main: Add support for cgroup v2

Support for cgroup v2 is very similar to cgroup v1 just checking (and
writing) different file.

Testing this feature is a bit harder than with cgroup v1 so it's
probably worh noting in this commit message.

1. Copy some service file (I've used httpd service) and set
   CPUQuota=30% in the [service] section.
2. Check /sys/fs/cgroup/cgroup.subtree_control - there should be no
   "cpu"
3. Start modified service
4. Check /sys/fs/cgroup/cgroup.subtree_control - there should be "cpu"
5. Start corosync - It should be able to get rt priority

When move_to_root_cgroup is disabled, behavior differs:
- If corosync is started before modified service, so
  there is no "cpu" in /sys/fs/cgroup/cgroup.subtree_control
  corosync starts without problem and gets rt priority.
  Starting modified service later will never add "cpu" into
  /sys/fs/cgroup/cgroup.subtree_control (because corosync is holding
  rt priority and it is placed in the non-root cgroup by systemd).

- When corosync is started after modified service, so "cpu"
  is in /sys/fs/cgroup/cgroup.subtree_control, corosync is not
  able to get RT priority.

Signed-off-by: Jan Friesse <jfriesse@redhat.com>
Reviewed-by: Christine Caulfield <ccaulfie@redhat.com>

exec/main.c

@@ -1173,6 +1173,7 @@ error_close:
 static int corosync_move_to_root_cgroup(void) {
 	FILE *f;
 	int res = -1;
+	const char *cgroup_task_fname = NULL;
 
 	/*
 	 * /sys/fs/cgroup is hardcoded, because most of Linux distributions are now
@@ -1183,15 +1184,25 @@ static int corosync_move_to_root_cgroup(void) {
 	 */
 	f = fopen("/sys/fs/cgroup/cpu/cpu.rt_runtime_us", "rt");
 	if (f == NULL) {
-		log_printf(LOGSYS_LEVEL_DEBUG, "cpu.rt_runtime_us doesn't exists -> "
-		    "system without cgroup or with disabled CONFIG_RT_GROUP_SCHED");
+		/*
+		 * Try cgroup v2
+		 */
+		f = fopen("/sys/fs/cgroup/cgroup.procs", "rt");
+		if (f == NULL) {
+			log_printf(LOG_DEBUG, "cpu.rt_runtime_us or cgroup.procs doesn't exist -> "
+			    "system without cgroup or with disabled CONFIG_RT_GROUP_SCHED");
 
-		res = 0;
-		goto exit_res;
+			res = 0;
+			goto exit_res;
+		} else {
+			cgroup_task_fname = "/sys/fs/cgroup/cgroup.procs";
+		}
+	} else {
+		cgroup_task_fname = "/sys/fs/cgroup/cpu/tasks";
 	}
 	(void)fclose(f);
 
-	f = fopen("/sys/fs/cgroup/cpu/tasks", "w");
+	f = fopen(cgroup_task_fname, "w");
 	if (f == NULL) {
 		log_printf(LOGSYS_LEVEL_WARNING, "Can't open cgroups tasks file for writing");
 

man/corosync.conf.5

@@ -1,6 +1,6 @@
 .\"/*
 .\" * Copyright (c) 2005 MontaVista Software, Inc.
-.\" * Copyright (c) 2006-2020 Red Hat, Inc.
+.\" * Copyright (c) 2006-2021 Red Hat, Inc.
 .\" *
 .\" * All rights reserved.
 .\" *
@@ -32,7 +32,7 @@
 .\" * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF
 .\" * THE POSSIBILITY OF SUCH DAMAGE.
 .\" */
-.TH COROSYNC_CONF 5 2021-04-09 "corosync Man Page" "Corosync Cluster Engine Programmer's Manual"
+.TH COROSYNC_CONF 5 2021-05-03 "corosync Man Page" "Corosync Cluster Engine Programmer's Manual"
 .SH NAME
 corosync.conf - corosync executive configuration file
 
@@ -800,8 +800,14 @@ meaning maximal / minimal priority (so minimal / maximal nice value).
 .TP
 move_to_root_cgroup
 Should be set to yes (default) if corosync should try to move itself to root
-cgroup. This feature is available only for systems with cgroups with RT
-sched enabled (Linux with CONFIG_RT_GROUP_SCHED kernel option).
+cgroup. This feature is available only for systems with cgroups v1 with RT
+sched enabled (Linux with CONFIG_RT_GROUP_SCHED kernel option) and cgroups v2.
+
+It's worth noting that currently (May 3 2021) cgroup2 doesn’t yet
+support control of realtime processes and the cpu controller can only be
+enabled when all RT processes are in the root cgroup. So when move_to_root_cgroup
+is disabled and systemd is used, it may be impossible to make systemd options
+like CPUQuota working correctly until corosync is stopped.
 
 .TP
 allow_knet_handle_fallback