cgroup: replace "cgroup.populated" with "cgroup.events"

memcg already uses "memory.events" for event reporting and other
controllers may need event reporting too.  Let's standardize on
"$SUBSYS.events" interface file for reporting events which don't
happen too frequently and thus can share event notification.

"cgroup.populated" is replaced with "populated" field in
"cgroup.events" and documentation is updated accordingly.
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Li Zefan <lizefan@huawei.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>

Documentation/cgroups/unified-hierarchy.txt

@@ -341,11 +341,11 @@ is riddled with issues.
   unnecessarily complicated and probably done this way because event
   delivery itself was expensive.
 
-Unified hierarchy implements an interface file "cgroup.populated"
-which can be used to monitor whether the cgroup's subhierarchy has
-tasks in it or not.  Its value is 0 if there is no task in the cgroup
-and its descendants; otherwise, 1.  poll and [id]notify events are
-triggered when the value changes.
+Unified hierarchy implements "populated" field in "cgroup.events"
+interface file which can be used to monitor whether the cgroup's
+subhierarchy has tasks in it or not.  Its value is 0 if there is no
+task in the cgroup and its descendants; otherwise, 1.  poll and
+[id]notify events are triggered when the value changes.
 
 This is significantly lighter and simpler and trivially allows
 delegating management of subhierarchy - subhierarchy monitoring can
@@ -435,6 +435,11 @@ may be specified in any order and not all pairs have to be specified.
   the first entry in the file.  Specific entries can use "default" as
   its value to indicate inheritance of the default value.
 
+- For events which are not very high frequency, an interface file
+  "events" should be created which lists event key value pairs.
+  Whenever a notifiable event happens, file modified event should be
+  generated on the file.
+
 
 5-4. Per-Controller Changes
 

include/linux/cgroup-defs.h

@@ -226,7 +226,7 @@ struct cgroup {
 
 	struct kernfs_node *kn;		/* cgroup kernfs entry */
 	struct kernfs_node *procs_kn;	/* kn for "cgroup.procs" */
-	struct kernfs_node *populated_kn; /* kn for "cgroup.subtree_populated" */
+	struct kernfs_node *events_kn;	/* kn for "cgroup.events" */
 
 	/*
 	 * The bitmask of subsystems enabled on the child cgroups.

kernel/cgroup.c

@@ -611,8 +611,8 @@ static void cgroup_update_populated(struct cgroup *cgrp, bool populated)
 		if (!trigger)
 			break;
 
-		if (cgrp->populated_kn)
-			kernfs_notify(cgrp->populated_kn);
+		if (cgrp->events_kn)
+			kernfs_notify(cgrp->events_kn);
 		cgrp = cgroup_parent(cgrp);
 	} while (cgrp);
 }
@@ -3045,9 +3045,10 @@ static ssize_t cgroup_subtree_control_write(struct kernfs_open_file *of,
 	goto out_unlock;
 }
 
-static int cgroup_populated_show(struct seq_file *seq, void *v)
+static int cgroup_events_show(struct seq_file *seq, void *v)
 {
-	seq_printf(seq, "%d\n", (bool)seq_css(seq)->cgroup->populated_cnt);
+	seq_printf(seq, "populated %d\n",
+		   (bool)seq_css(seq)->cgroup->populated_cnt);
 	return 0;
 }
 
@@ -3214,8 +3215,8 @@ static int cgroup_add_file(struct cgroup *cgrp, struct cftype *cft)
 
 	if (cft->write == cgroup_procs_write)
 		cgrp->procs_kn = kn;
-	else if (cft->seq_show == cgroup_populated_show)
-		cgrp->populated_kn = kn;
+	else if (cft->seq_show == cgroup_events_show)
+		cgrp->events_kn = kn;
 	return 0;
 }
 
@@ -4388,9 +4389,9 @@ static struct cftype cgroup_dfl_base_files[] = {
 		.write = cgroup_subtree_control_write,
 	},
 	{
-		.name = "cgroup.populated",
+		.name = "cgroup.events",
 		.flags = CFTYPE_NOT_ON_ROOT,
-		.seq_show = cgroup_populated_show,
+		.seq_show = cgroup_events_show,
 	},
 	{ }	/* terminate */
 };