linux-kernel - Re: Attaching PID 0 to a cgroup

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Date:	Tue, 01 Jul 2008 23:48:31 +0200
From:	Andrea Righi <righi.andrea@...il.com>
To:	Li Zefan <lizf@...fujitsu.com>
CC:	Dhaval Giani <dhaval@...ux.vnet.ibm.com>,
	Paul Menage <menage@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Balbir Singh <balbir@...ibm.com>,
	Sudhir Kumar <skumar@...ux.vnet.ibm.com>,
	lkml <linux-kernel@...r.kernel.org>, containers@...ts.osdl.org,
	Paul Jackson <pj@....com>
Subject: Re: Attaching PID 0 to a cgroup

Li Zefan wrote:
> CC: Paul Jackson <pj@....com>
> 
> Dhaval Giani wrote:
>> [put in the wrong alias for containers list correcting it.]
>>
>> On Tue, Jul 01, 2008 at 03:15:45PM +0530, Dhaval Giani wrote:
>>> Hi Paul,
>>>
>>> Attaching PID 0 to a cgroup caused the current task to be attached to
>>> the cgroup. Looking at the code,
>>>
> 
> [...]
> 
>>> I was wondering, why this was done. It seems to be unexpected behavior.
>>> Wouldn't something like the following be a better response? (I've used
>>> EINVAL, but I can change it to ESRCH if that is better.)
>>>
> 
> Why is it unexpected? it follows the behavior of cpuset, so this patch will
> break backward compatibility of cpuset.
> 
> But it's better to document this.
> 
> -----------------------------------------
> 
> Document the following cgroup usage:
>  # echo 0 > /dev/cgroup/tasks
> 
> Signed-off-by: Li Zefan <lizf@...fujitsu.com>
> ---
>  cgroups.txt |    4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/Documentation/cgroups.txt b/Documentation/cgroups.txt
> index 824fc02..213f533 100644
> --- a/Documentation/cgroups.txt
> +++ b/Documentation/cgroups.txt
> @@ -390,6 +390,10 @@ If you have several tasks to attach, you have to do it one after another:
>  	...
>  # /bin/echo PIDn > tasks
>  
> +You can attach the current task by echoing 0:
> +
> +# /bin/echo 0 > tasks
> +
>  3. Kernel API
>  =============

Wouldn't be more meaningful to specify the bash's builtin echo here
even if it doesn't opportunely handle write() errors?

Using /bin/echo would attach /bin/echo itself to the cgroup, that just
exists, so it seems like a kind of noop, isn't it?

-Andrea
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