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=-5.4 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,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 A95C1C433E7 for ; Thu, 15 Oct 2020 03:38:12 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 513C820691 for ; Thu, 15 Oct 2020 03:38:12 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=oracle.com header.i=@oracle.com header.b="Pao98oSA" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2387867AbgJODiL (ORCPT ); Wed, 14 Oct 2020 23:38:11 -0400 Received: from userp2120.oracle.com ([156.151.31.85]:57902 "EHLO userp2120.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2387414AbgJODiL (ORCPT ); Wed, 14 Oct 2020 23:38:11 -0400 Received: from pps.filterd (userp2120.oracle.com [127.0.0.1]) by userp2120.oracle.com (8.16.0.42/8.16.0.42) with SMTP id 09F3V1VA164274; Thu, 15 Oct 2020 03:37:52 GMT DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=oracle.com; h=subject : to : cc : references : from : message-id : date : mime-version : in-reply-to : content-type : content-transfer-encoding; s=corp-2020-01-29; bh=TdkRaI1K2emwA4ULWa95up2TNJiDjREeVjcHKnTnlc8=; b=Pao98oSADgnAZP4FqoZcmL520aM6F8URtxqADMzzibOeJu+0eMdV3tlmakSkxiSUJW8a gVgTQyf5XgUqxCnE3afABhc8rdXc/wdAb4efSpga6qsrIMi/iQacxA18I3nUhJJVs54l vJosugKlU/sgJpC3flxW7Z9CWUBfesGrbKztvMsret6XYq/Z1vRz2me1yxdDFogWVKak W9DzHtSElGx6ZbwjwBtFlr0kDV3HZsv6d++0eLCX2psVaUxd3Sltrmud7+3G1eKy2LnD tsLRzTBtPO/hMjSBL9oPStfCpo2EeFgGjU8/RNlS5mkfDOLLyCG0Rhb3H9sfloufUgWo 1w== Received: from aserp3030.oracle.com (aserp3030.oracle.com [141.146.126.71]) by userp2120.oracle.com with ESMTP id 343vaegxm3-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=FAIL); Thu, 15 Oct 2020 03:37:52 +0000 Received: from pps.filterd (aserp3030.oracle.com [127.0.0.1]) by aserp3030.oracle.com (8.16.0.42/8.16.0.42) with SMTP id 09F3TwkV122677; Thu, 15 Oct 2020 03:37:51 GMT Received: from aserv0122.oracle.com (aserv0122.oracle.com [141.146.126.236]) by aserp3030.oracle.com with ESMTP id 343phqfj42-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Thu, 15 Oct 2020 03:37:51 +0000 Received: from abhmp0016.oracle.com (abhmp0016.oracle.com [141.146.116.22]) by aserv0122.oracle.com (8.14.4/8.14.4) with ESMTP id 09F3bmcK019734; Thu, 15 Oct 2020 03:37:48 GMT Received: from [192.168.0.108] (/70.36.60.91) by default (Oracle Beehive Gateway v4.0) with ESMTP ; Wed, 14 Oct 2020 20:37:48 -0700 Subject: Re: [PATCH 5/8] x86/clear_page: add clear_page_uncached() To: Borislav Petkov , Andy Lutomirski Cc: Andy Lutomirski , LKML , Linux-MM , "Kirill A. Shutemov" , Michal Hocko , Boris Ostrovsky , Konrad Rzeszutek Wilk , Thomas Gleixner , Ingo Molnar , X86 ML , "H. Peter Anvin" , Arnd Bergmann , Andrew Morton , Ira Weiny , linux-arch References: <20201014195823.GC18196@zn.tnic> <22E29783-F1F5-43DA-B35F-D75FB247475D@amacapital.net> <20201014211214.GD18196@zn.tnic> From: Ankur Arora Message-ID: <3de58840-1f4c-566b-3a66-46d57475820c@oracle.com> Date: Wed, 14 Oct 2020 20:37:44 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.11.0 MIME-Version: 1.0 In-Reply-To: <20201014211214.GD18196@zn.tnic> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit X-Proofpoint-Virus-Version: vendor=nai engine=6000 definitions=9774 signatures=668682 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 phishscore=0 spamscore=0 adultscore=0 bulkscore=0 mlxlogscore=999 malwarescore=0 mlxscore=0 suspectscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2009150000 definitions=main-2010150025 X-Proofpoint-Virus-Version: vendor=nai engine=6000 definitions=9774 signatures=668682 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 mlxlogscore=999 clxscore=1015 impostorscore=0 phishscore=0 malwarescore=0 bulkscore=0 priorityscore=1501 mlxscore=0 suspectscore=0 spamscore=0 adultscore=0 lowpriorityscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2009150000 definitions=main-2010150025 Precedence: bulk List-ID: X-Mailing-List: linux-arch@vger.kernel.org On 2020-10-14 2:12 p.m., Borislav Petkov wrote: > On Wed, Oct 14, 2020 at 02:07:30PM -0700, Andy Lutomirski wrote: >> I assume it’s for a little optimization of clearing more than one >> page per SFENCE. >> >> In any event, based on the benchmark data upthread, we only want to do >> NT clears when they’re rather large, so this shouldn’t be just an >> alternative. I assume this is because a page or two will fit in cache >> and, for most uses that allocate zeroed pages, we prefer cache-hot >> pages. When clearing 1G, on the other hand, cache-hot is impossible >> and we prefer the improved bandwidth and less cache trashing of NT >> clears. > > Yeah, use case makes sense but people won't know what to use. At the > time I was experimenting with this crap, I remember Linus saying that > that selection should be made based on the size of the area cleared, so > users should not have to know the difference. I don't disagree but I think the selection of cached/uncached route should be made where we have enough context available to be able to choose to do this. This could be for example, done in mm_populate() or gup where if say the extent is larger than LLC-size, it takes the uncached path. > > Which perhaps is the only sane use case I see for this. > >> Perhaps SFENCE is so fast that this is a silly optimization, though, >> and we don’t lose anything measurable by SFENCEing once per page. > > Yes, I'd like to see real use cases showing improvement from this, not > just microbenchmarks. Sure will add. Thanks Ankur >