Re: [v8 0/4] cgroup-aware OOM killer

[Roman Gushchin <guro@fb.com>]

On Wed, Sep 27, 2017 at 09:37:44AM +0200, Michal Hocko wrote:
> On Tue 26-09-17 14:04:41, David Rientjes wrote:
> > On Tue, 26 Sep 2017, Michal Hocko wrote:
> > 
> > > > No, I agree that we shouldn't compare sibling memory cgroups based on 
> > > > different criteria depending on whether group_oom is set or not.
> > > > 
> > > > I think it would be better to compare siblings based on the same criteria 
> > > > independent of group_oom if the user has mounted the hierarchy with the 
> > > > new mode (I think we all agree that the mount option is needed).  It's 
> > > > very easy to describe to the user and the selection is simple to 
> > > > understand. 
> > > 
> > > I disagree. Just take the most simplistic example when cgroups reflect
> > > some other higher level organization - e.g. school with teachers,
> > > students and admins as the top level cgroups to control the proper cpu
> > > share load. Now you want to have a fair OOM selection between different
> > > entities. Do you consider selecting students all the time as an expected
> > > behavior just because their are the largest group? This just doesn't
> > > make any sense to me.
> > > 
> > 
> > Are you referring to this?
> > 
> > 	root
> >        /    \
> > students    admins
> > /      \    /    \
> > A      B    C    D
> > 
> > If the cumulative usage of all students exceeds the cumulative usage of 
> > all admins, yes, the choice is to kill from the /students tree.
> 
> Which is wrong IMHO because the number of stutends is likely much more
> larger than admins (or teachers) yet it might be the admins one to run
> away. This example simply shows how comparing siblinks highly depends
> on the way you organize the hierarchy rather than the actual memory
> consumer runaways which is the primary goal of the OOM killer to handle.
> 
> > This has been Roman's design from the very beginning.
> 
> I suspect this was the case because deeper hierarchies for
> organizational purposes haven't been considered.
> 
> > If the preference is to kill 
> > the single largest process, which may be attached to either subtree, you 
> > would not have opted-in to the new heuristic.
> 
> I believe you are making a wrong assumption here. The container cleanup
> is sound reason to opt in and deeper hierarchies are simply required in
> the cgroup v2 world where you do not have separate hierarchies.
>  
> > > > Then, once a cgroup has been chosen as the victim cgroup, 
> > > > kill the process with the highest badness, allowing the user to influence 
> > > > that with /proc/pid/oom_score_adj just as today, if group_oom is disabled; 
> > > > otherwise, kill all eligible processes if enabled.
> > > 
> > > And now, what should be the semantic of group_oom on an intermediate
> > > (non-leaf) memcg? Why should we compare it to other killable entities?
> > > Roman was mentioning a setup where a _single_ workload consists of a
> > > deeper hierarchy which has to be shut down at once. It absolutely makes
> > > sense to consider the cumulative memory of that hierarchy when we are
> > > going to kill it all.
> > > 
> > 
> > If group_oom is enabled on an intermediate memcg, I think the intuitive 
> > way to handle it would be that all descendants are also implicitly or 
> > explicitly group_oom.
> 
> This is an interesting point. I would tend to agree here. If somebody
> requires all-in clean up up the hierarchy it feels strange that a
> subtree would disagree (e.g. during memcg oom on the subtree). I can
> hardly see a usecase that would really need a different group_oom policy
> depending on where in the hierarchy the oom happened to be honest.
> Roman?

Yes, I'd say that it's strange to apply settings from outside the OOMing
cgroup to the subtree, but actually it's not. The oom_group setting should
basically mean that the OOM killer will not kill a random task in the subtree.
And it doesn't matter if it was global or memcg-wide OOM.

Applied to v9. Thanks!

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