Re: [v11 3/6] mm, oom: cgroup-aware OOM killer

[Roman Gushchin <guro@fb.com>]

On Tue, Oct 10, 2017 at 02:13:00PM -0700, David Rientjes wrote:
> On Tue, 10 Oct 2017, Roman Gushchin wrote:
> 
> > > This seems to unfairly bias the root mem cgroup depending on process size.  
> > > It isn't treated fairly as a leaf mem cgroup if they are being compared 
> > > based on different criteria: the root mem cgroup as (mostly) the largest 
> > > rss of a single process vs leaf mem cgroups as all anon, unevictable, and 
> > > unreclaimable slab pages charged to it by all processes.
> > > 
> > > I imagine a configuration where the root mem cgroup has 100 processes 
> > > attached each with rss of 80MB, compared to a leaf cgroup with 100 
> > > processes of 1MB rss each.  How does this logic prevent repeatedly oom 
> > > killing the processes of 1MB rss?
> > > 
> > > In this case, "the root cgroup is treated as a leaf memory cgroup" isn't 
> > > quite fair, it can simply hide large processes from being selected.  Users 
> > > who configure cgroups in a unified hierarchy for other resource 
> > > constraints are penalized for this choice even though the mem cgroup with 
> > > 100 processes of 1MB rss each may not be limited itself.
> > > 
> > > I think for this comparison to be fair, it requires accounting for the 
> > > root mem cgroup itself or for a different accounting methodology for leaf 
> > > memory cgroups.
> > 
> > This is basically a workaround, because we don't have necessary stats for root
> > memory cgroup. If we'll start gathering them at some point, we can change this
> > and treat root memcg exactly as other leaf cgroups.
> > 
> 
> I understand why it currently cannot be an apples vs apples comparison 
> without, as I suggest in the last paragraph, that the same accounting is 
> done for the root mem cgroup, which is intuitive if it is to be considered 
> on the same basis as leaf mem cgroups.
> 
> I understand for the design to work that leaf mem cgroups and the root mem 
> cgroup must be compared if processes can be attached to the root mem 
> cgroup.  My point is that it is currently completely unfair as I've 
> stated: you can have 10000 processes attached to the root mem cgroup with 
> rss of 80MB each and a leaf mem cgroup with 100 processes of 1MB rss each 
> and the oom killer is going to target the leaf mem cgroup as a result of 
> this apples vs oranges comparison.
> 
> In case it's not clear, the 10000 processes of 80MB rss each is the most 
> likely contributor to a system-wide oom kill.  Unfortunately, the 
> heuristic introduced by this patchset is broken wrt a fair comparison of 
> the root mem cgroup usage.
> 
> > Or, if someone will come with an idea of a better approximation, it can be
> > implemented as a separate enhancement on top of the initial implementation.
> > This is more than welcome.
> > 
> 
> We don't need a better approximation, we need a fair comparison.  The 
> heuristic that this patchset is implementing is based on the usage of 
> individual mem cgroups.  For the root mem cgroup to be considered 
> eligible, we need to understand its usage.  That usage is _not_ what is 
> implemented by this patchset, which is the largest rss of a single 
> attached process.  This, in fact, is not an "approximation" at all.  In 
> the example of 10000 processes attached with 80MB rss each, the usage of 
> the root mem cgroup is _not_ 80MB.

It's hard to imagine a "healthy" setup with 10000 process in the root
memory cgroup, and even if we kill 1 process we will still have 9999
remaining process. I agree with you at some point, but it's not
a real world example.

> 
> I'll restate that oom killing a process is a last resort for the kernel, 
> but it also must be able to make a smart decision.  Targeting dozens of 
> 1MB processes instead of 80MB processes because of a shortcoming in this 
> implementation is not the appropriate selection, it's the opposite of the 
> correct selection.
> 
> > > I'll reiterate what I did on the last version of the patchset: considering 
> > > only leaf memory cgroups easily allows users to defeat this heuristic and 
> > > bias against all of their memory usage up to the largest process size 
> > > amongst the set of processes attached.  If the user creates N child mem 
> > > cgroups for their N processes and attaches one process to each child, the 
> > > _only_ thing this achieved is to defeat your heuristic and prefer other 
> > > leaf cgroups simply because those other leaf cgroups did not do this.
> > > 
> > > Effectively:
> > > 
> > > for i in $(cat cgroup.procs); do mkdir $i; echo $i > $i/cgroup.procs; done
> > > 
> > > will radically shift the heuristic from a score of all anonymous + 
> > > unevictable memory for all processes to a score of the largest anonymous +
> > > unevictable memory for a single process.  There is no downside or 
> > > ramifaction for the end user in doing this.  When comparing cgroups based 
> > > on usage, it only makes sense to compare the hierarchical usage of that 
> > > cgroup so that attaching processes to descendants or splitting the 
> > > implementation of a process into several smaller individual processes does 
> > > not allow this heuristic to be defeated.
> > 
> > To all previously said words I can only add that cgroup v2 allows to limit
> > the amount of cgroups in the sub-tree:
> > 1a926e0bbab8 ("cgroup: implement hierarchy limits").
> > 
> 
> So the solution to 
> 
> for i in $(cat cgroup.procs); do mkdir $i; echo $i > $i/cgroup.procs; done
> 
> evading all oom kills for your mem cgroup is to limit the number of 
> cgroups that can be created by the user?  With a unified cgroup hierarchy, 
> that doesn't work well if I wanted to actually constrain these individual 
> processes to different resource limits like cpu usage.  In fact, the user 
> may not know it is effectively evading the oom killer entirely because it 
> has constrained the cpu of individual processes because its a side-effect 
> of this heuristic.
> 
> 
> You chose not to respond to my reiteration of userspace having absolutely 
> no control over victim selection with the new heuristic without setting 
> all processes to be oom disabled via /proc/pid/oom_score_adj.  If I have a 
> very important job that is running on a system that is really supposed to 
> use 80% of memory, I need to be able to specify that it should not be oom 
> killed based on user goals.  Setting all processes to be oom disabled in 
> the important mem cgroup to avoid being oom killed unless absolutely 
> necessary in a system oom condition is not a robust solution: (1) the mem 
> cgroup livelocks if it reaches its own mem cgroup limit and (2) the system 
> panic()'s if these preferred mem cgroups are the only consumers left on 
> the system.  With overcommit, both of these possibilities exist in the 
> wild and the problem is only a result of the implementation detail of this 
> patchset.
> 
> For these reasons: unfair comparison of root mem cgroup usage to bias 
> against that mem cgroup from oom kill in system oom conditions, the 
> ability of users to completely evade the oom killer by attaching all 
> processes to child cgroups either purposefully or unpurposefully, and the 
> inability of userspace to effectively control oom victim selection:
> 
> Nacked-by: David Rientjes <rientjes@google.com>

So, if we'll sum the oom_score of tasks belonging to the root memory cgroup,
will it fix the problem?

It might have some drawbacks as well (especially around oom_score_adj),
but it's doable, if we'll ignore tasks which are not owners of their's mm struct.

> 
> > > This is racy because mem_cgroup_select_oom_victim() found an eligible 
> > > oc->chosen_memcg that is not INFLIGHT_VICTIM with at least one eligible 
> > > process but mem_cgroup_scan_task(oc->chosen_memcg) did not.  It means if a 
> > > process cannot be killed because of oom_unkillable_task(), the only 
> > > eligible processes moved or exited, or the /proc/pid/oom_score_adj of the 
> > > eligible processes changed, we end up falling back to the complete 
> > > tasklist scan.  It would be better for oom_evaluate_memcg() to consider 
> > > oom_unkillable_task() and also retry in the case where 
> > > oom_kill_memcg_victim() returns NULL.
> > 
> > I agree with you here. The fallback to the existing mechanism is implemented
> > to be safe for sure, especially in a case of a global OOM. When we'll get
> > more confidence in cgroup-aware OOM killer reliability, we can change this
> > behavior. Personally, I would prefer to get rid of looking at all tasks just
> > to find a pre-existing OOM victim, but it might be quite tricky to implement.
> > 
> 
> I'm not sure what this has to do with confidence in this patchset's 
> reliability?  The race obviously exists: mem_cgroup_select_oom_victim() 
> found an eligible process in oc->chosen_memcg but it was either ineligible 
> later because of oom_unkillable_task(), it moved, or it exited.  It's a 
> race.  For users who opt-in to this new heuristic, they should not be 
> concerned with a process exiting and thus killing a completely unexpected 
> process from an unexpected memcg when it should be possible to retry and 
> select the correct victim.

Yes, I have to agree here.
Looks like we can't fallback to the original policy.

Thanks!

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