From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751337AbdJCB1N (ORCPT ); Mon, 2 Oct 2017 21:27:13 -0400 Received: from ozlabs.org ([103.22.144.67]:57809 "EHLO ozlabs.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750983AbdJCB1M (ORCPT ); Mon, 2 Oct 2017 21:27:12 -0400 From: Michael Ellerman To: Laurent Dufour , Andrew Morton Cc: Alexei Starovoitov , Paul McKenney , Peter Zijlstra , kirill@shutemov.name, Andi Kleen , Michal Hocko , dave@stgolabs.net, Jan Kara , Matthew Wilcox , Benjamin Herrenschmidt , Paul Mackerras , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , Will Deacon , Sergey Senozhatsky , "linux-kernel\@vger.kernel.org" , linux-mm , haren@linux.vnet.ibm.com, Anshuman Khandual , npiggin@gmail.com, bsingharora@gmail.com, Tim Chen , linuxppc-dev@lists.ozlabs.org, "x86\@kernel.org" Subject: Re: [PATCH v3 00/20] Speculative page faults In-Reply-To: <64e9759b-a4fb-63d3-a811-3e35ae5a1028@linux.vnet.ibm.com> References: <20170925163443.260d6092160ec704e2b04653@linux-foundation.org> <924a79af-6d7a-316a-1eee-3aebbfd4addf@linux.vnet.ibm.com> <20170928133850.90c5bf2aac0f1a63e29c01a3@linux-foundation.org> <64e9759b-a4fb-63d3-a811-3e35ae5a1028@linux.vnet.ibm.com> Date: Tue, 03 Oct 2017 12:27:10 +1100 Message-ID: <873771dnrl.fsf@concordia.ellerman.id.au> MIME-Version: 1.0 Content-Type: text/plain Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Laurent Dufour writes: > Hi Andrew, > > On 28/09/2017 22:38, Andrew Morton wrote: >> On Thu, 28 Sep 2017 14:29:02 +0200 Laurent Dufour wrote: >> >>>> Laurent's [0/n] provides some nice-looking performance benefits for >>>> workloads which are chosen to show performance benefits(!) but, alas, >>>> no quantitative testing results for workloads which we may suspect will >>>> be harmed by the changes(?). Even things as simple as impact upon >>>> single-threaded pagefault-intensive workloads and its effect upon >>>> CONFIG_SMP=n .text size? >>> >>> I forgot to mention in my previous email the impact on the .text section. >>> >>> Here are the metrics I got : >>> >>> .text size UP SMP Delta >>> 4.13-mmotm 8444201 8964137 6.16% >>> '' +spf 8452041 8971929 6.15% >>> Delta 0.09% 0.09% >>> >>> No major impact as you could see. >> >> 8k text increase seems rather a lot actually. That's a lot more >> userspace cacheclines that get evicted during a fault... >> >> Is the feature actually beneficial on uniprocessor? > > This is useless on uniprocessor, and I will disable it on x86 when !SMP > by not defining __HAVE_ARCH_CALL_SPF. > So the speculative page fault handler will not be built but the vm > sequence counter and the SCRU stuff will still be there. I may also make > it disabled through macro when __HAVE_ARCH_CALL_SPF is not defined, but > this may obfuscated the code a bit... > > On ppc64, as this feature requires book3s, it can't be built without SMP > support. Book3S doesn't force SMP, eg. PMAC is Book3S but can be built with SMP=n. It's true that POWERNV and PSERIES both force SMP, and those are the platforms used on modern Book3S CPUs. cheers