From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752636AbaJKTcs (ORCPT ); Sat, 11 Oct 2014 15:32:48 -0400 Received: from mail-pa0-f48.google.com ([209.85.220.48]:56117 "EHLO mail-pa0-f48.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751907AbaJKTcr (ORCPT ); Sat, 11 Oct 2014 15:32:47 -0400 Message-ID: <1413055964.9362.50.camel@edumazet-glaptop2.roam.corp.google.com> Subject: Re: [bisected] e341694e3eb5 netlink_lookup() rcu conversion causes latencies From: Eric Dumazet To: Heiko Carstens , Sasha Levin , paulmck@linux.vnet.ibm.com Cc: Thomas Graf , Nikolay Aleksandrov , "David S. Miller" , netdev@vger.kernel.org, linux-kernel@vger.kernel.org, Ursula Braun Date: Sat, 11 Oct 2014 12:32:44 -0700 In-Reply-To: <20141011083627.GB5074@osiris> References: <20141011083627.GB5074@osiris> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.10.4-0ubuntu2 Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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