Re: [RFC] [PATCH] Cgroup based OOM killer controller

From: David Rientjes
Date: Fri Jan 23 2009 - 05:34:40 EST

On Fri, 23 Jan 2009, Nikanth Karthikesan wrote:

> > Of course, because the oom killer must be aware that tasks in disjoint
> > cpusets are more likely than not to result in no memory freeing for
> > current's subsequent allocations.
> >
> Yes, the problem is cpuset does not track the tasks which has allocated from
> this node - who has either moved or changed it set of allowable nodes. And
> because of that it does not limit oom killing to the tasks with in those tasks
> and could kill some innocent tasks at times.

Right, the logic to prefer tasks that share the same set of allowable
nodes as the oom-triggering task is implemented in badness() and being in
a completely disjoint cpuset does not specifically exclude a task from
being chosen as the oom killer's victim. That's because, as you said, it
could have allocated memory elsewhere before changing cpusets or its set
of allowable mems.

> As it is unable to take deterministic decision as memcg does, it plays with
> badness value and only suggests but does not restricts within those tasks that
> has to be killed.
> This bug is present even without this patch.

It's not a bug, it can actually help in a couple instances:

- a much larger memory hogging task is identified in a disjoint cpuset
and the liklihood it has allocated memory elsewhere either previously
or atomically, or

- an administrator tunes the oom_adj value for such a task to describe
the above behavior even for smaller tasks and their liklihood to
allocate outside of their exclusive cpuset.

> This patch adds one more easier way for the administrator to over-ride.

Yeah, I know. But the problem with the approach is that it specifies an
oom priority for both global unconstrained ooms and cpuset-constrained

It's quite possible with your patch to identify an aggregate of tasks that
should be killed first whenever the system is completely out of memory.
That's great, and solves your problem. But that same system cannot
correctly use cpusets that have the potential to ever oom because your
patch completely overrides the victim priority and could needlessly kill
tasks that will not lead to future memory freeing.

That's my objection to the proposal: it doesn't behave appropriately for
both global unconstrained ooms and cpuset-constrained ooms at the same

I think if you look at the cgroup oom notifier patch I referred you to,
you will find it trivial to implement a handler that can issue a SIGKILL
for an aggregate of tasks and implement the functional equivalent of this
patch in userspace where it belongs. It's nearly impossible to code oom
killer heuristics in the kernel that work for every possible workload and
configuration without some unfortunate casualties.

> The current cpuset oom handling has to be fixed and the exact problem of
> killing innocent processes exists even without the oom-controller.

I think the heuristic could be tuned to penalize tasks in disjoint cpusets
a little more, but its implementation as a factor of the badness score is
actually very well placed.
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