From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753443AbcALWVR (ORCPT ); Tue, 12 Jan 2016 17:21:17 -0500 Received: from mx1.redhat.com ([209.132.183.28]:50526 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753033AbcALWVP (ORCPT ); Tue, 12 Jan 2016 17:21:15 -0500 Date: Wed, 13 Jan 2016 00:21:10 +0200 From: "Michael S. Tsirkin" To: Andy Lutomirski Cc: Linus Torvalds , Andy Lutomirski , Davidlohr Bueso , Davidlohr Bueso , Peter Zijlstra , the arch/x86 maintainers , Linux Kernel Mailing List , virtualization , "H. Peter Anvin" , Thomas Gleixner , "Paul E. McKenney" , Ingo Molnar Subject: Re: [PATCH 3/4] x86,asm: Re-work smp_store_mb() Message-ID: <20160113001824-mutt-send-email-mst@redhat.com> References: <1445975631-17047-4-git-send-email-dave@stgolabs.net> <20151027223744.GB11242@worktop.amr.corp.intel.com> <20151102201535.GB1707@linux-uzut.site> <20160112150032-mutt-send-email-mst@redhat.com> <56956276.1090705@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jan 12, 2016 at 12:59:58PM -0800, Andy Lutomirski wrote: > On Tue, Jan 12, 2016 at 12:54 PM, Linus Torvalds > wrote: > > On Tue, Jan 12, 2016 at 12:30 PM, Andy Lutomirski wrote: > >> > >> I recall reading somewhere that lock addl $0, 32(%rsp) or so (maybe even 64) > >> was better because it avoided stomping on very-likely-to-be-hot write > >> buffers. > > > > I suspect it could go either way. You want a small constant (for the > > isntruction size), but any small constant is likely to be within the > > current stack frame anyway. I don't think 0(%rsp) is particularly > > likely to have a spill on it right then and there, but who knows.. > > > > And 64(%rsp) is possibly going to be cold in the L1 cache, especially > > if it's just after a deep function call. Which it might be. So it > > might work the other way. > > > > So my guess would be that you wouldn't be able to measure the > > difference. It might be there, but probably too small to really see in > > any noise. > > > > But numbers talk, bullshit walks. It would be interesting to be proven wrong. > > Here's an article with numbers: > > http://shipilev.net/blog/2014/on-the-fence-with-dependencies/ > > I think they're suggesting using a negative offset, which is safe as > long as it doesn't page fault, even though we have the redzone > disabled. > > --Andy OK so I'll have to tweak the test to put something on stack to measure the difference: my test tweaks a global variable instead. I'll try that by tomorrow. I couldn't measure any difference between mfence and lock+addl except in a micro-benchmark, but hey since we are tweaking this, let's do the optimal thing. -- MST