Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751637AbbKCAGt (ORCPT ); Mon, 2 Nov 2015 19:06:49 -0500 Received: from mail-io0-f171.google.com ([209.85.223.171]:34254 "EHLO mail-io0-f171.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750801AbbKCAGq (ORCPT ); Mon, 2 Nov 2015 19:06:46 -0500 MIME-Version: 1.0 In-Reply-To: <20151102201535.GB1707@linux-uzut.site> References: <1445975631-17047-1-git-send-email-dave@stgolabs.net> <1445975631-17047-4-git-send-email-dave@stgolabs.net> <20151027223744.GB11242@worktop.amr.corp.intel.com> <20151102201535.GB1707@linux-uzut.site> Date: Mon, 2 Nov 2015 16:06:46 -0800 X-Google-Sender-Auth: X5KWsVJ82l2nCMFdVXd5EkngKIk Message-ID: Subject: Re: [PATCH 3/4] x86,asm: Re-work smp_store_mb() From: Linus Torvalds To: Davidlohr Bueso Cc: Peter Zijlstra , Ingo Molnar , Thomas Gleixner , "Paul E. McKenney" , Linux Kernel Mailing List , "the arch/x86 maintainers" , Davidlohr Bueso Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 907 Lines: 21 On Mon, Nov 2, 2015 at 12:15 PM, Davidlohr Bueso wrote: > > So I ran some experiments on an IvyBridge (2.8GHz) and the cost of XCHG is > constantly cheaper (by at least half the latency) than MFENCE. While there > was a decent amount of variation, this difference remained rather constant. Mind testing "lock addq $0,0(%rsp)" instead of mfence? That's what we use on old cpu's without one (ie 32-bit). I'm not actually convinced that mfence is necessarily a good idea. I could easily see it being microcode, for example. At least on my Haswell, the "lock addq" is pretty much exactly half the cost of "mfence". Linus -- 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/