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

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Message-ID: <47AC0E25.1060603@openvz.org>
Date:	Fri, 08 Feb 2008 11:09:09 +0300
From:	Pavel Emelyanov <xemul@...nvz.org>
To:	Paul Menage <menage@...gle.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Containers <containers@...ts.osdl.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH][DOCUMENTATION] Minimal controller code for a quick start

Paul Menage wrote:
> On Feb 7, 2008 7:37 AM, Pavel Emelyanov <xemul@...nvz.org> wrote:
>> 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).
> 
> Good idea.
> 
>> +
>> +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,
>> +       },
>> +};
> 
> Can you structure this example so as to encourage people to use the
> more formatted read/write routines, such as read_int64 and
> write_int64?

And one more for ->read_strategy hints :) Well, I thought that
it should be the _minimal_ controller.

>> +
>> +static struct cgroup_subsys_state *foo_create(struct cgroup_subsys *cs,
>> +               struct cgroup *cg)
>> +{
>> +       struct foo_cgroup *foo;
>> +
> 
> Maybe add a comment here that mentions that if your cgroup needs very
> early initialization, you can check for cg->parent being NULL, and
> return a statically-constructed structure here. (And set
> foo_subsys.early_init = 1)

Yet again - this is rather a special feature, that your controller
needs an early initialization - for a very minimal one this is not
required.

Maybe we can have two examples? One is the minimal one and the other
is an advanced one with ->attach callbacks, etc?

> Paul
> 

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