linux-kernel - Re: [RFC][PATCH 2/7][memcg] cgroup arbitarary ID allocation

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Message-ID: <20100728023027.GD12642@redhat.com>
Date:	Tue, 27 Jul 2010 22:30:27 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"nishimura@....nes.nec.co.jp" <nishimura@....nes.nec.co.jp>,
	"balbir@...ux.vnet.ibm.com" <balbir@...ux.vnet.ibm.com>,
	gthelen@...gle.com, m-ikeda@...jp.nec.com,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH 2/7][memcg] cgroup arbitarary ID allocation

On Tue, Jul 27, 2010 at 04:54:17PM +0900, KAMEZAWA Hiroyuki wrote:
> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> 
> When a subsystem want to make use of "id" more, it's necessary to
> manage the id at cgroup subsystem creation time. But, now,
> because of the order of cgroup creation callback, subsystem can't
> declare the id it wants. This patch allows subsystem to use customized
> ID for themselves.
> 
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>

[..]
> Index: mmotm-2.6.35-0719/Documentation/cgroups/cgroups.txt
> ===================================================================
> --- mmotm-2.6.35-0719.orig/Documentation/cgroups/cgroups.txt
> +++ mmotm-2.6.35-0719/Documentation/cgroups/cgroups.txt
> @@ -621,6 +621,15 @@ and root cgroup. Currently this will onl
>  the default hierarchy (which never has sub-cgroups) and a hierarchy
>  that is being created/destroyed (and hence has no sub-cgroups).
>  
> +void custom_id(struct cgroup_subsys *ss, struct cgroup *cgrp)
> +
> +Called at assigning a new ID to cgroup subsystem state struct. This
> +is called when ss->use_id == true. If this function is not provided,
> +a new ID is automatically assigned. If you enable ss->use_id,
> +you can use css_lookup()  and css_get_next() to access "css" objects
> +via IDs.
> +

Couple of lines to explain why a subsystem would like to assign its
own ids and not be happy with generic cgroup assigned id be helpful.
In this case, I think you are using this id as index into array
and want to control the index, hence you seem to be doing it.

But I am not sure again why do you want to control index?

Vivek
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