Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758533AbZDKRsX (ORCPT ); Sat, 11 Apr 2009 13:48:23 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1757590AbZDKRsG (ORCPT ); Sat, 11 Apr 2009 13:48:06 -0400 Received: from e9.ny.us.ibm.com ([32.97.182.139]:45616 "EHLO e9.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757440AbZDKRsD (ORCPT ); Sat, 11 Apr 2009 13:48:03 -0400 Date: Sat, 11 Apr 2009 10:48:01 -0700 From: "Paul E. McKenney" To: Ingo Molnar 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: <20090411174801.GG6822@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20090410095246.4fdccb56@s6510> <20090410.182507.140306636.davem@davemloft.net> <20090411041533.GB6822@linux.vnet.ibm.com> <20090411070854.GC11799@elte.hu> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20090411070854.GC11799@elte.hu> User-Agent: Mutt/1.5.15+20070412 (2007-04-11) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1545 Lines: 38 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. Thanx, Paul -- 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/