Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932380AbcCCPe7 (ORCPT ); Thu, 3 Mar 2016 10:34:59 -0500 Received: from casper.infradead.org ([85.118.1.10]:42243 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753240AbcCCPe5 (ORCPT ); Thu, 3 Mar 2016 10:34:57 -0500 Date: Thu, 3 Mar 2016 16:34:53 +0100 From: Peter Zijlstra To: Ingo Molnar Cc: Dexuan Cui , "linux-x86_64@vger.kernel.org" , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , David Howells , "Paul E. McKenney" , "linux-kernel@vger.kernel.org" , "Michael S. Tsirkin" Subject: Re: x86 memory barrier: why does Linux prefer MFENCE to Locked ADD? Message-ID: <20160303153453.GR6356@twins.programming.kicks-ass.net> References: <20160303152739.GA16303@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20160303152739.GA16303@gmail.com> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2095 Lines: 53 On Thu, Mar 03, 2016 at 04:27:39PM +0100, Ingo Molnar wrote: > > * Dexuan Cui wrote: > > > Hi, > > My understanding about arch/x86/include/asm/barrier.h is: obviously Linux > > more likes {L,S,M}FENCE -- Locked ADD is only used in x86_32 platforms that > > don't support XMM2. > > > > However, it looks people say Locked Add is much faster than the FENCE > > instructions, even on modern Intel CPUs like Haswell, e.g., please see > > the three sources: > > > > " 11.5.1 Locked Instructions as Memory Barriers > > Optimization > > Use locked instructions to implement Store/Store and Store/Load barriers. > > " > > http://support.amd.com/TechDocs/47414_15h_sw_opt_guide.pdf > > > > "lock addl %(rsp), 0 is a better solution for StoreLoad barrier ": > > http://shipilev.net/blog/2014/on-the-fence-with-dependencies/ > > > > "...locked instruction are more efficient barriers...": > > http://www.pvk.ca/Blog/2014/10/19/performance-optimisation-~-writing-an-essay/ > > > > I also found that FreeBSD prefers Locked Add. > > > > So, I'm curious why Linux prefers MFENCE. > > I guess I may be missing something. > > > > I tried to google the question, but didn't find an answer. > > It's being worked on, see this thread on lkml from a few weeks ago: > > C Jan 13 Michael S. Tsir | [PATCH v3 0/4] x86: faster mb()+documentation tweaks > C Jan 13 Michael S. Tsir | ├─>[PATCH v3 1/4] x86: add cc clobber for addl > C Jan 13 Michael S. Tsir | ├─>[PATCH v3 2/4] x86: drop a comment left over from X86_OOSTORE > C Jan 13 Michael S. Tsir | ├─>[PATCH v3 3/4] x86: tweak the comment about use of wmb for IO > C Jan 13 Michael S. Tsir | ├─>[PATCH v3 4/4] x86: drop mfence in favor of lock+addl > > The 4th patch changes MFENCE to a LOCK ADDL locked instruction. Lots of additional chatter here: lkml.kernel.org/r/20160112150032-mutt-send-email-mst@redhat.com And some useful bits here: lkml.kernel.org/r/56957D54.5000602@zytor.com latest version here: lkml.kernel.org/r/1453921746-16178-1-git-send-email-mst@redhat.com