[Devel] Re: [PATCH][DOCUMENTATION] Minimal controller code for a quick start

[Serge E. Hallyn]

Quoting Pavel Emelyanov (xemul at 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 at openvz.org>

Actually I thought that was the main point of kernel/cgroup_debug.c?

-serge

> 
> ---
> 
> 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,
> +};
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