Teaching the OOM killer about control groups

AFAIU this is not about cgroups memory limit, but a system-wide OOM in a situation where there are cgroups. There might be per-cgroup limits, but it's not that the sum of these limits would equal the system memory, because that would lead to underutilized system. So instead there's overcommit and the workloads can tolerate being killed on the level of a whole group.