From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752450AbdIETQp (ORCPT ); Tue, 5 Sep 2017 15:16:45 -0400 Received: from mx0a-00082601.pphosted.com ([67.231.145.42]:60496 "EHLO mx0a-00082601.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752064AbdIETQl (ORCPT ); Tue, 5 Sep 2017 15:16:41 -0400 Date: Tue, 5 Sep 2017 20:16:09 +0100 From: Roman Gushchin To: Michal Hocko CC: , Vladimir Davydov , Johannes Weiner , Tetsuo Handa , David Rientjes , Andrew Morton , Tejun Heo , , , , Subject: Re: [v7 5/5] mm, oom: cgroup v2 mount option to disable cgroup-aware OOM killer Message-ID: <20170905191609.GA19687@castle.dhcp.TheFacebook.com> References: <20170904142108.7165-1-guro@fb.com> <20170904142108.7165-6-guro@fb.com> <20170905134412.qdvqcfhvbdzmarna@dhcp22.suse.cz> <20170905143021.GA28599@castle.dhcp.TheFacebook.com> <20170905151251.luh4wogjd3msfqgf@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; 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IMHO the original oom logic > > > should be preserved by default and specific workloads should opt in for > > > the cgroup aware logic. Changing the global behavior depending on > > > whether cgroup v2 interface is in use is more than unexpected and IMHO > > > wrong approach to take. I think we should instead go with > > > oom_strategy=[alloc_task,biggest_task,cgroup] > > > > > > we currently have alloc_task (via sysctl_oom_kill_allocating_task) and > > > biggest_task which is the default. You are adding cgroup and the more I > > > think about the more I agree that it doesn't really make sense to try to > > > fit thew new semantic into the existing one (compare tasks to kill-all > > > memcgs). Just introduce a new strategy and define a new semantic from > > > scratch. Memcg priority and kill-all are a natural extension of this new > > > strategy. This will make the life easier and easier to understand by > > > users. > > > > > > Does that make sense to you? > > > > Absolutely. > > > > The only thing: I'm not sure that we have to preserve the existing logic > > as default option. For most users (except few very specific usecases), > > it should be at least as good, as the existing one. > > But this is really an unexpected change. Users even might not know that > they are using cgroup v2 and memcg is in use. > > > Making it opt-in means that corresponding code will be executed only > > by few users, who cares. > > Yeah, which is the way we should introduce new features no? > > > Then we should probably hide corresponding > > cgroup interface (oom_group and oom_priority knobs) by default, > > and it feels as unnecessary complication and is overall against > > cgroup v2 interface design. > > Why. If we care enough, we could simply return EINVAL when those knobs > are written while the corresponding strategy is not used. It doesn't look as a nice default interface. > > > > I think we should instead go with > > > oom_strategy=[alloc_task,biggest_task,cgroup] > > > > It would be a really nice interface; although I've no idea how to implement it: > > "alloc_task" is an existing sysctl, which we have to preserve; > > I would argue that we should simply deprecate and later drop the sysctl. > I _strongly_ suspect anybody is using this. If yes it is not that hard > to change the kernel command like rather than select the sysctl. I agree. And if so, why do we need a new interface for an useless feature? > > > while "cgroup" depends on cgroup v2. > > Which is not a big deal either. Simply fall back to default if there are > no cgroup v2. The implementation would have essentially the same effect > because there won't be any kill-all cgroups and so we will select the > largest task. I'd agree with you, if there are use cases (excluding pure legacy), when the per-process algorithm is preferable over the cgroup-aware OOM. I really doubt, and hope, that with oom_priorities the suggested algorithm should cover almost all reasonable use cases. Thanks!