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Date: Tue, 31 Oct 2017 14:44:11 -0400 From: Johannes Weiner <hannes@...xchg.org> To: Shakeel Butt <shakeelb@...gle.com> Cc: Roman Gushchin <guro@...com>, Linux MM <linux-mm@...ck.org>, Vladimir Davydov <vdavydov.dev@...il.com>, Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>, David Rientjes <rientjes@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>, Tejun Heo <tj@...nel.org>, kernel-team@...com, Cgroups <cgroups@...r.kernel.org>, linux-doc@...r.kernel.org, LKML <linux-kernel@...r.kernel.org> Subject: Re: [RESEND v12 3/6] mm, oom: cgroup-aware OOM killer On Tue, Oct 31, 2017 at 10:50:43AM -0700, Shakeel Butt wrote: > On Tue, Oct 31, 2017 at 9:40 AM, Johannes Weiner <hannes@...xchg.org> wrote: > > On Tue, Oct 31, 2017 at 08:04:19AM -0700, Shakeel Butt wrote: > >> > + > >> > +static void select_victim_memcg(struct mem_cgroup *root, struct oom_control *oc) > >> > +{ > >> > + struct mem_cgroup *iter; > >> > + > >> > + oc->chosen_memcg = NULL; > >> > + oc->chosen_points = 0; > >> > + > >> > + /* > >> > + * The oom_score is calculated for leaf memory cgroups (including > >> > + * the root memcg). > >> > + */ > >> > + rcu_read_lock(); > >> > + for_each_mem_cgroup_tree(iter, root) { > >> > + long score; > >> > + > >> > + if (memcg_has_children(iter) && iter != root_mem_cgroup) > >> > + continue; > >> > + > >> > >> Cgroup v2 does not support charge migration between memcgs. So, there > >> can be intermediate nodes which may contain the major charge of the > >> processes in their leave descendents. Skipping such intermediate nodes > >> will kind of protect such processes from oom-killer (lower on the list > >> to be killed). Is it ok to not handle such scenario? If yes, shouldn't > >> we document it? > > > > Tasks cannot be in intermediate nodes, so the only way you can end up > > in a situation like this is to start tasks fully, let them fault in > > their full workingset, then create child groups and move them there. > > > > That has attribution problems much wider than the OOM killer: any > > local limits you would set on a leaf cgroup like this ALSO won't > > control the memory of its tasks - as it's all sitting in the parent. > > > > We created the "no internal competition" rule exactly to prevent this > > situation. > > Rather than the "no internal competition" restriction I think "charge > migration" would have resolved that situation? Also "no internal > competition" restriction (I am assuming 'no internal competition' is > no tasks in internal nodes, please correct me if I am wrong) has made > "charge migration" hard to implement and thus not added in cgroup v2. > > I know this is parallel discussion and excuse my ignorance, what are > other reasons behind "no internal competition" specifically for memory > controller? Sorry, but this is completely off-topic. The rationale for this decisions is in Documentation/cgroup-v2.txt.
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