From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754361Ab1LIPfk (ORCPT ); Fri, 9 Dec 2011 10:35:40 -0500 Received: from mx1.redhat.com ([209.132.183.28]:27093 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751346Ab1LIPfj (ORCPT ); Fri, 9 Dec 2011 10:35:39 -0500 Date: Fri, 9 Dec 2011 16:30:09 +0100 From: Oleg Nesterov To: Andrew Morton Cc: Cyrill Gorcunov , Pavel Emelyanov , Serge Hallyn , KAMEZAWA Hiroyuki , Tejun Heo , Vasiliy Kulikov , Andrew Vagin , LKML Subject: Re: [PATCH] fs, proc: Introduce the /proc//children entry v2 Message-ID: <20111209153009.GA20865@redhat.com> References: <20111206181026.GO29781@moon> <20111207185343.GA3209@redhat.com> <20111207190340.GP21678@moon> <20111208163535.GA25023@redhat.com> <20111208212853.GO21678@moon> <20111208135430.00730308.akpm@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20111208135430.00730308.akpm@linux-foundation.org> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 12/08, Andrew Morton wrote: > > On Fri, 9 Dec 2011 01:28:53 +0400 > Cyrill Gorcunov wrote: > > > On Thu, Dec 08, 2011 at 05:35:35PM +0100, Oleg Nesterov wrote: > > ... > > > > > > However, ->children list is not rcu-safe, this means that even > > > list_for_each() itself is not safe. Either you need tasklist or > > > we can probably make it rcu-safe... > > > > > > > Andrew, Oleg, does the below one look more less fine? Note the > > tasklist_lock is back and it worries me a bit since I imagine > > one could be endlessly reading some /proc//children file > > increasing contention over this lock on the whole system > > (regardless the fact that it's take for read only). > > It is a potential problem, from the lock-hold point of view and > also it can cause large scheduling latencies. What's involved in > making ->children an rcu-protected list? At first glance, this doesn't look trivial... forget_original_parent() abuses ->sibling. But yes, it is not really nice to hold tasklist_lock here. May be we can change this code so that every iteration records the reported task_struct and then tries to continue. This means we should verify that ->real_parent is still the same under tasklist, but at least this way we do not hold it throughout. > > From: Cyrill Gorcunov > > Subject: [PATCH] fs, proc: Introduce the /proc//children entry v4 > > > > There is no easy way to make a reverse parent->children chain > > from arbitrary (while parent pid is provided in "PPid" > > field of /proc//status). > > > > So instead of walking over all pids in the system to figure out which > > children a task have -- we add explicit /proc//children entry, > > because kernel already has this kind of information but it is not > > yet exported. This is a first level children, not the whole process > > tree, neither the process threads are identified with this interface. > > The changelog doesn't explain why we want the patch, so there's no > reason to merge it! Something to do with c/r, yes? > > If so, I guess the feature could/should be configurable. Probably with > a CONFIG_PROC_CHILDREN which is selected by CONFIG_CHECKPOINT_RESTORE. > Which is all getting a bit over the top, but I suppose we must do it. Heh. This is the rare case when I personally like the new feature ;) I mean, it looks "obviously useful" to me. If nothing else, it can help to debug the problems. Probably the tools like pstree can use it. Personally I'd even prefer /proc/pid/children/ directory (like /proc/pid/task), but I guess this needs much more complications. Oleg.