From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752989AbaJTIVR (ORCPT ); Mon, 20 Oct 2014 04:21:17 -0400 Received: from e06smtp14.uk.ibm.com ([195.75.94.110]:34907 "EHLO e06smtp14.uk.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752623AbaJTIVO (ORCPT ); Mon, 20 Oct 2014 04:21:14 -0400 Date: Mon, 20 Oct 2014 10:21:08 +0200 From: Heiko Carstens To: Thomas Graf Cc: Eric Dumazet , Sasha Levin , paulmck@linux.vnet.ibm.com, Nikolay Aleksandrov , "David S. Miller" , netdev@vger.kernel.org, linux-kernel@vger.kernel.org, Ursula Braun Subject: Re: [bisected] e341694e3eb5 netlink_lookup() rcu conversion causes latencies Message-ID: <20141020082107.GB4268@osiris> References: <20141011083627.GB5074@osiris> <1413055964.9362.50.camel@edumazet-glaptop2.roam.corp.google.com> <20141011222514.GA14186@casper.infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20141011222514.GA14186@casper.infradead.org> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 14102008-0017-0000-0000-0000017FF01D Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Oct 11, 2014 at 11:25:14PM +0100, Thomas Graf wrote: > On 10/11/14 at 12:32pm, Eric Dumazet wrote: > > On Sat, 2014-10-11 at 10:36 +0200, Heiko Carstens wrote: > > > Hi all, > > > > > > it just came to my attention that commit e341694e3eb5 > > > "netlink: Convert netlink_lookup() to use RCU protected hash table" > > > causes network latencies for me on s390. > > > > > > The testcase is quite simple and 100% reproducible on s390: > > > > > > Simply login via ssh to a remote system which has the above mentioned > > > patch applied. Any action like pressing return now has significant > > > latencies. Or in other words, working via such a connection becomes > > > a pain ;) > > > > > > I haven't debugged it, however I assume the problem is that a) the > > > commit introduces a synchronize_net() call und b) s390 kernels > > > usually get compiled with CONFIG_HZ_100 while most other architectures > > > use CONFIG_HZ_1000. > > > If I change the kernel config to CONFIG_HZ_1000 the problem goes away, > > > however I don't consider this a fix... > > > > > > Another reason why this hasn't been observed on x86 may or may not be > > > that we haven't implemented CONFIG_HAVE_CONTEXT_TRACKING on s390 (yet). > > > But that's just guessing... > > > > CC Paul and Sasha > > I think the issue here is obvious and a fix is on the way to move > the insertion and removal to a worker to no longer require the > synchronize_rcu(). > > What bothers me is that the synchronize_rcu() should only occur > on expand/shrink and not for every table update. The default table > size is 64. *ping* ... is there already any patch available?