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

[Roman Gushchin]

On Fri, Aug 25, 2017 at 10:14:03AM +0200, Michal Hocko wrote:
> On Thu 24-08-17 15:58:01, Roman Gushchin wrote:
> > On Thu, Aug 24, 2017 at 04:13:37PM +0200, Michal Hocko wrote:
> > > On Thu 24-08-17 14:58:42, Roman Gushchin wrote:
> [...]
> > > > 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.
> > 
> > It makes things even more complicated.
> > Right now task's oom_score can be in (~ -total_memory, ~ +2*total_memory) range,
> > and it you're starting summing it, it can be multiplied by number of tasks...
> > Weird.
> 
> oom_score_adj is just a normalized bias so if tasks inside oom will use
> it the whole memcg will get accumulated bias from all such tasks so it
> is not completely off. I agree that the more tasks use the bias the more
> biased the whole memcg will be. This might or might not be a problem.
> As you are trying to reimplement the existing oom killer implementation
> I do not think we cannot simply ignore API which people are used to.
> 
> If this was a configurable oom policy then I could see how ignoring
> oom_score_adj is acceptable because it would be an explicit opt-in.
>
> > It also will be different in case of system and memcg-wide OOM.
> 
> Why, we do honor oom_score_adj for the memcg OOM now and in fact the
> kernel memcg OOM killer shouldn't be very much different from the global
> one except for the tasks scope.

Assume, you have two tasks (2Gb and 1Gb) in a cgroup with limit 3Gb.
The second task has oom_score_adj +100. Total memory is 64Gb, for example.

I case of memcg-wide oom first task will be selected;
in case of system-wide OOM - the second.

Personally I don't like this, but it looks like we have to respect
oom_score_adj set to -1000, I'll alter my patch.

> 
> > > > 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.
> > 
> > Seriously speaking, do you have an example, when summing per-process
> > oom_score will work better?
> 
> The primary reason I am pushing for this is to have the common iterator
> code path (which we have since Vladimir has unified memcg and global oom
> paths) and only parametrize the value calculation and victim selection.

I agree, but I'm not sure that we can (and have to) totally unify the way,
how oom_score is calculated for processes and cgroups.

But I'd like to see an unified oom_priority approach. This will allow
to define an OOM killing order in a clear way, and use size-based tiebreaking
for items of the same priority. Root-cgroup processes will be compared with
other memory consumers by oom_priority first and oom_score afterwards.

What do you think about it?

Thanks!