This example subsystem exports debugging information as an aid to diagnosing
refcount leaks, etc, in the cgroup framework.

Signed-off-by: Paul Menage <menage@google.com>
---

 include/linux/cgroup_subsys.h |    4 +
 init/Kconfig                     |   10 ++
 kernel/Makefile                  |    1 
 kernel/cgroup_debug.c         |   97 +++++++++++++++++++++++++++++
 4 files changed, 112 insertions(+)

diff -puN include/linux/cgroup_subsys.h~task-cgroupsv11-simple-task-cgroup-debug-info-subsystem include/linux/cgroup_subsys.h
--- a/include/linux/cgroup_subsys.h~task-cgroupsv11-simple-task-cgroup-debug-info-subsystem
+++ a/include/linux/cgroup_subsys.h
@@ -19,4 +19,8 @@ SUBSYS(cpuacct)
 
 /* */
 
+#ifdef CONFIG_CGROUP_DEBUG
+SUBSYS(debug)
+#endif
+
 /* */
diff -puN init/Kconfig~task-cgroupsv11-simple-task-cgroup-debug-info-subsystem init/Kconfig
--- a/init/Kconfig~task-cgroupsv11-simple-task-cgroup-debug-info-subsystem
+++ a/init/Kconfig
@@ -282,6 +282,16 @@ config CGROUPS
 
 	  Say N if unsure.
 
+config CGROUP_DEBUG
+	bool "Example debug cgroup subsystem"
+	depends on CGROUPS
+	help
+	  This option enables a simple cgroup subsystem that
+	  exports useful debugging information about the cgroups
+	  framework
+
+	  Say N if unsure
+
 config CPUSETS
 	bool "Cpuset support"
 	depends on SMP && CGROUPS
diff -puN kernel/Makefile~task-cgroupsv11-simple-task-cgroup-debug-info-subsystem kernel/Makefile
--- a/kernel/Makefile~task-cgroupsv11-simple-task-cgroup-debug-info-subsystem
+++ a/kernel/Makefile
@@ -39,6 +39,7 @@ obj-$(CONFIG_BSD_PROCESS_ACCT) += acct.o
 obj-$(CONFIG_KEXEC) += kexec.o
 obj-$(CONFIG_COMPAT) += compat.o
 obj-$(CONFIG_CGROUPS) += cgroup.o
+obj-$(CONFIG_CGROUP_DEBUG) += cgroup_debug.o
 obj-$(CONFIG_CPUSETS) += cpuset.o
 obj-$(CONFIG_CGROUP_CPUACCT) += cpu_acct.o
 obj-$(CONFIG_IKCONFIG) += configs.o
diff -puN /dev/null kernel/cgroup_debug.c
--- /dev/null
+++ a/kernel/cgroup_debug.c
@@ -0,0 +1,97 @@
+/*
+ * kernel/ccontainer_debug.c - Example cgroup subsystem that
+ * exposes debug info
+ *
+ * Copyright (C) Google Inc, 2007
+ *
+ * Developed by Paul Menage (menage@google.com)
+ *
+ */
+
+#include <linux/cgroup.h>
+#include <linux/fs.h>
+#include <linux/slab.h>
+#include <linux/rcupdate.h>
+
+#include <asm/atomic.h>
+
+static struct cgroup_subsys_state *debug_create(struct cgroup_subsys *ss,
+						   struct cgroup *cont)
+{
+	struct cgroup_subsys_state *css = kzalloc(sizeof(*css), GFP_KERNEL);
+
+	if (!css)
+		return ERR_PTR(-ENOMEM);
+
+	return css;
+}
+
+static void debug_destroy(struct cgroup_subsys *ss, struct cgroup *cont)
+{
+	kfree(cont->subsys[debug_subsys_id]);
+}
+
+static u64 cgroup_refcount_read(struct cgroup *cont, struct cftype *cft)
+{
+	return atomic_read(&cont->count);
+}
+
+static u64 taskcount_read(struct cgroup *cont, struct cftype *cft)
+{
+	u64 count;
+
+	cgroup_lock();
+	count = cgroup_task_count(cont);
+	cgroup_unlock();
+	return count;
+}
+
+static u64 current_css_set_read(struct cgroup *cont, struct cftype *cft)
+{
+	return (u64)(long)current->cgroups;
+}
+
+static u64 current_css_set_refcount_read(struct cgroup *cont,
+					   struct cftype *cft)
+{
+	u64 count;
+
+	rcu_read_lock();
+	count = atomic_read(&current->cgroups->ref.refcount);
+	rcu_read_unlock();
+	return count;
+}
+
+static struct cftype files[] =  {
+	{
+		.name = "cgroup_refcount",
+		.read_uint = cgroup_refcount_read,
+	},
+	{
+		.name = "taskcount",
+		.read_uint = taskcount_read,
+	},
+
+	{
+		.name = "current_css_set",
+		.read_uint = current_css_set_read,
+	},
+
+	{
+		.name = "current_css_set_refcount",
+		.read_uint = current_css_set_refcount_read,
+	},
+};
+
+static int debug_populate(struct cgroup_subsys *ss, struct cgroup *cont)
+{
+	return cgroup_add_files(cont, ss, files, ARRAY_SIZE(files));
+}
+
+struct cgroup_subsys debug_subsys = {
+	.name = "debug",
+	.create = debug_create,
+	.destroy = debug_destroy,
+	.populate = debug_populate,
+	.subsys_id = debug_subsys_id,
+};
_

--
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