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Date: Fri, 2 Dec 2011 09:40:10 +0900
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
To: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: 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>,
        Pavel Emelyanov <xemul@parallels.com>,
        Vasiliy Kulikov <segoon@openwall.com>
Subject: Re: [rfc 2/3] fs, proc: Introduce the Children: line in
 /proc/<pid>/status
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On Thu, 1 Dec 2011 13:54:34 +0400
Cyrill Gorcunov <gorcunov@gmail.com> wrote:

> On Wed, Nov 30, 2011 at 10:05:37AM +0400, Cyrill Gorcunov wrote:
> > 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@openvz.org> wrote:
> > > 
> > > > From: Pavel Emelyanov <xemul@parallels.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@parallels.com>
> > > > Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
> > > > Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.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
> > > 
> 
> What about this one?
> ---
> fs, proc: Introduce the /proc/<pid>/children entry
> 
> 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 /proc/<pid>/children entry,
> since kernel already knows this kind of information but it was not
> yet exported.
> 
> Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
> Cc: Pavel Emelyanov <xemul@parallels.com>
> Cc: Serge Hallyn <serge.hallyn@canonical.com>
> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

Yes, I like /children file. other points seems to be pointed out by other
reviewers. 

Thanks,
-Kame