,
-                                int controllers);
+                                int controllers,
+                                virCgroupPtr parent);
 
 typedef bool
 (*virCgroupHasControllerCB)(virCgroupPtr cgroup,

src/util/vircgroupv1.c

@@ -420,7 +420,8 @@ virCgroupV1StealPlacement(virCgroupPtr group)
 
 static int
 virCgroupV1DetectControllers(virCgroupPtr group,
-                             int controllers)
+                             int controllers,
+                             virCgroupPtr parent ATTRIBUTE_UNUSED)
 {
     size_t i;
     size_t j;

src/util/vircgroupv2.c

@@ -242,7 +242,8 @@ virCgroupV2StealPlacement(virCgroupPtr group)
 
 
 static int
-virCgroupV2ParseControllersFile(virCgroupPtr group)
+virCgroupV2ParseControllersFile(virCgroupPtr group,
+                                virCgroupPtr parent)
 {
     int rc;
     VIR_AUTOFREE(char *) contStr = NULL;
@@ -250,10 +251,17 @@ virCgroupV2ParseControllersFile(virCgroupPtr group)
     char **contList = NULL;
     char **tmp;
 
-    if (virAsprintf(&contFile, "%s%s/cgroup.controllers",
-                    group->unified.mountPoint,
-                    NULLSTR_EMPTY(group->unified.placement)) < 0)
-        return -1;
+    if (parent) {
+        if (virAsprintf(&contFile, "%s%s/cgroup.subtree_control",
+                        parent->unified.mountPoint,
+                        NULLSTR_EMPTY(parent->unified.placement)) < 0)
+            return -1;
+    } else {
+        if (virAsprintf(&contFile, "%s%s/cgroup.controllers",
+                        group->unified.mountPoint,
+                        NULLSTR_EMPTY(group->unified.placement)) < 0)
+            return -1;
+    }
 
     rc = virFileReadAll(contFile, 1024 * 1024, &contStr);
     if (rc < 0) {
@@ -286,11 +294,12 @@ virCgroupV2ParseControllersFile(virCgroupPtr group)
 
 static int
 virCgroupV2DetectControllers(virCgroupPtr group,
-                             int controllers)
+                             int controllers,
+                             virCgroupPtr parent)
 {
     size_t i;
 
-    if (virCgroupV2ParseControllersFile(group) < 0)
+    if (virCgroupV2ParseControllersFile(group, parent) < 0)
         return -1;
 
     /* In cgroup v2 there is no cpuacct controller, the cpu.stat file always