[RFC 3/4] cgroup: add cgroup.stat interface with basic hierarchy stats

From: Roman Gushchin
Date: Wed Aug 02 2017 - 12:58:33 EST

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 <guro@xxxxxx>
Suggested-by: Tejun Heo <tj@xxxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: Zefan Li <lizefan@xxxxxxxxxx>
Cc: Waiman Long <longman@xxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: kernel-team@xxxxxx
Cc: cgroups@xxxxxxxxxxxxxxx
Cc: linux-doc@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
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 9d53d69e44bb..f58e1fe8bebd 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 */