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 44B29C43465 for ; Mon, 21 Sep 2020 11:36:51 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id BB6A420BED for ; Mon, 21 Sep 2020 11:36:50 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=suse.com header.i=@suse.com header.b="mLlvyqS1" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org BB6A420BED 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 0EBF4900051; Mon, 21 Sep 2020 07:36:50 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 09CF8900046; Mon, 21 Sep 2020 07:36:50 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id ECD62900051; Mon, 21 Sep 2020 07:36:49 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0068.hostedemail.com [216.40.44.68]) by kanga.kvack.org (Postfix) with ESMTP id D123D900046 for ; Mon, 21 Sep 2020 07:36:49 -0400 (EDT) Received: from smtpin03.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay04.hostedemail.com (Postfix) with ESMTP id 9351B1EE6 for ; Mon, 21 Sep 2020 11:36:49 +0000 (UTC) X-FDA: 77286866538.03.wound35_1812a2127144 Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin03.hostedemail.com (Postfix) with ESMTP id 73E6228A4E8 for ; Mon, 21 Sep 2020 11:36:49 +0000 (UTC) X-HE-Tag: wound35_1812a2127144 X-Filterd-Recvd-Size: 5948 Received: from mx2.suse.de (mx2.suse.de [195.135.220.15]) by imf29.hostedemail.com (Postfix) with ESMTP for ; Mon, 21 Sep 2020 11:36:48 +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=1600688207; 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=gv9Fy+RbnmwfMLme8gdEClJLpssZdj3OhXBnDTz3OTA=; b=mLlvyqS1BiqU/o7NuvXRKRHsOo9lchqjppDk38OO43W1KxzKSEb47qXBt8k243SgdqID49 KDNluBwTXxKX1L3dLcRJNj/TkgIoy4Z233u+MeSgAz35Lss7mWjyKxzgri2kvySqQd+14H wiExpQN1VnsKXAbBRmCeylFmKQZEV24= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id B70F9ACB7; Mon, 21 Sep 2020 11:37:23 +0000 (UTC) Date: Mon, 21 Sep 2020 13:36:46 +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: <20200921113646.GJ12990@dhcp22.suse.cz> References: <20200921080255.15505-1-zangchunxin@bytedance.com> <20200921081200.GE12990@dhcp22.suse.cz> <20200921110505.GH12990@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 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. > So container users want to use it in containers as > well. > If this feature is not implemented in cgroup, they will ask you why > but there is no explanation in the kernel. There is no usecase that would really require it so far. > Regarding the memory.high, it is not perfect as well, because you have > to set it to 0 to drop_caches, and the processes in the containers > have to reclaim pages as well because they reach the memory.high, but > memory.force_empty won't make other processes to reclaim. Since 536d3bf261a2 ("mm: memcontrol: avoid workload stalls when lowering memory.high") the limit is set after the reclaim so the race window when somebody would be pushed to high limit reclaim is reduced. But I do agree this is just a workaround. > That doesn't mean I agree to add this interface, while I really mean > that if we discard one feature we'd better explain why. We need to understand why somebody wants an interface because once it is added it will have to be maintained for ever. -- Michal Hocko SUSE Labs