From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752258AbdIFIcK (ORCPT ); Wed, 6 Sep 2017 04:32:10 -0400 Received: from mx2.suse.de ([195.135.220.15]:51616 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1752122AbdIFIcD (ORCPT ); Wed, 6 Sep 2017 04:32:03 -0400 Date: Wed, 6 Sep 2017 10:31:58 +0200 From: Michal Hocko To: Roman Gushchin Cc: linux-mm@kvack.org, Vladimir Davydov , Johannes Weiner , Tetsuo Handa , David Rientjes , Andrew Morton , Tejun Heo , kernel-team@fb.com, cgroups@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [v7 2/5] mm, oom: cgroup-aware OOM killer Message-ID: <20170906083158.gvqx6pekrsy2ya47@dhcp22.suse.cz> References: <20170904142108.7165-1-guro@fb.com> <20170904142108.7165-3-guro@fb.com> <20170905145700.fd7jjd37xf4tb55h@dhcp22.suse.cz> <20170905202357.GA10535@castle.DHCP.thefacebook.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170905202357.GA10535@castle.DHCP.thefacebook.com> User-Agent: NeoMutt/20170609 (1.8.3) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue 05-09-17 21:23:57, Roman Gushchin wrote: > On Tue, Sep 05, 2017 at 04:57:00PM +0200, Michal Hocko wrote: [...] > > Hmm. The changelog says "By default, it will look for the biggest leaf > > cgroup, and kill the largest task inside." But you are accumulating > > oom_score up the hierarchy and so parents will have higher score than > > the layer of their children and the larger the sub-hierarchy the more > > biased it will become. Say you have > > root > > /\ > > / \ > > A D > > / \ > > B C > > > > B (5), C(15) thus A(20) and D(20). Unless I am missing something we are > > going to go down A path and then chose C even though D is the largest > > leaf group, right? > > You're right, changelog is not accurate, I'll fix it. > The behavior is correct, IMO. Please explain why. This is really a non-intuitive semantic. Why should larger hierarchies be punished more than shallow ones? I would completely agree if the whole hierarchy would be a killable entity (aka A would be kill-all). [...] > > I do not understand why do we have to handle root cgroup specially here. > > select_victim_memcg already iterates all memcgs in the oom hierarchy > > (including root) so if the root memcg is the largest one then we > > should simply consider it no? > > We don't have necessary stats for the root cgroup, so we can't calculate > it's oom_score. We used to charge pages to the root memcg as well so we might resurrect that idea. In any case this is something that could be hidden in memcg_oom_badness rather then special cased somewhere else. > > You are skipping root there because of > > memcg_has_children but I suspect this and the whole accumulate up the > > hierarchy approach just makes the whole thing more complex than necessary. With > > "tasks only in leafs" cgroup policy we should only see any pages on LRUs > > on the global root memcg and leaf cgroups. The same applies to memcg > > stats. So why cannot we simply do the tree walk, calculate > > badness/check the priority and select the largest memcg in one go? > > We have to traverse from top to bottom to make priority-based decision, > but size-based oom_score is calculated as sum of descending leaf cgroup scores. > > For example: > root > /\ > / \ > A D > / \ > B C > A and D have same priorities, B has larger priority than C. > > In this case we need to calculate size-based score for A, which requires > summing oom_score of the sub-tree (B an C), despite we don't need it > for choosing between B and C. > > Maybe I don't see it, but I don't know how to implement it more optimal. I have to think about the priority based oom killing some more to be honest. Do we really want to allow setting a priority to non-leaf memcgs? How are you going to manage the whole tree consistency? Say your above example have prio(A) < prio(D) && prio(C) > prio(D). Your current implementation would kill D, right? Isn't that counter intuitive behavior again. If anything we should prio(A) = max(tree_prio(A)). Again I could understand comparing priorities only on killable entities. -- Michal Hocko SUSE Labs