Re: [RFC PATCH] mm, oom: cgroup-aware OOM-killer

[Johannes Weiner <hannes@cmpxchg.org>]

On Thu, May 18, 2017 at 07:30:04PM +0200, Michal Hocko wrote:
> On Thu 18-05-17 17:28:04, Roman Gushchin wrote:
> > Traditionally, the OOM killer is operating on a process level.
> > Under oom conditions, it finds a process with the highest oom score
> > and kills it.
> > 
> > This behavior doesn't suit well the system with many running
> > containers. There are two main issues:
> > 
> > 1) There is no fairness between containers. A small container with
> > a few large processes will be chosen over a large one with huge
> > number of small processes.
> > 
> > 2) Containers often do not expect that some random process inside
> > will be killed. So, in general, a much safer behavior is
> > to kill the whole cgroup. Traditionally, this was implemented
> > in userspace, but doing it in the kernel has some advantages,
> > especially in a case of a system-wide OOM.
> > 
> > To address these issues, cgroup-aware OOM killer is introduced.
> > Under OOM conditions, it looks for a memcg with highest oom score,
> > and kills all processes inside.
> > 
> > Memcg oom score is calculated as a size of active and inactive
> > anon LRU lists, unevictable LRU list and swap size.
> > 
> > For a cgroup-wide OOM, only cgroups belonging to the subtree of
> > the OOMing cgroup are considered.
> 
> While this might make sense for some workloads/setups it is not a
> generally acceptable policy IMHO. We have discussed that different OOM
> policies might be interesting few years back at LSFMM but there was no
> real consensus on how to do that. One possibility was to allow bpf like
> mechanisms. Could you explore that path?

OOM policy is an orthogonal discussion, though.

The OOM killer's job is to pick a memory consumer to kill. Per default
the unit of the memory consumer is a process, but cgroups allow
grouping processes into compound consumers. Extending the OOM killer
to respect the new definition of "consumer" is not a new policy.

I don't think it's reasonable to ask the person who's trying to make
the OOM killer support group-consumers to design a dynamic OOM policy
framework instead.

All we want is the OOM policy, whatever it is, applied to cgroups.

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