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,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 97E91C433DF for ; Thu, 15 Oct 2020 03:22:30 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 3CF6322241 for ; Thu, 15 Oct 2020 03:22:30 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=oracle.com header.i=@oracle.com header.b="sjySP3ga" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728598AbgJODW3 (ORCPT ); Wed, 14 Oct 2020 23:22:29 -0400 Received: from aserp2120.oracle.com ([141.146.126.78]:58832 "EHLO aserp2120.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725919AbgJODW3 (ORCPT ); Wed, 14 Oct 2020 23:22:29 -0400 Received: from pps.filterd (aserp2120.oracle.com [127.0.0.1]) by aserp2120.oracle.com (8.16.0.42/8.16.0.42) with SMTP id 09F3EnEJ087623; Thu, 15 Oct 2020 03:22:13 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=5jq4EdyxsgnsfxJk2c/wmbun8kFNHfpd+axuEGHJq9I=; b=sjySP3ga5v8ftnocYB2D+xwQ/joDb/mZcEZsbMpmRTEH3oHBkTmvEAhgOpAAQGXOZfSK 5+Dwnf3uymC/LfXnAFIHnnqMKulPh6iiTL9HwxPe/iibv5JrsajHiD3eMwTPKepPwDxz xOQyRWypFLWszDPSzOr9axbEjsq8Tu5MjDBYI+joS6K0Ya92KmS9C7fSaUf3yPy1izjT 7ZdLKsbck222qdK4WvKdQqtQ1MthtKtFgnAawVJXYSOfVJd9XlnTi9mIqjyFeLtJbwtH jsnv0E8EiAVH/gVPMudbBm+IlAdyyjgFWGZ7wr6j7K08myffQUjwJLePIuR+SET8nu1F jQ== Received: from userp3030.oracle.com (userp3030.oracle.com [156.151.31.80]) by aserp2120.oracle.com with ESMTP id 3434wktkcs-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=FAIL); Thu, 15 Oct 2020 03:22:13 +0000 Received: from pps.filterd (userp3030.oracle.com [127.0.0.1]) by userp3030.oracle.com (8.16.0.42/8.16.0.42) with SMTP id 09F3AbpF075099; Thu, 15 Oct 2020 03:22:12 GMT Received: from userv0121.oracle.com (userv0121.oracle.com [156.151.31.72]) by userp3030.oracle.com with ESMTP id 343pvyqw3j-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Thu, 15 Oct 2020 03:22:12 +0000 Received: from abhmp0009.oracle.com (abhmp0009.oracle.com [141.146.116.15]) by userv0121.oracle.com (8.14.4/8.13.8) with ESMTP id 09F3LxCC011235; Thu, 15 Oct 2020 03:22:00 GMT Received: from [192.168.0.108] (/70.36.60.91) by default (Oracle Beehive Gateway v4.0) with ESMTP ; Wed, 14 Oct 2020 20:21:59 -0700 Subject: Re: [PATCH 5/8] x86/clear_page: add clear_page_uncached() To: Andy Lutomirski , Borislav Petkov 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> From: Ankur Arora Message-ID: <50286c32-2869-cbd5-b178-0ad0c13584ea@oracle.com> Date: Wed, 14 Oct 2020 20:21:57 -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: <22E29783-F1F5-43DA-B35F-D75FB247475D@amacapital.net> 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 spamscore=0 mlxscore=0 adultscore=0 bulkscore=0 mlxlogscore=999 suspectscore=0 malwarescore=0 phishscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2009150000 definitions=main-2010150024 X-Proofpoint-Virus-Version: vendor=nai engine=6000 definitions=9774 signatures=668682 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 lowpriorityscore=0 mlxscore=0 malwarescore=0 phishscore=0 suspectscore=0 impostorscore=0 clxscore=1011 spamscore=0 priorityscore=1501 bulkscore=0 adultscore=0 mlxlogscore=999 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2009150000 definitions=main-2010150024 Precedence: bulk List-ID: X-Mailing-List: linux-arch@vger.kernel.org On 2020-10-14 2:07 p.m., Andy Lutomirski wrote: > > > >> On Oct 14, 2020, at 12:58 PM, Borislav Petkov wrote: >> >> On Wed, Oct 14, 2020 at 08:45:37AM -0700, Andy Lutomirski wrote: >>>> On Wed, Oct 14, 2020 at 1:33 AM Ankur Arora wrote: >>>> >>>> Define clear_page_uncached() as an alternative_call() to clear_page_nt() >>>> if the CPU sets X86_FEATURE_NT_GOOD and fallback to clear_page() if it >>>> doesn't. >>>> >>>> Similarly define clear_page_uncached_flush() which provides an SFENCE >>>> if the CPU sets X86_FEATURE_NT_GOOD. >>> >>> As long as you keep "NT" or "MOVNTI" in the names and keep functions >>> in arch/x86, I think it's reasonable to expect that callers understand >>> that MOVNTI has bizarre memory ordering rules. But once you give >>> something a generic name like "clear_page_uncached" and stick it in >>> generic code, I think the semantics should be more obvious. >> >> Why does it have to be a separate call? Why isn't it behind the >> clear_page() alternative machinery so that the proper function is >> selected at boot? IOW, why does a user of clear_page functionality need >> to know at all about an "uncached" variant? > > 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. Also, if we did extend clear_page() to take the page-size as parameter we still might not have enough information (ex. a 4K or a 2MB page that clear_page() sees could be part of a GUP of a much larger extent) to decide whether to go uncached or not. > Perhaps SFENCE is so fast that this is a silly optimization, though, > and we don’t lose anything measurable by SFENCEing once per page. Alas, no. I tried that and dropped about 15% performance on Rome. Thanks Ankur