From: Michal Hocko <mhocko@kernel.org>
To: Roman Gushchin <guro@fb.com>
Cc: linux-mm@kvack.org, Vladimir Davydov <vdavydov.dev@gmail.com>,
Johannes Weiner <hannes@cmpxchg.org>,
Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
David Rientjes <rientjes@google.com>,
Andrew Morton <akpm@linux-foundation.org>,
Tejun Heo <tj@kernel.org>,
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
Date: Wed, 6 Sep 2017 10:31:58 +0200 [thread overview]
Message-ID: <20170906083158.gvqx6pekrsy2ya47@dhcp22.suse.cz> (raw)
In-Reply-To: <20170905202357.GA10535@castle.DHCP.thefacebook.com>
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
next prev parent reply other threads:[~2017-09-06 8:32 UTC|newest]
Thread overview: 44+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-09-04 14:21 [v7 0/5] cgroup-aware OOM killer Roman Gushchin
2017-09-04 14:21 ` [v7 1/5] mm, oom: refactor the oom_kill_process() function Roman Gushchin
2017-09-05 13:34 ` Michal Hocko
2017-09-04 14:21 ` [v7 2/5] mm, oom: cgroup-aware OOM killer Roman Gushchin
2017-09-05 14:57 ` Michal Hocko
2017-09-05 20:23 ` Roman Gushchin
2017-09-06 8:31 ` Michal Hocko [this message]
2017-09-06 12:57 ` Roman Gushchin
2017-09-06 13:22 ` Michal Hocko
2017-09-06 13:41 ` Roman Gushchin
2017-09-06 14:10 ` Michal Hocko
2017-09-06 8:34 ` Michal Hocko
2017-09-06 12:33 ` Roman Gushchin
2017-09-07 16:18 ` Christopher Lameter
2017-09-11 8:49 ` Michal Hocko
2017-09-04 14:21 ` [v7 3/5] mm, oom: introduce oom_priority for memory cgroups Roman Gushchin
2017-09-04 14:21 ` [v7 4/5] mm, oom, docs: describe the cgroup-aware OOM killer Roman Gushchin
2017-09-04 14:21 ` [v7 5/5] mm, oom: cgroup v2 mount option to disable " Roman Gushchin
2017-09-04 17:32 ` Shakeel Butt
2017-09-04 17:51 ` Roman Gushchin
2017-09-05 13:44 ` Michal Hocko
2017-09-05 14:30 ` Roman Gushchin
2017-09-05 15:12 ` Michal Hocko
2017-09-05 19:16 ` Roman Gushchin
2017-09-06 8:42 ` Michal Hocko
2017-09-06 17:40 ` Roman Gushchin
2017-09-06 17:59 ` Michal Hocko
2017-09-06 20:59 ` David Rientjes
2017-09-07 14:43 ` Christopher Lameter
2017-09-07 14:52 ` Roman Gushchin
2017-09-07 15:03 ` Christopher Lameter
2017-09-07 16:42 ` Roman Gushchin
2017-09-07 17:03 ` Christopher Lameter
2017-09-07 21:55 ` David Rientjes
2017-09-07 16:21 ` Christopher Lameter
2017-09-05 21:53 ` Johannes Weiner
2017-09-06 8:28 ` Michal Hocko
2017-09-07 16:14 ` Johannes Weiner
2017-09-11 9:05 ` Michal Hocko
2017-09-11 12:50 ` Roman Gushchin
2017-09-07 16:27 ` Christopher Lameter
2017-09-07 22:03 ` David Rientjes
2017-09-08 21:07 ` Christopher Lameter
2017-09-09 8:45 ` David Rientjes
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