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=-4.0 required=3.0 tests=INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 BAEA0C54FD0 for ; Fri, 24 Apr 2020 16:21:07 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 7685320700 for ; Fri, 24 Apr 2020 16:21:07 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 7685320700 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 0885F8E0008; Fri, 24 Apr 2020 12:21:07 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 0388C8E0003; Fri, 24 Apr 2020 12:21:06 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id E698A8E0008; Fri, 24 Apr 2020 12:21:06 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0126.hostedemail.com [216.40.44.126]) by kanga.kvack.org (Postfix) with ESMTP id CC5408E0003 for ; Fri, 24 Apr 2020 12:21:06 -0400 (EDT) Received: from smtpin12.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay02.hostedemail.com (Postfix) with ESMTP id 915464995ED for ; Fri, 24 Apr 2020 16:21:06 +0000 (UTC) X-FDA: 76743262932.12.wind32_26bfeadd41719 X-HE-Tag: wind32_26bfeadd41719 X-Filterd-Recvd-Size: 8184 Received: from mail-wr1-f66.google.com (mail-wr1-f66.google.com [209.85.221.66]) by imf44.hostedemail.com (Postfix) with ESMTP for ; Fri, 24 Apr 2020 16:21:06 +0000 (UTC) Received: by mail-wr1-f66.google.com with SMTP id i10so11571000wrv.10 for ; Fri, 24 Apr 2020 09:21:06 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=q0pJPiNahulzXjepW4Fsah9pOIeUNBqbsHnVMKibHTw=; b=IYoXlagMIuKY8ztpQr6MzYrjLdc4hFVuWDYFHmQ9uVK9+v3kR+2/5ZLBEbmwh3TPlH WxAb04ElknE8yR2ztIp73vXuQabWSbmRZiMo9b0yKPYQVoZ8s/7OhfREYLja3n6LCmX+ gT1oZUGS17e4oCHVny6/sAvd/hvzxqprFyQAozcubE+u3BHaOYUyJvMHkRoVoMsqSPBy Ya1INkDA6AFxaW6P4aCwOZQWKXI2/vVpT3JC1JMsUypzyHo1DeXXCMpDvLpEEbuekUq4 8WJR6NF6KVmtydf/WIX2BOkuxmkg4TRHzC/Li3xj6dp9md2+k5AXj3/qU70oCMZKzVkk w4tA== X-Gm-Message-State: AGi0PuZDsp31WL26+xaZBr55/Bgz3m+75ei0FmXqzY3ZP5Icdi4js2Gh cFviypQ8YmFlJS4XP5cPFtM= X-Google-Smtp-Source: APiQypJ27Vk63KNaOL2/qa5F5TtZbSWQUXj0MofRfX8qGY/2+/C3iSSnS4Z0g2l7Zh6htzuVY4e3cQ== X-Received: by 2002:adf:db4d:: with SMTP id f13mr11434693wrj.289.1587745265085; Fri, 24 Apr 2020 09:21:05 -0700 (PDT) Received: from localhost (ip-37-188-130-62.eurotel.cz. [37.188.130.62]) by smtp.gmail.com with ESMTPSA id z76sm4035330wmc.9.2020.04.24.09.21.03 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 24 Apr 2020 09:21:04 -0700 (PDT) Date: Fri, 24 Apr 2020 18:21:03 +0200 From: Michal Hocko To: Johannes Weiner Cc: Yafang Shao , akpm@linux-foundation.org, vdavydov.dev@gmail.com, linux-mm@kvack.org, Chris Down , Roman Gushchin , stable@vger.kernel.org Subject: Re: [PATCH] mm, memcg: fix wrong mem cgroup protection Message-ID: <20200424162103.GK11591@dhcp22.suse.cz> References: <20200423061629.24185-1-laoar.shao@gmail.com> <20200424131450.GA495720@cmpxchg.org> <20200424142958.GF11591@dhcp22.suse.cz> <20200424151013.GA525165@cmpxchg.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200424151013.GA525165@cmpxchg.org> 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 Fri 24-04-20 11:10:13, Johannes Weiner wrote: > On Fri, Apr 24, 2020 at 04:29:58PM +0200, Michal Hocko wrote: > > On Fri 24-04-20 09:14:50, Johannes Weiner wrote: > > > On Thu, Apr 23, 2020 at 02:16:29AM -0400, Yafang Shao wrote: > > > > This patch is an improvement of a previous version[1], as the previous > > > > version is not easy to understand. > > > > This issue persists in the newest kernel, I have to resend the fix. As > > > > the implementation is changed, I drop Roman's ack from the previous > > > > version. > > > > > > Now that I understand the problem, I much prefer the previous version. > > > > > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > > > index 745697906ce3..2bf91ae1e640 100644 > > > --- a/mm/memcontrol.c > > > +++ b/mm/memcontrol.c > > > @@ -6332,8 +6332,19 @@ enum mem_cgroup_protection mem_cgroup_protected(struct mem_cgroup *root, > > > > > > if (!root) > > > root = root_mem_cgroup; > > > - if (memcg == root) > > > + if (memcg == root) { > > > + /* > > > + * The cgroup is the reclaim root in this reclaim > > > + * cycle, and therefore not protected. But it may have > > > + * stale effective protection values from previous > > > + * cycles in which it was not the reclaim root - for > > > + * example, global reclaim followed by limit reclaim. > > > + * Reset these values for mem_cgroup_protection(). > > > + */ > > > + memcg->memory.emin = 0; > > > + memcg->memory.elow = 0; > > > return MEMCG_PROT_NONE; > > > + } > > > > Could you be more specific why you prefer this over the > > mem_cgroup_protection which doesn't change the effective value? > > Isn't it easier to simply ignore effective value for the reclaim roots? > > Because now both mem_cgroup_protection() and mem_cgroup_protected() > have to know about the reclaim root semantics, instead of just the one > central place. Yes this is true but it is also potentially overwriting the state with a parallel reclaim which can lead to surprising results beacause parent's effective protection is used to define protection distribution for children. Let's have global and A's reclaim in parallel: | A (low=2G, usage = 3G, max = 3G, children_low_usage = 1.5G) |\ | C (low = 1G, usage = 2.5G) B (low = 1G, usage = 0.5G) for A reclaim we have B.elow = B.low C.elow = C.low For the global reclaim A.elow = A.low B.elow = min(B.usage, B.low) because children_low_usage <= A.elow C.elow = min(C.usage, C.low) With the effective values reseting we have A reclaim A.elow = 0 B.elow = B.low C.elow = C.low [...] and global reclaim could see the above and then B.elow = C.elow = 0 because children_low_usage > A.elow > And the query function has to know additional rules about when the > emin/elow values are uptodate or it could silently be looking at stale > data, which isn't very robust. > > "The effective protection values are uptodate after calling > mem_cgroup_protected() inside the reclaim cycle - UNLESS the group > you're looking at happens to be..." > > It's much easier to make the rule: The values are uptodate after you > called mem_cgroup_protected(). > > Or mem_cgroup_calculate_protection(), if we go with that later. > > > > As others have noted, it's fairly hard to understand the problem from > > > the above changelog. How about the following: > > > > > > A cgroup can have both memory protection and a memory limit to isolate > > > it from its siblings in both directions - for example, to prevent it > > > from being shrunk below 2G under high pressure from outside, but also > > > from growing beyond 4G under low pressure. > > > > > > 9783aa9917f8 ("mm, memcg: proportional memory.{low,min} reclaim") > > > implemented proportional scan pressure so that multiple siblings in > > > excess of their protection settings don't get reclaimed equally but > > > instead in accordance to their unprotected portion. > > > > > > During limit reclaim, this proportionality shouldn't apply of course: > > > there is no competition, all pressure is from within the cgroup and > > > should be applied as such. Reclaim should operate at full efficiency. > > > > > > However, mem_cgroup_protected() never expected anybody to look at the > > > effective protection values when it indicated that the cgroup is above > > > its protection. As a result, a query during limit reclaim may return > > > stale protection values that were calculated by a previous reclaim > > > cycle in which the cgroup did have siblings. > > > > This is better. Thanks! > > > > > When this happens, reclaim is unnecessarily hesitant and potentially > > > slow to meet the desired limit. In theory this could lead to premature > > > OOM kills, although it's not obvious this has occurred in practice. > > > > I do not see how this would lead all the way to OOM killer but it > > certainly can lead to unnecessary increase of the reclaim priority. The > > smaller the difference between the reclaim target and protection the > > more visible the effect would be. But if there are reclaimable pages > > then the reclaim should see them sooner or later > > It would be a pretty extreme case, but not impossible AFAICS, because > OOM is just a sampled state, not deterministic. > > If memory.max is 64G and memory.low is 64G minus one page, this bug > could cause limit reclaim to look at no more than SWAP_CLUSTER_MAX > pages at priority 0. It's possible it wouldn't get through the full > 64G worth of memory before giving up and declaring OOM. Yes, my bad I didn't really realize that there won't be a full scan even under priority 0. -- Michal Hocko SUSE Labs