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 17:16:52 +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 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).

I'd accept the symbolic links, but how would they look like? Like this:
   # ls -l /proc/123/children
            234 -> ../../234
?

Thanks,
Pavel
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