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

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Message-Id: <20100728113529.f086716d.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Wed, 28 Jul 2010 11:35:29 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Vivek Goyal <vgoyal@...hat.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, 27 Jul 2010 22:30:27 -0400
Vivek Goyal <vgoyal@...hat.com> wrote:

> > 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?
> 

Now, the subsystem allocation/id-allocation order is

	->create()
	alloc_id.

Otherwise "id" of memory cgroup is just determined by the place in virtual-indexed
array. 
As
	memcg =	mem_cgroup_base + id

This "id" is determined at create().

If "id" is determined regardless of memory cgroup's placement, it's of no use.
My original design of css_id() allocates id in create() but it was moved to
generic part. So, this is expected change in my plan.

We have 2 choices.
	id = alloc_id()
	create(id)
or
	this patch.

Both are okay for me. But alloc id before create() may add some ugly rollback.

Thanks,
-Kame

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