Re: [RFC 3/3] oom: Do not try to sacrifice small children

[Michal Hocko]

On Tue 12-01-16 16:51:43, David Rientjes wrote:
> On Tue, 12 Jan 2016, Michal Hocko wrote:
> 
> > diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> > index 8bca0b1e97f7..b5c0021c6462 100644
> > --- a/mm/oom_kill.c
> > +++ b/mm/oom_kill.c
> > @@ -721,8 +721,16 @@ try_to_sacrifice_child(struct oom_control *oc, struct task_struct *victim,
> >  	if (!child_victim)
> >  		goto out;
> >  
> > -	put_task_struct(victim);
> > -	victim = child_victim;
> > +	/*
> > +	 * Protecting the parent makes sense only if killing the child
> > +	 * would release at least some memory (at least 1MB).
> > +	 */
> > +	if (K(victim_points) >= 1024) {
> > +		put_task_struct(victim);
> > +		victim = child_victim;
> > +	} else {
> > +		put_task_struct(child_victim);
> > +	}
> >  
> >  out:
> >  	return victim;
> 
> The purpose of sacrificing a child has always been to prevent a process 
> that has been running with a substantial amount of work done from being 
> terminated and losing all that work if it can be avoided.  This happens a 
> lot: imagine a long-living front end client forking a child which simply 
> collects stats and malloc information at a regular intervals and writes 
> them out to disk or over the network.  These processes may be quite small, 
> and we're willing to happily sacrifice them if it will save the parent.  
> This was, and still is, the intent of the sacrifice in the first place.

Yes I understand the intention of the heuristic. I am just contemplating
about what is way too small to sacrifice because it clearly doesn't make
much sense to kill a task which is sitting on basically no memory (well
just few pages backing page tables and stack) because this would just
prolong the OOM agony.

> We must be able to deal with oom victims that are very small, since 
> userspace has complete control in prioritizing these processes in the 
> first place.

Sure the patch is not great but I would like to come up with some
threshold when children are way too small to be worthwhile considering.
Or maybe there is other measure we can use.

-- 
Michal Hocko
SUSE Labs