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=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=no 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 CA186C43331 for ; Fri, 27 Mar 2020 08:46:43 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id AB86C20716 for ; Fri, 27 Mar 2020 08:46:43 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727263AbgC0Iqm (ORCPT ); Fri, 27 Mar 2020 04:46:42 -0400 Received: from mga14.intel.com ([192.55.52.115]:2058 "EHLO mga14.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726096AbgC0Iqm (ORCPT ); Fri, 27 Mar 2020 04:46:42 -0400 IronPort-SDR: ajW9eLjF99Q94bOokbHTbdWnOBaSGGUHCprt79fBeGnxIn6QeBWuEh3HGF4nr+Q7Q0G15MoTns MjK3YPfgeQcg== X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from fmsmga004.fm.intel.com ([10.253.24.48]) by fmsmga103.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 27 Mar 2020 01:46:42 -0700 IronPort-SDR: m9bWDS0smC6fU10zjO7AJfBOg/gNnNGCaVr0F8Otjg4VJbEX2WXZ+lrW0mtLUCYuWtbLiux74X oPnqGlHqj3iQ== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.72,311,1580803200"; d="scan'208";a="271497408" Received: from shao2-debian.sh.intel.com (HELO [10.239.13.3]) ([10.239.13.3]) by fmsmga004.fm.intel.com with ESMTP; 27 Mar 2020 01:46:40 -0700 Subject: Re: [mm] fd4d9c7d0c: stress-ng.switch.ops_per_sec -30.5% regression To: Linus Torvalds Cc: Jann Horn , LKML , lkp@lists.01.org References: <20200326055723.GL11705@shao2-debian> From: Rong Chen Message-ID: Date: Fri, 27 Mar 2020 16:46:22 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 3/27/20 12:57 AM, Linus Torvalds wrote: > On Wed, Mar 25, 2020 at 10:57 PM kernel test robot > wrote: >> FYI, we noticed a -30.5% regression of stress-ng.switch.ops_per_sec due to commit: >> >> commit: fd4d9c7d0c71866ec0c2825189ebd2ce35bd95b8 ("mm: slub: add missing TID bump in kmem_cache_alloc_bulk()") > This looks odd. > > I would not expect the update of c->tid to have that noticeable an > impact, even on a big machine that might be close to some scaling > limit. > > It doesn't add any expensive atomic ops, and while it _could_ make a > percpu cacheline dirty, I think that cacheline should already be dirty > anyway under any load where this is noticeable. Plus this should be a > relatively cold path anyway. > > So mind humoring me, and double-check that regression? > > Of course, it might be another "just magic cache placement" detail > where code moved enough to make a difference. > > Or maybe it really ends up causing new tid mismatches and we end up > failing the fast path in slub as a result. But looking at the stats > that changed in your message doesn't make me go "yeah, that looks like > a slub difference". > > So before we look more at this, I'd like to make sure that the > regression is actually real, and not noise. > > Please? > > Linus Hi Linus, We rebuilt the kernels and tested more times, but the data is constant, we are still checking this case. Best Regards, Rong Chen