From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758887AbYCZKdR (ORCPT ); Wed, 26 Mar 2008 06:33:17 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753832AbYCZKdE (ORCPT ); Wed, 26 Mar 2008 06:33:04 -0400 Received: from e28smtp01.in.ibm.com ([59.145.155.1]:55210 "EHLO e28smtp01.in.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753849AbYCZKdB (ORCPT ); Wed, 26 Mar 2008 06:33:01 -0400 Message-ID: <47EA2592.7090600@linux.vnet.ibm.com> Date: Wed, 26 Mar 2008 15:59:38 +0530 From: Balbir Singh Reply-To: balbir@linux.vnet.ibm.com Organization: IBM User-Agent: Thunderbird 2.0.0.12 (X11/20080226) MIME-Version: 1.0 To: Paul Menage CC: balbir@linux.vnet.ibm.com, linux-mm@kvack.org, Hugh Dickins , Sudhir Kumar , YAMAMOTO Takashi , lizf@cn.fujitsu.com, linux-kernel@vger.kernel.org, taka@valinux.co.jp, David Rientjes , Pavel Emelianov , Andrew Morton , KAMEZAWA Hiroyuki Subject: Re: [RFC][-mm] Memory controller add mm->owner References: <20080324140142.28786.97267.sendpatchset@localhost.localdomain> <6599ad830803240803s5160101bi2bf68b36085f777f@mail.gmail.com> <47E7D51E.4050304@linux.vnet.ibm.com> <6599ad830803240934g2a70d904m1ca5548f8644c906@mail.gmail.com> <47E7E5D0.9020904@linux.vnet.ibm.com> <6599ad830803241046l61e2965t52fd28e165d5df7a@mail.gmail.com> <47E8E4F3.6090604@linux.vnet.ibm.com> In-Reply-To: <47E8E4F3.6090604@linux.vnet.ibm.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Balbir Singh wrote: > Paul Menage wrote: >> On Mon, Mar 24, 2008 at 10:33 AM, Balbir Singh >> wrote: >>> > OK, so we don't need to handle this for NPTL apps - but for anything >>> > still using LinuxThreads or manually constructed clone() calls that >>> > use CLONE_VM without CLONE_PID, this could still be an issue. >>> >>> CLONE_PID?? Do you mean CLONE_THREAD? >> Yes, sorry - CLONE_THREAD. >> >>> For the case you mentioned, mm->owner is a moving target and we don't want to >>> spend time finding the successor, that can be expensive when threads start >>> exiting one-by-one quickly and when the number of threads are high. I wonder if >>> there is an efficient way to find mm->owner in that case. >>> >> But: >> >> - running a high-threadcount LinuxThreads process is by definition >> inefficient and expensive (hence the move to NPTL) >> >> - any potential performance hit is only paid at exit time >> >> - in the normal case, any of your children or one of your siblings >> will be a suitable alternate owner >> >> - in the worst case, it's not going to be worse than doing a >> for_each_thread() loop >> This will have to be the common case, since you never know what combination of clone calls did CLONE_VM and what did CLONE_THREAD. At exit time, we need to pay a for_each_process() overhead. Although very unlikely, an application can call pthread_* functions (NPTL) and then do a clone with CLONE_VM, thus forcing threads in a thread group and another process to share the mm_struct. This makes mm->owner struct approach hard to implement. >> so I don't think this would be a major problem >> > > I've been looking at zap_threads, I suspect we'll end up implementing a similar > loop, which makes me very uncomfortable. Adding code for the least possible > scenario. It will not get invoked for CLONE_THREAD, but will get invoked for the > case when CLONE_VM is set without CLONE_THREAD. > > I'll try and experiment a bit more and see what I come up with I am yet to benchmark the cost of doing for_each_process() on every exit. I suspect, we'll see a big drop in performance. I am not sure anymore if mm->owner is worth the overhead. -- Warm Regards, Balbir Singh Linux Technology Center IBM, ISTL