Linux Kernels

cgroup: add cgroup.stat interface with basic hierarchy stats

This change “cgroup: add cgroup.stat interface with basic hierarchy stats” (commit ec39225) in Linux kernel is authored by Roman Gushchin <guro [at]> on Wed Aug 2 17:55:31 2017 +0100.

Description of "cgroup: add cgroup.stat interface with basic hierarchy stats"

The change “cgroup: add cgroup.stat interface with basic hierarchy stats” introduces changes as follows.

cgroup: add cgroup.stat interface with basic hierarchy stats

A cgroup can consume resources even after being deleted by a user.
For example, writing back dirty pages should be accounted and
limited, despite the corresponding cgroup might contain no processes
and being deleted by a user.

In the current implementation a cgroup can remain in such "dying" state
for an undefined amount of time. For instance, if a memory cgroup
contains a pge, mlocked by a process belonging to an other cgroup.

Although the lifecycle of a dying cgroup is out of user's control,
it's important to have some insight of what's going on under the hood.

In particular, it's handy to have a counter which will allow
to detect css leaks.

To solve this problem, add a cgroup.stat interface to
the base cgroup control files with the following metrics:

nr_descendants          total number of visible descendant cgroups
nr_dying_descendants    total number of dying descendant cgroups

Signed-off-by: Roman Gushchin <>
Suggested-by: Tejun Heo <>
Signed-off-by: Tejun Heo <>
Cc: Zefan Li <>
Cc: Waiman Long <>
Cc: Johannes Weiner <>

Linux kernel releases containing commit ec39225

The Linux kernel releases containing this commit are as follows.

Linux kernel code changes from "cgroup: add cgroup.stat interface with basic hierarchy stats"

There are 34 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 Documentation/cgroup-v2.txt | 18 ++++++++++++++++++
 kernel/cgroup/cgroup.c      | 16 ++++++++++++++++
 2 files changed, 34 insertions(+)
diff --git a/Documentation/cgroup-v2.txt b/Documentation/cgroup-v2.txt
index 46ec3f76211c..dc44785dc0fa 100644
--- a/Documentation/cgroup-v2.txt
+++ b/Documentation/cgroup-v2.txt
@@ -868,6 +868,24 @@ All cgroup core files are prefixed with "cgroup."
 	If the actual descent depth is equal or larger,
 	an attempt to create a new child cgroup will fail.
+  cgroup.stat
+	A read-only flat-keyed file with the following entries:
+	  nr_descendants
+		Total number of visible descendant cgroups.
+	  nr_dying_descendants
+		Total number of dying descendant cgroups. A cgroup becomes
+		dying after being deleted by a user. The cgroup will remain
+		in dying state for some time undefined time (which can depend
+		on system load) before being completely destroyed.
+		A process can't enter a dying cgroup under any circumstances,
+		a dying cgroup can't revive.
+		A dying cgroup can consume system resources not exceeding
+		limits, which were active at the moment of cgroup deletion.
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 0fd9134e1720..a06755a610e1 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -3304,6 +3304,18 @@ static int cgroup_events_show(struct seq_file *seq, void *v)
 	return 0;
+static int cgroup_stats_show(struct seq_file *seq, void *v)
+	struct cgroup *cgroup = seq_css(seq)->cgroup;
+	seq_printf(seq, "nr_descendants %d\n",
+		   cgroup->nr_descendants);
+	seq_printf(seq, "nr_dying_descendants %d\n",
+		   cgroup->nr_dying_descendants);
+	return 0;
 static int cgroup_file_open(struct kernfs_open_file *of)
 	struct cftype *cft = of->kn->priv;
@@ -4407,6 +4419,10 @@ static struct cftype cgroup_base_files[] = {
 		.seq_show = cgroup_max_depth_show,
 		.write = cgroup_max_depth_write,
+	{
+		.name = "cgroup.stat",
+		.seq_show = cgroup_stats_show,
+	},
 	{ }	/* terminate */

The commit for this change in Linux stable tree is ec39225 (patch).

Last modified: 2020/01/11 06:54