Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760175AbZDLKzK (ORCPT ); Sun, 12 Apr 2009 06:55:10 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1757418AbZDLKyw (ORCPT ); Sun, 12 Apr 2009 06:54:52 -0400 Received: from mx2.mail.elte.hu ([157.181.151.9]:47647 "EHLO mx2.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755673AbZDLKyv (ORCPT ); Sun, 12 Apr 2009 06:54:51 -0400 Date: Sun, 12 Apr 2009 12:54:16 +0200 From: Ingo Molnar To: "Paul E. McKenney" 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, paulus@samba.org, benh@kernel.crashing.org Subject: Re: iptables very slow after commit 784544739a25c30637397ace5489eeb6e15d7d49 Message-ID: <20090412105416.GB25392@elte.hu> References: <20090410095246.4fdccb56@s6510> <20090410.182507.140306636.davem@davemloft.net> <20090411041533.GB6822@linux.vnet.ibm.com> <20090411070854.GC11799@elte.hu> <20090411174801.GG6822@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20090411174801.GG6822@linux.vnet.ibm.com> User-Agent: Mutt/1.5.18 (2008-05-17) X-ELTE-VirusStatus: clean X-ELTE-SpamScore: -1.5 X-ELTE-SpamLevel: X-ELTE-SpamCheck: no X-ELTE-SpamVersion: ELTE 2.0 X-ELTE-SpamCheck-Details: score=-1.5 required=5.9 tests=BAYES_00 autolearn=no SpamAssassin version=3.2.3 -1.5 BAYES_00 BODY: Bayesian spam probability is 0 to 1% [score: 0.0000] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1923 Lines: 48 * Paul E. McKenney wrote: > On Sat, Apr 11, 2009 at 09:08:54AM +0200, Ingo Molnar wrote: > > > > * 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. > > If the generic implementation is needed only on !SMP systems, that > could work. The architectures I would be worried about include > powerpc and ia64, which I believe support 32-bit SMP builds. ia64 would naturally support the CMPXCHG8B instructions. Not sure about powerpc32. Having a lock for the library implementation is not _that_ much of a problem. We obviously dont want the design of Linux to be dictated by the weakest link of all platforms, right? Ingo -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/