linux-kernel - Re: [PATCH v4 0/4] cgroups: support for module-loadable subsystems

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Date:	Thu, 7 Jan 2010 16:16:27 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Li Zefan <lizf@...fujitsu.com>
Cc:	Ben Blum <bblum@...rew.cmu.edu>,
	Andrew Morton <akpm@...ux-foundation.org>, menage@...gle.com,
	containers@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 0/4] cgroups: support for module-loadable subsystems

On Thu, 07 Jan 2010 14:42:19 +0800
Li Zefan <lizf@...fujitsu.com> wrote:

> KAMEZAWA Hiroyuki wrote:
> > On Wed, 6 Jan 2010 20:26:06 -0500
> > Ben Blum <bblum@...rew.cmu.edu> wrote:
> > 
> >> On Wed, Jan 06, 2010 at 04:04:14PM -0800, Andrew Morton wrote:
> >>> On Thu, 31 Dec 2009 00:10:50 -0500
> >>> Ben Blum <bblum@...rew.cmu.edu> wrote:
> >>>
> >>>> This patch series implements support for building, loading, and
> >>>> unloading subsystems as modules, both within and outside the kernel
> >>>> source tree. It provides an interface cgroup_load_subsys() and
> >>>> cgroup_unload_subsys() which modular subsystems can use to register and
> >>>> depart during runtime. The net_cls classifier subsystem serves as the
> >>>> example for a subsystem which can be converted into a module using these
> >>>> changes.
> >>> What is the value in this?  What are the usage scenarios?  Why does the
> >>> benefit of this change exceed the cost/risk/etc of merging it?
> >> As discussed in the first posting of these patches, this provides the
> >> ability for arbitrary subsystems to be used with cgroups.. cls_cgroup
> >> would have already been a module except for a lack of support from
> >> cgroups, and the change also allows other module-loadable classifiers
> >> to add subsystems of their own.
> > 
> > Hmm, do you have your own module in plan ?
> > 
> 
> Maybe the new blkio_cgroup can also be made module-able.
> 

Hmm, I read the patch slightly. I'm not enough expert to review this patch..

I requst following as TODO.
(No objection to the direction/patch.)

 1. Add documentation about load/unlod module.
   It seems module unloading will not succuess while subsystem is mounted.
   Right ?

 2. Making this to be reasonable value.
#define CGROUP_SUBSYS_COUNT (BITS_PER_BYTE*sizeof(unsigned long))
   I can't find why.

 3. show whehter a subsys is a loadable module or not via /proc/cgroups

 4. how to test ? load/unload NET_CLS is enough ?

Last one is question.

 5. Is following path is safe ?

    find_css_set() {
     ....
          read_lock(&css_set_lock);
          get template including pointer
          read_unlock(&css_set_lock);

          use template to build new css_set.


Thanks,
-Kame


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