Re: [patch -mm v2] mm: introduce oom_adj_child

[KOSAKI Motohiro]

Hi

> On Thu, 30 Jul 2009, KOSAKI Motohiro wrote:
> 
> > > diff --git a/kernel/fork.c b/kernel/fork.c
> > > --- a/kernel/fork.c
> > > +++ b/kernel/fork.c
> > > @@ -426,7 +426,7 @@ static struct mm_struct * mm_init(struct mm_struct * mm, struct task_struct *p)
> > >  	init_rwsem(&mm->mmap_sem);
> > >  	INIT_LIST_HEAD(&mm->mmlist);
> > >  	mm->flags = (current->mm) ? current->mm->flags : default_dump_filter;
> > > -	mm->oom_adj = (current->mm) ? current->mm->oom_adj : 0;
> > > +	mm->oom_adj = p->oom_adj_child;
> > 
> > This code doesn't fix anything.
> > mm->oom_adj assignment still change vfork() parent process oom_adj value.
> > (Again, vfork() parent and child use the same mm)
> > 
> 
> That's because the oom killer only really considers the highest oom_adj 
> value amongst all threads that share the same mm.  Allowing those threads 
> to each have different oom_adj values leads (i) to an inconsistency in 
> reporting /proc/pid/oom_score for how the oom killer selects a task to 
> kill and (ii) the oom killer livelock that it fixes when one thread 
> happens to be OOM_DISABLE.

I agree both. again I only disagree ABI breakage regression and
stupid new /proc interface.
Paul already pointed out this issue can be fixed without ABI change.


> So, yes, changing the oom_adj value for a thread may have side-effects 
> on other threads that didn't exist prior to 2.6.31-rc1 because the oom_adj 
> value now represents a killable quantity of memory instead of a being a 
> characteristic of the task itself.  But we now provide the inheritance 
> property in a new way, via /proc/pid/oom_adj_child, that gives you all the 
> functionality that the previous way did but without the potential for 
> livelock.

maybe, I should say my stand-point obviously. I don't dislike your
per-process oom_adj concept.
I only oppose vfork breakage.

if you feel my stand point is double standard, I need explain me more.
So, I don't think per-process oom_adj makes any regression on _real_ world.
but vfork()'s one is real world issue.

I think they are totally different thing.


And, May I explay why I think your oom_adj_child is wrong idea?
The fact is: new feature introducing never fix regression. yes, some
application use new interface and disappear the problem. but other
application still hit the problem. that's not correct development style
in kernel.


> 
> > IOW, in vfork case, oom_adj_child parameter doesn't only change child oom_adj,
> > but also parent oom_adj value.
> 
> Changing oom_adj_child for a task never changes oom_adj for any mm, it 
> simply specifies what default value shall be given for a child's newly 
> initialized mm.  Chaning oom_adj, on the other hand, will 

Ah, ok. I miunderstood.
However, We can fix this issue without new interface, isn't it?


> > IOW, oom_adj_child is NOT child effective parameter.
> > 
> 
> It's not meant to be, it's only meant to specify a default value for newly 
> initialized mm's of its descendants.  What happens after that is governed 
> completely by the child's own /proc/pid/oom_adj.  That's pretty clearly 
> explained in Documentation/filesystems/proc.txt.



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