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

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Message-ID: <20080701215448.GD5893@linux.vnet.ibm.com>
Date:	Wed, 2 Jul 2008 03:24:48 +0530
From:	Dhaval Giani <dhaval@...ux.vnet.ibm.com>
To:	Andrea Righi <righi.andrea@...il.com>
Cc:	Li Zefan <lizf@...fujitsu.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

On Tue, Jul 01, 2008 at 11:48:31PM +0200, Andrea Righi wrote:
> 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?
>

Yes, you are right. this example should use bash's builtin echo.

-- 
regards,
Dhaval
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