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

[Pavel Emelyanov <xemul-GEFAQzZX7r8dnm+yROfE0A@public.gmane.org>]

Serge E. Hallyn wrote:
> Quoting Pavel Emelyanov (xemul-GEFAQzZX7r8dnm+yROfE0A@public.gmane.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-GEFAQzZX7r8dnm+yROfE0A@public.gmane.org>
> 
> Actually I thought that was the main point of kernel/cgroup_debug.c?

This one doesn't show how to use generic read/write for files,
but a bit more advanced read_uint/write_uiant, it doesn't declare
its own cgroup with extra fields and doesn't show how to get the
one from task.

Hmm... This one is even more minimal than my :)

> -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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