linux-kernel - Re: [rfc 2/3] fs, proc: Introduce the Children: line in /proc/<pid>/status

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Date:	Fri, 02 Dec 2011 17:52:43 +0400
From:	Pavel Emelyanov <xemul@...allels.com>
To:	Pedro Alves <pedro@...esourcery.com>
CC:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Cyrill Gorcunov <gorcunov@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>, Andrew Vagin <avagin@...nvz.org>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	Vasiliy Kulikov <segoon@...nwall.com>
Subject: Re: [rfc 2/3] fs, proc: Introduce the Children: line in /proc/<pid>/status

On 12/02/2011 05:44 PM, Pedro Alves wrote:
> On Friday 02 December 2011 13:16:52, Pavel Emelyanov wrote:
>> On 12/02/2011 04:58 PM, Pedro Alves wrote:
>>> On Friday 02 December 2011 12:43:10, Pavel Emelyanov wrote:
>>>
>>>>>> Yes, I like /children file. other points seems to be pointed out by other
>>>>>> reviewers. 
>>>>>
>>>>> Any reason this is a file instead of a directory like /proc/PID/task/ ?
>>>>>
>>>>> $ sudo ls /proc/8167/task/
>>>>> 8167  854  855  856  857  858  859
>>>>> $ sudo ls /proc/8167/task/855/
>>>>> attr    clear_refs  cpuset   exe     io       loginuid  mountinfo  oom_adj        pagemap      sched      smaps  statm    wchan
>>>>> auxv    cmdline     cwd      fd      latency  maps      mounts     oom_score      personality  schedstat  stack  status
>>>>> cgroup  comm        environ  fdinfo  limits   mem       numa_maps  oom_score_adj  root         sessionid  stat   syscall
>>>>>
>>>>> Much easier to follow the chain from the command line this way.
>>>>
>>>> What do you propose to put into these directories? Another directories named with
>>>> children pid-s?
>>>
>>> Yes, just like the task/ dir gives you directories named with the
>>> processes's thread ids.  Opening /proc/PID/children/PID-CHILD1/ would get
>>> you the same as opening /proc/PID-CHILD1/.  Just like
>>> opening /proc/PID/task/PID-CHILD1/ gets you (almost) the same as opening
>>> /proc/PID-CHILD1/.
>>
>> You cannot make the dentry named /proc/<pid1>/children/<pid2> be a hardlink on
>> the /proc/<pid2>. Thus you have to make arbitrary amount of inodes to point to
>> a single task. This brings unnecessary complexity and memory usage (by dentries
>> and proc inodes).
> 
> How is this different from the _already existing_ /proc/<pid1>/task/ directory?

Those living in /proc/<pid1>/task do not live in /proc. At all. This explains
everything below.

> I can imagine that 98% of the code would be shared even?  It's "just" a matter of
> listing thread group children (child/), instead of clone children (task/),
> isn't it?
> 
> They are not symbolic links under task/.  /proc/<pid1>/task/<pid2>/ does not
> have a task/ subdir, only /proc/<pid1>/ does, I guess to avoid the memory usage
> issue you raise.
> 
>> I'd accept the symbolic links, but how would they look like? Like this:
>>    # ls -l /proc/123/children
>>             234 -> ../../234
>> ?
> 
> That'd work for me...  but really, why not reuse tasks/'s code and
> behave the same?

Why wouldn't /proc/<pid>/children work for you? This is as simple as

  # ls -l $(cat /proc/<pid>/children)

:)

Thanks,
Pavel
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