From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-10.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,NICE_REPLY_A, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 6AD3FC07E94 for ; Fri, 4 Jun 2021 08:38:02 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5490961411 for ; Fri, 4 Jun 2021 08:38:02 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230055AbhFDIjq (ORCPT ); Fri, 4 Jun 2021 04:39:46 -0400 Received: from mga06.intel.com ([134.134.136.31]:50572 "EHLO mga06.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229956AbhFDIjq (ORCPT ); Fri, 4 Jun 2021 04:39:46 -0400 IronPort-SDR: uLbcEjWgX2ZijfDg376+/Dz70qKitHflCob+7PWMmxr4d8efxQNISGGG5zjZ2YfgpNaAPKbf8g WjChheiKfjDw== X-IronPort-AV: E=McAfee;i="6200,9189,10004"; a="265415523" X-IronPort-AV: E=Sophos;i="5.83,247,1616482800"; d="scan'208";a="265415523" Received: from orsmga001.jf.intel.com ([10.7.209.18]) by orsmga104.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 04 Jun 2021 01:37:59 -0700 IronPort-SDR: AclLJia9BD0aIRsL+zozbyKJTa3DCdxowysJiikvTY/1MDyAvZt7qqiiQje0PVoYbvgDjdSGId X96xh0/Sr7KA== X-IronPort-AV: E=Sophos;i="5.83,247,1616482800"; d="scan'208";a="480561157" Received: from xingzhen-mobl.ccr.corp.intel.com (HELO [10.238.5.220]) ([10.238.5.220]) by orsmga001-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 04 Jun 2021 01:37:55 -0700 Subject: Re: [LKP] Re: [mm/gup] 57efa1fe59: will-it-scale.per_thread_ops -9.2% regression To: Linus Torvalds , kernel test robot Cc: Jason Gunthorpe , John Hubbard , Jan Kara , Peter Xu , Andrea Arcangeli , "Aneesh Kumar K.V" , Christoph Hellwig , Hugh Dickins , Jann Horn , Kirill Shutemov , Kirill Tkhai , Leon Romanovsky , Michal Hocko , Oleg Nesterov , Andrew Morton , LKML , lkp@lists.01.org, kernel test robot References: <20210525031636.GB7744@xsang-OptiPlex-9020> From: Xing Zhengjun Message-ID: <2a1977e1-4fd1-065a-214f-51c6b40cc3d2@linux.intel.com> Date: Fri, 4 Jun 2021 16:37:53 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Linus, On 5/25/2021 11:11 AM, Linus Torvalds wrote: > On Mon, May 24, 2021 at 5:00 PM kernel test robot wrote: >> FYI, we noticed a -9.2% regression of will-it-scale.per_thread_ops due to commit: >> commit: 57efa1fe5957694fa541c9062de0a127f0b9acb0 ("mm/gup: prevent gup_fast from racing with COW during fork") > Hmm. This looks like one of those "random fluctuations" things. > > It would be good to hear if other test-cases also bisect to the same > thing, but this report already says: > >> In addition to that, the commit also has significant impact on the following tests: >> >> +------------------+---------------------------------------------------------------------------------+ >> | testcase: change | will-it-scale: will-it-scale.per_thread_ops 3.7% improvement | > which does kind of reinforce that "this benchmark gives unstable numbers". > > The perf data doesn't even mention any of the GUP paths, and on the > pure fork path the biggest impact would be: > > (a) maybe "struct mm_struct" changed in size or had a different cache layout I move "write_protect_seq" to the tail of the "struct mm_struct", the regression reduced to -3.6%. The regression should relate to the cache layout. ========================================================================================= tbox_group/testcase/rootfs/kconfig/compiler/nr_task/mode/test/cpufreq_governor/ucode: lkp-icl-2sp1/will-it-scale/debian-10.4-x86_64-20200603.cgz/x86_64-rhel-8.3/gcc-9/50%/thread/mmap1/performance/0xb000280 commit:   c28b1fc70390df32e29991eedd52bd86e7aba080   57efa1fe5957694fa541c9062de0a127f0b9acb0   f6a9c27882d51ff551e15522992d3725c342372d  (the test patch) c28b1fc70390df32 57efa1fe5957694fa541c9062de f6a9c27882d51ff551e15522992 ---------------- --------------------------- ---------------------------          %stddev     %change         %stddev     %change %stddev              \          |                \          | \     341938            -9.0%     311218 ±  2%      -3.6% 329513        will-it-scale.48.threads       7123            -9.0%       6483 ±  2%      -3.6% 6864        will-it-scale.per_thread_ops     341938            -9.0%     311218 ±  2%      -3.6% 329513        will-it-scale.workload diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h index 915f4f100383..34bb2a01806c 100644 --- a/include/linux/mm_types.h +++ b/include/linux/mm_types.h @@ -447,13 +447,6 @@ struct mm_struct {                  */                 atomic_t has_pinned; -               /** -                * @write_protect_seq: Locked when any thread is write -                * protecting pages mapped by this mm to enforce a later COW, -                * for instance during page table copying for fork(). -                */ -               seqcount_t write_protect_seq; -  #ifdef CONFIG_MMU                 atomic_long_t pgtables_bytes;   /* PTE page table pages */  #endif @@ -564,6 +557,12 @@ struct mm_struct {  #ifdef CONFIG_IOMMU_SUPPORT                 u32 pasid;  #endif +                /** +                 * @write_protect_seq: Locked when any thread is write +                 * protecting pages mapped by this mm to enforce a later COW, +                 * for instance during page table copying for fork(). +                 */ +                seqcount_t write_protect_seq;         } __randomize_layout;         /* > > (b) two added (nonatomic) increment operations in the fork path due > to the seqcount > > and I'm not seeing what would cause that 9% change. Obviously cache > placement has done it before. > > If somebody else sees something that I'm missing, please holler. But > I'll ignore this as "noise" otherwise. > > Linus > _______________________________________________ > LKP mailing list -- lkp@lists.01.org > To unsubscribe send an email to lkp-leave@lists.01.org -- Zhengjun Xing