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=-12.9 required=3.0 tests=BAYES_00,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,USER_IN_DEF_DKIM_WL 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 7555CC4346E for ; Mon, 21 Sep 2020 19:07:27 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 27CC62054F for ; Mon, 21 Sep 2020 19:07:27 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=linux.microsoft.com header.i=@linux.microsoft.com header.b="NLm6cj99" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728014AbgIUTH0 (ORCPT ); Mon, 21 Sep 2020 15:07:26 -0400 Received: from linux.microsoft.com ([13.77.154.182]:52410 "EHLO linux.microsoft.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727727AbgIUTHZ (ORCPT ); Mon, 21 Sep 2020 15:07:25 -0400 Received: from [192.168.0.121] (unknown [209.134.121.133]) by linux.microsoft.com (Postfix) with ESMTPSA id 056FB20B7179; Mon, 21 Sep 2020 12:07:23 -0700 (PDT) DKIM-Filter: OpenDKIM Filter v2.11.0 linux.microsoft.com 056FB20B7179 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.microsoft.com; s=default; t=1600715244; bh=xFQyWZ09JYTz1ea8EvmYsjd9gkpv8PinmSTBjsmoFsU=; h=Subject:To:Cc:References:From:Date:In-Reply-To:From; b=NLm6cj99WAXyA5dNxCeKJSK4CUlX6HEGfBxod83mGICxcHqbylk7abpAxQ60kb50/ FsA1RqKicfvg4Hheun7VAdiNx7TJzVhErLZfkYH9R0xcnqRZvIitmB7gyBD8+iHl30 DiKWpyG2STH2fEzdl+tXhJCdWSchHv1H34DRO84c= Subject: Re: [v4] mm: khugepaged: avoid overriding min_free_kbytes set by user To: Michal Hocko Cc: Andrew Morton , "Kirill A. Shutemov" , Oleg Nesterov , Song Liu , Andrea Arcangeli , Pavel Tatashin , Allen Pais , linux-kernel@vger.kernel.org, linux-mm@kvack.org References: <1600324779-6782-1-git-send-email-vijayb@linux.microsoft.com> <20200917092805.GA29887@dhcp22.suse.cz> <21c4788c-2516-2e47-5885-486c352c2b6e@linux.microsoft.com> <20200917175217.GD29887@dhcp22.suse.cz> <20200918055637.GB28827@dhcp22.suse.cz> From: Vijay Balakrishna Message-ID: Date: Mon, 21 Sep 2020 12:07:23 -0700 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.2.2 MIME-Version: 1.0 In-Reply-To: <20200918055637.GB28827@dhcp22.suse.cz> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-GB Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 9/17/2020 10:56 PM, Michal Hocko wrote: > On Thu 17-09-20 11:16:55, Vijay Balakrishna wrote: >> >> >> On 9/17/2020 10:52 AM, Michal Hocko wrote: >>> On Thu 17-09-20 10:27:16, Vijay Balakrishna wrote: >>>> >>>> >>>> On 9/17/2020 2:28 AM, Michal Hocko wrote: >>>>> On Wed 16-09-20 23:39:39, Vijay Balakrishna wrote: >>>>>> set_recommended_min_free_kbytes need to honor min_free_kbytes set by the >>>>>> user. Post start-of-day THP enable or memory hotplug operations can >>>>>> lose user specified min_free_kbytes, in particular when it is higher than >>>>>> calculated recommended value. >>>>> >>>>> I was about to recommend a more detailed explanation when I have >>>>> realized that this patch is not really needed after all. Unless I am >>>>> missing something. >>>>> >>>>> init_per_zone_wmark_min ignores the newly calculated min_free_kbytes if >>>>> it is lower than user_min_free_kbytes. So calculated min_free_kbytes >= >>>>> user_min_free_kbytes. >>>>> >>>>> Except for value clamping when the value is reduced and this likely >>>>> needs fixing. But set_recommended_min_free_kbytes should be fine. >>>>> >>>> >>>> IIUC, after start-of-day if a user performs >>>> - THP disable >>>> - modifies min_free_bytes >>>> - THP enable >>>> above sequence currently wouldn't result in calling init_per_zone_wmark_min. >>> >>> I will not, but why do you think this matters? All we should care about >>> is that auto-tuning shouldn't reduce user provided value [1] and that >>> the memory hotplug should be consistent with the boot time heuristic. >>> init_per_zone_wmark_min should make sure that the user value is not >>> reduced and thp heuristic makes sure it will not reduce this value. >>> So the property should be transitive with the existing code (modulo the >>> problem I have highlighted). >>> >>> [1] one could argue that it shouldn't even increase the value strictly >>> speaking because an admin might have a very good reason to decrease the >>> value but this has never been the semantic and changing it now might be >>> problematic >>> >> >> I made an attempt to address Kirill A. Shutemov's comment. > > This is for Kirill to comment on but my take would be that memory > hotplug really has to alter the user defined min_free_kbytes because it > is manipulating the amount of memory. There are usecases which are > adding a lot of memory. > > We are trying to not decrease the value which is arguably a weird semantic > but this is what've been doing for years. We would need to hear a > specific usecase where this matters (e.g. memory hotremove heavy > workalod with manually tuned min_free_kbytes) that misbehaves. In our use case memory hotremove done normally during shutdown and we aren't manually tuning min_free_kbytes. > >> And incrased >> min_free_kbytes to see the issue in my testing and attempted a fix. I'm ok >> leaving as it is. Do not want introduce any changes that may cause >> regression. > > I would recommend reposting the patch which adds heuristic for THP (if > THP is enabled) into the hotplug path, arguing with the consistency and > surprising results when adding memory decreases the value. I hope my reposted patch ([v3 1/2] mm: khugepaged: recalculate min_free_kbytes after memory hotplug as expected by khugepaged) change log is ok: When memory is hotplug added or removed the min_free_kbytes must be recalculated based on what is expected by khugepaged. Currently after hotplug, min_free_kbytes will be set to a lower default and higher default set when THP enabled is lost. This change restores min_free_kbytes as expected for THP consumers. > Your initial > problem is in sizing as mentioned in other email thread and you should > be investigating more but this inconsistency might really come as a > surprise. > > All that if Kirill is reconsidering his initial position of course. > Kirill, can you comment or share your opinion? Thanks, Vijay