[PATCH][DOCUMENTATION] Minimal controller code for a quick start

[Pavel Emelyanov <xemul@openvz.org>]

The Documentation/cgroups.txt file contains the info on how
to write some controller for cgroups subsystem, but even with
this, one need to write quite a lot of code before developing
the core (or copy-n-paste it from some other place).

I propose to put this minimal controller into Documentation
directory to let people copy-n-paste a) from a known place and 
b) a small piece of code.

Besides, many people learn better reading an example rather
than/along with a document.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>

---

diff --git a/Documentation/cgroups.txt b/Documentation/cgroups.txt
index 42d7c4c..66068dc 100644
--- a/Documentation/cgroups.txt
+++ b/Documentation/cgroups.txt
@@ -531,6 +531,9 @@ and root cgroup. Currently this will only involve movement between
 the default hierarchy (which never has sub-cgroups) and a hierarchy
 that is being created/destroyed (and hence has no sub-cgroups).
 
+For a quick start you may want to look at the
+Documentation/controllers/example.c file.
+
 4. Questions
 ============
 
diff --git a/Documentation/controllers/example.c b/Documentation/controllers/example.c
new file mode 100644
index 0000000..4a73c77
--- /dev/null
+++ b/Documentation/controllers/example.c
@@ -0,0 +1,134 @@
+/*
+ * Documentation/controllers/example.c - A simple controller
+ *
+ * Copy and make s/foo/$SUBSYS_NAME/g in it to get a minimal
+ * working code. Don't forget to add a SUBSYS(foo) line in the
+ * include/linux/cgroup_subsys.h file.
+ *
+ */
+
+#include <linux/cgroup.h>
+
+/*
+ * the foo main structure - it is used to store any info, that
+ * is required from the group of tasks
+ */
+
+struct foo_cgroup {
+	/*
+	 * put your fields here
+	 */
+
+	struct cgroup_subsys_state css;
+
+	/*
+	 * ... or/and here
+	 */
+};
+
+/*
+ * helpers to get the foo_cgroup from a task and a control group
+ */
+
+static inline struct foo_cgroup *foo_from_css(struct cgroup_subsys_state *css)
+{
+	return container_of(css, struct foo_cgroup, css);
+}
+
+static inline struct foo_cgroup *foo_from_cgroup(struct cgroup *cg)
+{
+	return foo_from_css(cgroup_subsys_state(cg, foo_subsys_id));
+}
+
+static inline struct foo_cgroup *foo_from_task(struct task_struct *p)
+{
+	return foo_from_css(task_subsys_state(p, foo_subsys_id));
+}
+
+/*
+ * foo files
+ */
+
+static ssize_t foo_bar_read(struct cgroup *cg, struct cftype *cft,
+		struct file *file, char __user *userbuf,
+		size_t nbytes, loff_t *ppos)
+{
+	struct foo_cgroup *foo;
+
+	foo = foo_from_cgroup(cg);
+
+	/*
+	 * produce some output
+	 */
+
+	return nbytes;
+}
+
+static ssize_t foo_bar_write(struct cgroup *cg, struct cftype *cft,
+		struct file *file, const char __user *userbuf,
+		size_t nbytes, loff_t *ppos)
+{
+	struct foo_cgroup *foo;
+
+	foo = foo_from_cgroup(cg);
+
+	/*
+	 * read and tune the foo
+	 */
+
+	return nbytes;
+}
+
+static struct cftype foo_files[] = {
+	{
+		.name = "bar",
+		.read = foo_bar_read,
+		.write = foo_bar_write,
+	},
+};
+
+/*
+ * foo subsystem basic callbacks
+ */
+
+static struct cgroup_subsys_state *foo_create(struct cgroup_subsys *cs,
+		struct cgroup *cg)
+{
+	struct foo_cgroup *foo;
+
+	foo = kmalloc(sizeof(struct foo_cgroup), GFP_KERNEL);
+	if (foo == NULL)
+		return NULL;
+
+	/*
+	 * initialize your fields
+	 */
+
+	return &foo->css;
+}
+
+static void foo_destroy(struct cgroup_subsys *cs, struct cgroup *cg)
+{
+	struct foo_cgroup *foo;
+
+	foo = foo_from_cgroup(cg);
+
+	/*
+	 * clean your fields
+	 */
+
+	kfree(foo);
+}
+
+static int foo_populate(struct cgroup_subsys *cs, struct cgroup *cg)
+{
+	return cgroup_add_files(cg, cs, foo_files, ARRAY_SIZE(foo_files));
+}
+
+struct cgroup_subsys foo_subsys = {
+	.name = "foo",
+	.subsys_id = foo_subsys_id,
+	.create = foo_create,
+	.destroy = foo_destroy,
+	.populate = foo_populate,
+};