From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ingo Molnar Subject: Re: iptables very slow after commit 784544739a25c30637397ace5489eeb6e15d7d49 Date: Sat, 11 Apr 2009 09:08:54 +0200 Message-ID: <20090411070854.GC11799@elte.hu> References: <20090410095246.4fdccb56@s6510> <20090410.182507.140306636.davem@davemloft.net> <20090411041533.GB6822@linux.vnet.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Linus Torvalds , David Miller , Lai Jiangshan , shemminger@vyatta.com, jeff.chua.linux@gmail.com, dada1@cosmosbay.com, jengelh@medozas.de, kaber@trash.net, r000n@r000n.net, Linux Kernel Mailing List , netfilter-devel@vger.kernel.org, netdev@vger.kernel.org To: "Paul E. McKenney" Return-path: Received: from mx2.mail.elte.hu ([157.181.151.9]:57247 "EHLO mx2.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752244AbZDKHJW (ORCPT ); Sat, 11 Apr 2009 03:09:22 -0400 Content-Disposition: inline In-Reply-To: <20090411041533.GB6822@linux.vnet.ibm.com> Sender: netfilter-devel-owner@vger.kernel.org List-ID: * Paul E. McKenney wrote: > I will nevertheless suggest the following egregious hack to > get a consistent sample of one counter for some other CPU: > > a. Disable interrupts > b. Atomically exchange the bottom 32 bits of the > counter with the value zero. > c. Atomically exchange the top 32 bits of the counter > with the value zero. > d. Concatenate the values obtained in (b) and (c), which > is the snapshot value. Note, i have recently implemented full atomic64_t support on 32-bit x86, for the perfcounters code, based on the CMPXCHG8B instruction. Which, while not the lightest of instructions, is still much better than the sequence above. So i think a better approach would be to also add a dumb generic implementation for atomic64_t (using a global lock or so), and then generic code could just assume that atomic64_t always exists. It is far nicer - and faster as well - as the hack above, even on 32-bit x86. Ingo