Re: cgroup-aware OOM killer, how to move forward

[Roman Gushchin <guro@fb.com>]

On Wed, Jul 18, 2018 at 10:12:30AM +0200, Michal Hocko wrote:
> On Tue 17-07-18 13:06:42, Roman Gushchin wrote:
> > On Tue, Jul 17, 2018 at 09:49:46PM +0200, Michal Hocko wrote:
> > > On Tue 17-07-18 10:38:45, Roman Gushchin wrote:
> > > [...]
> > > > Let me show my proposal on examples. Let's say we have the following hierarchy,
> > > > and the biggest process (or the process with highest oom_score_adj) is in D.
> > > > 
> > > >   /
> > > >   |
> > > >   A
> > > >   |
> > > >   B
> > > >  / \
> > > > C   D
> > > > 
> > > > Let's look at different examples and intended behavior:
> > > > 1) system-wide OOM
> > > >   - default settings: the biggest process is killed
> > > >   - D/memory.group_oom=1: all processes in D are killed
> > > >   - A/memory.group_oom=1: all processes in A are killed
> > > > 2) memcg oom in B
> > > >   - default settings: the biggest process is killed
> > > >   - A/memory.group_oom=1: the biggest process is killed
> > > 
> > > Huh? Why would you even consider A here when the oom is below it?
> > > /me confused
> > 
> > I do not.
> > This is exactly a counter-example: A's memory.group_oom
> > is not considered at all in this case,
> > because A is above ooming cgroup.
> 
> OK, it confused me.
> 
> > > 
> > > >   - B/memory.group_oom=1: all processes in B are killed
> > > 
> > >     - B/memory.group_oom=0 &&
> > > >   - D/memory.group_oom=1: all processes in D are killed
> > > 
> > > What about?
> > >     - B/memory.group_oom=1 && D/memory.group_oom=0
> > 
> > All tasks in B are killed.
> 
> so essentially find a task, traverse the memcg hierarchy from the
> victim's memcg up to the oom root as long as memcg.group_oom = 1?
> If the resulting memcg.group_oom == 1 then kill the whole sub tree.
> Right?

Yes.

> 
> > Group_oom set to 1 means that the workload can't tolerate
> > killing of a random process, so in this case it's better
> > to guarantee consistency for B.
> 
> OK, but then if D itself is OOM then we do not care about consistency
> all of the sudden? I have hard time to think about a sensible usecase.

I mean if traversing the hierarchy up to the oom root we meet
a memcg with group_oom set to 0, we shouldn't stop traversing.

Thanks!