Date: Thu, 15 Jul 2010 09:00:48 +0900
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
To: Vivek Goyal <vgoyal@redhat.com>
Cc: Nauman Rafique <nauman@google.com>, Munehiro Ikeda <m-ikeda@ds.jp.nec.com>,
        linux-kernel@vger.kernel.org, Ryo Tsuruta <ryov@valinux.co.jp>,
        taka@valinux.co.jp, Andrea Righi <righi.andrea@gmail.com>,
        Gui Jianfeng <guijianfeng@cn.fujitsu.com>, akpm@linux-foundation.org,
        balbir@linux.vnet.ibm.com
Subject: Re: [RFC][PATCH 00/11] blkiocg async support
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On Wed, 14 Jul 2010 10:29:19 -0400
Vivek Goyal <vgoyal@redhat.com> wrote:
> > 
> > Cgroup's feature as mounting several subsystems at a mount point at once
> > is very useful in many case.
> 
> I agree that it is useful but if some controllers are not supporting
> hierarchy, it just adds to more confusion. And later when hierarchy
> support comes in, there will be additional issue of keeping this file
> "use_hierarchy" like memory controller.
> 
> So at this point of time , I am not too inclined towards allowing hierarchical
> cgroup creation but treating them as flat in CFQ. I think it adds to the
> confusion and user space should handle this situation.
> 

Hmm. 

Could you fix error code in create blkio cgroup ? It returns -EINVAL now.
IIUC, mkdir(2) doesn't return -EINVAL as error code (from man.)
Then, it's very confusing. I think -EPERM or -ENOMEM will be much better.

Anyway, I need to see source code of blk-cgroup.c to know why libvirt fails
to create cgroup. Where is the user-visible information (in RHEL or Fedora)
about "you can't use blkio-cgroup via libvirt or libcgroup" ?

Thanks,
-Kame

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