cgroup aware oom killer (was Re: [PATCH 0/3] introduce memory.oom.group)

[David Rientjes]

Roman, have you had time to go through this?


On Tue, 7 Aug 2018, David Rientjes wrote:

> On Mon, 6 Aug 2018, Roman Gushchin wrote:
> 
> > > In a cgroup-aware oom killer world, yes, we need the ability to specify 
> > > that the usage of the entire subtree should be compared as a single 
> > > entity with other cgroups.  That is necessary for user subtrees but may 
> > > not be necessary for top-level cgroups depending on how you structure your 
> > > unified cgroup hierarchy.  So it needs to be configurable, as you suggest, 
> > > and you are correct it can be different than oom.group.
> > > 
> > > That's not the only thing we need though, as I'm sure you were expecting 
> > > me to say :)
> > > 
> > > We need the ability to preserve existing behavior, i.e. process based and 
> > > not cgroup aware, for subtrees so that our users who have clear 
> > > expectations and tune their oom_score_adj accordingly based on how the oom 
> > > killer has always chosen processes for oom kill do not suddenly regress.
> > 
> > Isn't the combination of oom.group=0 and oom.evaluate_together=1 describing
> > this case? This basically means that if memcg is selected as target,
> > the process inside will be selected using traditional per-process approach.
> > 
> 
> No, that would overload the policy and mechanism.  We want the ability to 
> consider user-controlled subtrees as a single entity for comparison with 
> other user subtrees to select which subtree to target.  This does not 
> imply that users want their entire subtree oom killed.
> 
> > > So we need to define the policy for a subtree that is oom, and I suggest 
> > > we do that as a characteristic of the cgroup that is oom ("process" vs 
> > > "cgroup", and process would be the default to preserve what currently 
> > > happens in a user subtree).
> > 
> > I'm not entirely convinced here.
> > I do agree, that some sub-tree may have a well tuned oom_score_adj,
> > and it's preferable to keep the current behavior.
> > 
> > At the same time I don't like the idea to look at the policy of the OOMing
> > cgroup. Why exceeding of one limit should be handled different to exceeding
> > of another? This seems to be a property of workload, not a limit.
> > 
> 
> The limit is the property of the mem cgroup, so it's logical that the 
> policy when reaching that limit is a property of the same mem cgroup.
> Using the user-controlled subtree example, if we have /david and /roman, 
> we can define our own policies on oom, we are not restricted to cgroup 
> aware selection on the entire hierarchy.  /david/oom.policy can be 
> "process" so that I haven't regressed with earlier kernels, and 
> /roman/oom.policy can be "cgroup" to target the largest cgroup in your 
> subtree.
> 
> Something needs to be oom killed when a mem cgroup at any level in the 
> hierarchy is reached and reclaim has failed.  What to do when that limit 
> is reached is a property of that cgroup.
> 
> > > Now, as users who rely on process selection are well aware, we have 
> > > oom_score_adj to influence the decision of which process to oom kill.  If 
> > > our oom subtree is cgroup aware, we should have the ability to likewise 
> > > influence that decision.  For example, we have high priority applications 
> > > that run at the top-level that use a lot of memory and strictly oom 
> > > killing them in all scenarios because they use a lot of memory isn't 
> > > appropriate.  We need to be able to adjust the comparison of a cgroup (or 
> > > subtree) when compared to other cgroups.
> > > 
> > > I've also suggested, but did not implement in my patchset because I was 
> > > trying to define the API and find common ground first, that we have a need 
> > > for priority based selection.  In other words, define the priority of a 
> > > subtree regardless of cgroup usage.
> > > 
> > > So with these four things, we have
> > > 
> > >  - an "oom.policy" tunable to define "cgroup" or "process" for that 
> > >    subtree (and plans for "priority" in the future),
> > > 
> > >  - your "oom.evaluate_as_group" tunable to account the usage of the
> > >    subtree as the cgroup's own usage for comparison with others,
> > > 
> > >  - an "oom.adj" to adjust the usage of the cgroup (local or subtree)
> > >    to protect important applications and bias against unimportant
> > >    applications.
> > > 
> > > This adds several tunables, which I didn't like, so I tried to overload 
> > > oom.policy and oom.evaluate_as_group.  When I referred to separating out 
> > > the subtree usage accounting into a separate tunable, that is what I have 
> > > referenced above.
> > 
> > IMO, merging multiple tunables into one doesn't make it saner.
> > The real question how to make a reasonable interface with fever tunables.
> > 
> > The reason behind introducing all these knobs is to provide
> > a generic solution to define OOM handling rules, but then the
> > question raises if the kernel is the best place for it.
> > 
> > I really doubt that an interface with so many knobs has any chances
> > to be merged.
> > 
> 
> This is why I attempted to overload oom.policy and oom.evaluate_as_group: 
> I could not think of a reasonable usecase where a subtree would be used to 
> account for cgroup usage but not use a cgroup aware policy itself.  You've 
> objected to that, where memory.oom_policy == "tree" implied cgroup 
> awareness in my patchset, so I've separated that out.
> 
> > IMO, there should be a compromise between the simplicity (basically,
> > the number of tunables and possible values) and functionality
> > of the interface. You nacked my previous version, and unfortunately
> > I don't have anything better so far.
> > 
> 
> If you do not agree with the overloading and have a preference for single 
> value tunables, then all three tunables are needed.  This functionality 
> could be represented as two or one tunable if they are not single value, 
> but from the oom.group discussion you preferred single values.
> 
> I assume you'd also object to adding and removing files based on 
> oom.policy since oom.evaluate_as_group and oom.adj is only needed for 
> oom.policy of "cgroup" or "priority", and they do not need to exist for 
> the default oom.policy of "process".
>