linux-kernel - Re: [RFC] [PATCH 2/2] cgroups: blkio subsystem as module

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Date:	Thu, 14 Jan 2010 06:42:43 -0500
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Balbir Singh <bsingharora@...il.com>
Cc:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Li Zefan <lizf@...fujitsu.com>, axboe@...nel.dk,
	ryov@...inux.co.jp, Andrew Morton <akpm@...ux-foundation.org>,
	menage@...gle.com, containers@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC] [PATCH 2/2] cgroups: blkio subsystem as module

On Thu, Jan 14, 2010 at 03:02:09PM +0530, Balbir Singh wrote:
> On Tue, Jan 12, 2010 at 5:51 AM, KAMEZAWA Hiroyuki
> <kamezawa.hiroyu@...fujitsu.com> wrote:
> > On Fri, 8 Jan 2010 10:10:38 -0500
> > Vivek Goyal <vgoyal@...hat.com> wrote:
> >
> >> On Fri, Jan 08, 2010 at 12:30:21AM -0500, Ben Blum wrote:
> >> > Convert blk-cgroup to be buildable as a module
> >> >
> >> > From: Ben Blum <bblum@...rew.cmu.edu>
> >> >
> >> > This patch modifies the Block I/O cgroup subsystem to be able to be built as a
> >> > module. As the CFQ disk scheduler optionally depends on blk-cgroup, config
> >> > options in block/Kconfig, block/Kconfig.iosched, and block/blk-cgroup.h are
> >> > enhanced to support the new module dependency.
> >> >
> >>
> >> Hi Ben,
> >>
> >> I will give this patch a try.
> >>
> >> So from blk-cgroup perspective, the advantage of allowing it as module
> >> will be that we can save some memory if we are not using the controller?
> >>
> > Is "moduled" blkio cgroup safe after page-tracking by page_cgroup is
> > introduced ?
> >
> 
> My guess is it won't be, unless we start exposing page_cgroup API and
> then make the module depend on memcg.

I think I agree. When we introduce page_cgroup based page tracking, either
we need to export page_cgroup API or we can force blkio controller to
compile as in-kernel if user selects the CONFIG_PAGE_TRACKING option.

So as of now, I can't think why we should not we allow compiling blkio as
module as long as core cgroup functionality supports it safely.

Thanks
Vivek
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