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

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Date:	Wed, 30 Nov 2011 10:05:37 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	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>,
	Pavel Emelyanov <xemul@...allels.com>,
	Vasiliy Kulikov <segoon@...nwall.com>
Subject: Re: [rfc 2/3] fs, proc: Introduce the Children: line in
 /proc/<pid>/status

On Wed, Nov 30, 2011 at 02:00:09PM +0900, KAMEZAWA Hiroyuki wrote:
> On Tue, 29 Nov 2011 23:12:54 +0400
> Cyrill Gorcunov <gorcunov@...nvz.org> wrote:
> 
> > From: Pavel Emelyanov <xemul@...allels.com>
> > 
> > There is no easy way to make a reverse parent->children chain
> > from the task status, in turn children->parent provided with "PPid"
> > field.
> > 
> > So instead of walking over all pids in system to figure out what
> > children the task have -- we add explicit "Children" member to
> > /proc/<pid>/status since kernel already knows this kind of information
> > but it was not yet exported.
> > 
> > Signed-off-by: Pavel Emelyanov <xemul@...allels.com>
> > Acked-by: Serge Hallyn <serge.hallyn@...onical.com>
> > Signed-off-by: Cyrill Gorcunov <gorcunov@...nvz.org>
> 
> I may be too pessimistic but what amount of overhead will this add to
> ps -elf/ top ? Assuming an environment 'ps -elf' is called once per a sec,
> if there are 2000 processes, task_list lock is taken 2000 times by this patch.
> 

Hi Kame, good point! Yes, it introduces latency on high loaded systems.
I must admit I tested this patch on a regular system, where not that much
processes were launched but technically I think more correct would be to
switch to children file. I'll tune up the patch. Thanks!

> Isn't it better to add /proc/<pid>/children file or dir (as task)?
> 
> Thanks,
> -Kame
> 
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