From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752146AbaJKIgf (ORCPT ); Sat, 11 Oct 2014 04:36:35 -0400 Received: from e06smtp14.uk.ibm.com ([195.75.94.110]:45155 "EHLO e06smtp14.uk.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752040AbaJKIgc (ORCPT ); Sat, 11 Oct 2014 04:36:32 -0400 Date: Sat, 11 Oct 2014 10:36:27 +0200 From: Heiko Carstens To: Thomas Graf , Nikolay Aleksandrov , "David S. Miller" Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org, Ursula Braun Subject: [bisected] e341694e3eb5 netlink_lookup() rcu conversion causes latencies Message-ID: <20141011083627.GB5074@osiris> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 14101108-0017-0000-0000-00000168AA3F Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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...