From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753630AbZEYQVn (ORCPT ); Mon, 25 May 2009 12:21:43 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752141AbZEYQVf (ORCPT ); Mon, 25 May 2009 12:21:35 -0400 Received: from e1.ny.us.ibm.com ([32.97.182.141]:46162 "EHLO e1.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751055AbZEYQVe (ORCPT ); Mon, 25 May 2009 12:21:34 -0400 Date: Mon, 25 May 2009 09:21:42 -0700 From: "Paul E. McKenney" To: Eric Dumazet Cc: Benjamin LaHaise , Denys Fedoryschenko , netdev@vger.kernel.org, linux kernel , damien.wyart@free.fr Subject: Re: regression: unregister_netdev() unusably slow Message-ID: <20090525162142.GC7168@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20090524192150.GE24757@kvack.org> <200905250023.31056.denys@visp.net.lb> <20090524213744.GG24757@kvack.org> <4A19BF39.4000305@cosmosbay.com> <20090524214433.GH24757@kvack.org> <4A19C50B.9040304@cosmosbay.com> <20090524221240.GI24757@kvack.org> <4A19CE8B.3070302@cosmosbay.com> <20090525000050.GJ24757@kvack.org> <4A1A2AFA.8020605@cosmosbay.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <4A1A2AFA.8020605@cosmosbay.com> User-Agent: Mutt/1.5.15+20070412 (2007-04-11) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, May 25, 2009 at 07:22:02AM +0200, Eric Dumazet wrote: > Benjamin LaHaise a écrit : > > On Mon, May 25, 2009 at 12:47:39AM +0200, Eric Dumazet wrote: > >> There is a strong dependancy against HZ > >> BTW, I am using TREE_RCU > > > > I'm using CLASSIC_RCU. The bisect just completed, and it points to RCU. > > It makes some degree of sense since I'm testing on an otherwise idle > > machine. That said, where is fixing it going to make sense? I'm not > > opposed to having device unregister take a few timer ticks, but there > > has to be some way of exposing parallelism to the system, and since the > > synchronize_net() calls are done under rntl_lock(), none is possible at > > present. Hrm. > > Thanks Ben, this bisection indeed confirms how nasty synchronize_rcu() is :) Yet another step in my learning what is required of RCU, it seems! ;-) > Time to include Paul and lkml in the discussion, and find a better solution than > one provided in February. One approach would be to convert the offending synchronize_rcu() to call_rcu(), but if this were straightforward, I would guess that you would have already done this. But if the code following the synchronize_rcu() does nothing but free up old data structures, this is an easy fix. If there are statistics or other state involved, then call_rcu() might not be the right tool for the job. Another approach is to apply the patch at: http://lkml.org/lkml/2009/5/22/332 Then replace the offending synchronize_rcu() with synchronize_rcu_expedited(). This code is still a bit on the experimental side, but tests have been going quite well, so, unlike a week or two ago, it is definitely worth trying out. Do either of these approaches work for you? Thanx, Paul > > -ben > > > > bf51935f3e988e0ed6f34b55593e5912f990750a is first bad commit > > commit bf51935f3e988e0ed6f34b55593e5912f990750a > > Author: Paul E. McKenney > > Date: Tue Feb 17 06:01:30 2009 -0800 > > > > x86, rcu: fix strange load average and ksoftirqd behavior > > > > Damien Wyart reported high ksoftirqd CPU usage (20%) on an > > otherwise idle system. > > > > The function-graph trace Damien provided: > > ... > > diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c > > > > index a546f55..bd4da2a 100644 > > --- a/arch/x86/kernel/process_32.c > > +++ b/arch/x86/kernel/process_32.c > > @@ -104,9 +104,6 @@ void cpu_idle(void) > > check_pgt_cache(); > > rmb(); > > > > - if (rcu_pending(cpu)) > > - rcu_check_callbacks(cpu, 0); > > - > > if (cpu_is_offline(cpu)) > > play_dead(); > > > > > > -- > > Paul, this commit makes net device unregister very slow (more than 100 ms > if CONFIG_NO_HZ is set), while it used to be pretty fast in previous kernels. > > Quoting Ben : > " I just ran a few L2TP tests against 2.6.30-rc7, and it looks like network > device deletion has become unusably slow. At least in 2.6.27.10, deleting > 1000 network interfaces takes less than 2 seconds of real time. The same > test run under 2.6.30-rc7 is taking hundreds of seconds to delete 1000 > interfaces at a rate of about 5 per second. The interfaces all share the > same local ip address, but each have a single route to a unique client > ip address." > > Device unregister is a synchronize_rcu() abuser (three calls to dismantle > a vlan...) so delaying rcu callbacks can be pretty expensive for it. > > I wonder if the real root of the problem was not discovered in the meantime, > by commit 64ca5ab913f1594ef316556e65f5eae63ff50cee > rcu: increment quiescent state counter in ksoftirqd() > > Maybe this commit solved Damien Wyart problem as well, and we can revert > commit bf51935f3e988e0ed6f34b55593e5912f990750a ? > > Thank you >