On Fri, 15 Sep 2017, Roman Gushchin wrote:

quoted

quoted

quoted

But then you just enforce a structural restriction on your configuration
because
	root
        /  \
       A    D
      /\   
     B  C

is a different thing than
	root
        / | \
       B  C  D

I actually don't have a strong argument against an approach to select
largest leaf or kill-all-set memcg. I think, in practice there will be
no much difference.

The only real concern I have is that then we have to do the same with
oom_priorities (select largest priority tree-wide), and this will limit
an ability to enforce the priority by parent cgroup.

Yes, oom_priority cannot select the largest priority tree-wide for exactly 
that reason.  We need the ability to control from which subtree the kill 
occurs in ancestor cgroups.  If multiple jobs are allocated their own 
cgroups and they can own memory.oom_priority for their own subcontainers, 
this becomes quite powerful so they can define their own oom priorities.   
Otherwise, they can easily override the oom priorities of other cgroups.

I believe, it's a solvable problem: we can require CAP_SYS_RESOURCE to set
the oom_priority below parent's value, or something like this.

But it looks more complex, and I'm not sure there are real examples,
when we have to compare memcgs, which are on different levels
(or in different subtrees).

It's actually much more complex because in our environment we'd need an 
"activity manager" with CAP_SYS_RESOURCE to control oom priorities of user 
subcontainers when today it need only be concerned with top-level memory 
cgroups.  Users can create their own hierarchies with their own oom 
priorities at will, it doesn't alter the selection heuristic for another 
other user running on the same system and gives them full control over the 
selection in their own subtree.  We shouldn't need to have a system-wide 
daemon with CAP_SYS_RESOURCE be required to manage subcontainers when 
nothing else requires it.  I believe it's also much easier to document: 
oom_priority is considered for all sibling cgroups at each level of the 
hierarchy and the cgroup with the lowest priority value gets iterated.

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