Re: [v6 2/4] mm, oom: cgroup-aware OOM killer

[Michal Hocko]

On Thu 24-08-17 14:58:42, Roman Gushchin wrote:
> On Thu, Aug 24, 2017 at 02:58:11PM +0200, Michal Hocko wrote:
> > On Thu 24-08-17 13:28:46, Roman Gushchin wrote:
> > > Hi Michal!
> > > 
> > There is nothing like a "better victim". We are pretty much in a
> > catastrophic situation when we try to survive by killing a userspace.
> 
> Not necessary, it can be a cgroup OOM.

memcg OOM is no different. The catastrophic is scoped to the specific
hierarchy but tasks in that hierarchy still fail to make a further
progress.

> > We try to kill the largest because that assumes that we return the
> > most memory from it. Now I do understand that you want to treat the
> > memcg as a single killable entity but I find it really questionable
> > to do a per-memcg metric and then do not treat it like that and kill
> > only a single task. Just imagine a single memcg with zillions of taks
> > each very small and you select it as the largest while a small taks
> > itself doesn't help to help to get us out of the OOM.
> 
> I don't think it's different from a non-containerized state: if you
> have a zillion of small tasks in the system, you'll meet the same issues.

Yes this is possible but usually you are comparing apples to apples so
you will kill the largest offender and then go on. To be honest I really
do hate how we try to kill a children rather than the selected victim
for the same reason.

> > > > I guess I have asked already and we haven't reached any consensus. I do
> > > > not like how you treat memcgs and tasks differently. Why cannot we have
> > > > a memcg score a sum of all its tasks?
> > > 
> > > It sounds like a more expensive way to get almost the same with less accuracy.
> > > Why it's better?
> > 
> > because then you are comparing apples to apples?
> 
> Well, I can say that I compare some number of pages against some other number
> of pages. And the relation between a page and memcg is more obvious, than a
> relation between a page and a process.

But you are comparing different accounting systems.
 
> Both ways are not ideal, and sum of the processes is not ideal too.
> Especially, if you take oom_score_adj into account. Will you respect it?

Yes, and I do not see any reason why we shouldn't.

> I've started actually with such approach, but then found it weird.
> 
> > Besides that you have
> > to check each task for over-killing anyway. So I do not see any
> > performance merits here.
> 
> It's an implementation detail, and we can hopefully get rid of it at some point.

Well, we might do some estimations and ignore oom scopes but I that
sounds really complicated and error prone. Unless we have anything like
that then I would start from tasks and build up the necessary to make a
decision at the higher level.
 
> > > > How do you want to compare memcg score with tasks score?
> > > 
> > > I have to do it for tasks in root cgroups, but it shouldn't be a common case.
> > 
> > How come? I can easily imagine a setup where only some memcgs which
> > really do need a kill-all semantic while all others can live with single
> > task killed perfectly fine.
> 
> I mean taking a unified cgroup hierarchy into an account, there should not
> be lot of tasks in the root cgroup, if any.

Is that really the case? I would assume that memory controller would be
enabled only in those subtrees which really use the functionality and
the rest will be sitting in the root memcg. It might be the case if you
are running only containers but I am not really sure this is true in
general.
-- 
Michal Hocko
SUSE Labs