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

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Message-Id: <20111130140009.1ccd5af0.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Wed, 30 Nov 2011 14:00:09 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Cyrill Gorcunov <gorcunov@...nvz.org>
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 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.

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

Thanks,
-Kame

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