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From: Pedro Alves <pedro@codesourcery.com>
Organization: CodeSourcery
To: Cyrill Gorcunov <gorcunov@gmail.com>
Subject: Re: [rfc 2/3] fs, proc: Introduce the Children: line in /proc/<pid>/status
Date: Fri, 2 Dec 2011 13:40:11 +0000
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Cc: Pavel Emelyanov <xemul@parallels.com>,
        KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>, Tejun Heo <tj@kernel.org>,
        Andrew Vagin <avagin@openvz.org>,
        Serge Hallyn <serge.hallyn@canonical.com>,
        Vasiliy Kulikov <segoon@openwall.com>
References: <20111129191252.769160532@openvz.org> <20111202124551.GD14515@moon> <201112021310.59972.pedro@codesourcery.com>
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On Friday 02 December 2011 13:10:59, Pedro Alves wrote:
> On Friday 02 December 2011 12:45:51, Cyrill Gorcunov wrote:
> > On Fri, Dec 02, 2011 at 04:43:10PM +0400, 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, I suppose additional directory/links and whatever would be just
> > noneeded overhead.
> 
> /proc/8167/task/ being a directory could also be claimed
> "noneeded overhead", yet it exists, and is very useful (I use
> it a lot).  Why diverge instead of being consistent?

Guess I should give an example.  Here's one.  While debugging
gdb stuff, I often do:

$ cat /proc/8167/task/*/status | grep State
State:  S (sleeping)
State:  S (sleeping)
State:  S (sleeping)
State:  S (sleeping)
State:  S (sleeping)
State:  S (sleeping)
State:  S (sleeping)

If children were a directory (or "child", for
consistency with singular "task"), you could do child
things in the same natural way, like for instance:

$ ls -als /proc/8167/child/*/exe 

It's like interactive pstree...

> (Note that listing /proc only shows thread group ids, but
> you can still open /proc/THREAD-ID/, so a /proc/PID/task file
> with lists of pids would have been "good enough".)

-- 
Pedro Alves