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 <guro@fb.com>
Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Zefan Li <lizefan@huawei.com>
Cc: Waiman Long <longman@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: kernel-team@fb.com
Cc: cgroups@vger.kernel.org
Cc: linux-doc@vger.kernel.org
Cc: linux-kernel@vger.kernel.org

diff --git a/Documentation/cgroup-v2.txt b/Documentation/cgroup-v2.txt
index 46ec3f7..dc44785 100644 (file)
--- 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.
+
 
 Controllers
 ===========

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 0fd9134..a06755a 100644 (file)
--- 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 */
 };