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

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Message-Id: <201112021344.40177.pedro@codesourcery.com>
Date:	Fri, 2 Dec 2011 13:44:39 +0000
From:	Pedro Alves <pedro@...esourcery.com>
To:	Pavel Emelyanov <xemul@...allels.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 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?
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?

-- 
Pedro Alves
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