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=-3.9 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 55895C4727C for ; Tue, 22 Sep 2020 07:27:47 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id DDC4E21548 for ; Tue, 22 Sep 2020 07:27:46 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=suse.com header.i=@suse.com header.b="SJu6HGOM" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org DDC4E21548 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=suse.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 46DC190002D; Tue, 22 Sep 2020 03:27:46 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 3F827900012; Tue, 22 Sep 2020 03:27:46 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 2BF0F90002D; Tue, 22 Sep 2020 03:27:46 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0085.hostedemail.com [216.40.44.85]) by kanga.kvack.org (Postfix) with ESMTP id 1051E900012 for ; Tue, 22 Sep 2020 03:27:46 -0400 (EDT) Received: from smtpin06.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay04.hostedemail.com (Postfix) with ESMTP id BC39A1EE6 for ; Tue, 22 Sep 2020 07:27:45 +0000 (UTC) X-FDA: 77289867690.06.birds36_3a175882714b Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin06.hostedemail.com (Postfix) with ESMTP id 9680B100C0CC1 for ; Tue, 22 Sep 2020 07:27:45 +0000 (UTC) X-HE-Tag: birds36_3a175882714b X-Filterd-Recvd-Size: 6830 Received: from mx2.suse.de (mx2.suse.de [195.135.220.15]) by imf26.hostedemail.com (Postfix) with ESMTP for ; Tue, 22 Sep 2020 07:27:44 +0000 (UTC) X-Virus-Scanned: by amavisd-new at test-mx.suse.de DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1600759663; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=Erw2OosUhNlUjJ/92tw/QfOb1ODXL2vdqOviv1OTGQE=; b=SJu6HGOM7mmekNJl2WAqQ2Z0OqgE18sdv4bZEjSiRqgybbfl14iLBvSa882Sg+JSj+TZhl UMLU1ibKgV/UqhrqCSyTiFlZ8zGyr3CeNw9z6X6gESLPqu69C9fKTiSuSmG4ObY2kuJbYo T6QzgAjoDLcehxHPNXIOeQcatmJPSdI= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 50C8AAA35; Tue, 22 Sep 2020 07:28:20 +0000 (UTC) Date: Tue, 22 Sep 2020 09:27:33 +0200 From: Michal Hocko To: Yafang Shao Cc: zangchunxin@bytedance.com, Johannes Weiner , Vladimir Davydov , Andrew Morton , Tejun Heo , lizefan@huawei.com, Jonathan Corbet , Alexei Starovoitov , Daniel Borkmann , kafai@fb.com, Song Liu , Yonghong Song , andriin@fb.com, john.fastabend@gmail.com, kpsingh@chromium.org, Cgroups , linux-doc@vger.kernel.org, Linux MM , LKML , netdev , bpf@vger.kernel.org Subject: Re: [PATCH] mm/memcontrol: Add the drop_cache interface for cgroup v2 Message-ID: <20200922072733.GT12990@dhcp22.suse.cz> References: <20200921080255.15505-1-zangchunxin@bytedance.com> <20200921081200.GE12990@dhcp22.suse.cz> <20200921110505.GH12990@dhcp22.suse.cz> <20200921113646.GJ12990@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Tue 22-09-20 12:20:52, Yafang Shao wrote: > On Mon, Sep 21, 2020 at 7:36 PM Michal Hocko wrote: > > > > On Mon 21-09-20 19:23:01, Yafang Shao wrote: > > > On Mon, Sep 21, 2020 at 7:05 PM Michal Hocko wrote: > > > > > > > > On Mon 21-09-20 18:55:40, Yafang Shao wrote: > > > > > On Mon, Sep 21, 2020 at 4:12 PM Michal Hocko wrote: > > > > > > > > > > > > On Mon 21-09-20 16:02:55, zangchunxin@bytedance.com wrote: > > > > > > > From: Chunxin Zang > > > > > > > > > > > > > > In the cgroup v1, we have 'force_mepty' interface. This is very > > > > > > > useful for userspace to actively release memory. But the cgroup > > > > > > > v2 does not. > > > > > > > > > > > > > > This patch reuse cgroup v1's function, but have a new name for > > > > > > > the interface. Because I think 'drop_cache' may be is easier to > > > > > > > understand :) > > > > > > > > > > > > This should really explain a usecase. Global drop_caches is a terrible > > > > > > interface and it has caused many problems in the past. People have > > > > > > learned to use it as a remedy to any problem they might see and cause > > > > > > other problems without realizing that. This is the reason why we even > > > > > > log each attempt to drop caches. > > > > > > > > > > > > I would rather not repeat the same mistake on the memcg level unless > > > > > > there is a very strong reason for it. > > > > > > > > > > > > > > > > I think we'd better add these comments above the function > > > > > mem_cgroup_force_empty() to explain why we don't want to expose this > > > > > interface in cgroup2, otherwise people will continue to send this > > > > > proposal without any strong reason. > > > > > > > > I do not mind people sending this proposal. "V1 used to have an > > > > interface, we need it in v2 as well" is not really viable without > > > > providing more reasoning on the specific usecase. > > > > > > > > _Any_ patch should have a proper justification. This is nothing really > > > > new to the process and I am wondering why this is coming as a surprise. > > > > > > > > > > Container users always want to drop cache in a specific container, > > > because they used to use drop_caches to fix memory pressure issues. > > > > This is exactly the kind of problems we have seen in the past. There > > should be zero reason to addre potential reclaim problems by dropping > > page cache on the floor. There is a huge cargo cult about this > > procedure and I have seen numerous reports when people complained about > > performance afterwards just to learn that the dropped page cache was one > > of the resons for that. > > > > > Although drop_caches can cause some unexpected issues, it could also > > > fix some issues. > > > > "Some issues" is way too general. We really want to learn about those > > issues and address them properly. > > > > One use case in our production environment is that some of our tasks > become very latency sensitive from 7am to 10am, so before these tasks > become active we will use drop_caches to drop page caches generated by > other tasks at night to avoid these tasks triggering direct reclaim. > > The best way to do it is to fix the latency in direct reclaim, but it > will take great effort. What is the latency triggered by the memory reclaim? It should be mostly a clean page cache right as drop_caches only drops clean pages. Or is this more about [id]cache? Do you have any profiles where is the time spent? > while drop_caches give us an easier way to achieve the same goal. It papers over real problems and that is my earlier comment about. > IOW, drop_caches give the users an option to achieve their goal before > they find a better solution. You can achieve the same by a different configuration already. You can isolate your page cache hungry overnight (likely a backup) workload into its own memcg. You can either use an aggressive high limit during the run or simply reduce the high/max limit after the work is done. If you cannot isolate that easily then you can lower high limit temporarily before your peak workload. Really throwing all the page cache away just to prepare for a peak workload sounds like a bad idea to me. You are potentially throwing data that you have to spend time to refault again. -- Michal Hocko SUSE Labs