Message-ID: <20090113233502.GA31490@ioremap.net>
Date:	Wed, 14 Jan 2009 02:35:02 +0300
From:	Evgeniy Polyakov <zbr@...emap.net>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: Linux killed Kenny, bastard!

On Tue, Jan 13, 2009 at 03:10:50PM -0800, David Rientjes (rientjes@...gle.com) wrote:
> On Wed, 14 Jan 2009, Evgeniy Polyakov wrote:
> 
> > > Your patch simply allows users to specify a task by name that will always 
> > > be killed first when the oom killer is invoked.  That's terribly 
> > > insufficient if another task uses an excessive amount of memory that you 
> > > didn't expect; a rogue task may be leaking memory and the task you've 
> > > identified by name with your patch is repeatedly forked and killed when 
> > > the rogue task goes untouched.
> > 
> > It is up to user to decide, exactly the same will happen if you tune the
> > oom_adj for the task.
> > 
> 
> It is up to the user to decide, using oom_adj scores as influence, how to 
> define when a task should be selected by the oom killer.  Your 
> name-parsing hack can do that for a single global task, but oom_adj scores 
> are actually much more powerful.
> 
> > No, you can not. Did you try that? The only sane way to use oom_adj is
> > to disable oom killer for the task or make its score very small or very
> > big, there is really no way to make a finegrained tuning, since score
> > changes and userspace does not know the algorithm.
> > 
> 
> We finely tune oom_adj scores so that we get the desired results, yes.  
> What you're complaining about here is purely a documentation issue.

Which does not work. Even besides documenation issue, which really means
that no one really tried to work with it :)

> > > Additionally, your patch completely breaks cpuset oom killing since 
> > > candidacy is determined in badness() because a task may have allocated 
> > > non-migrated memory elsewhere before being moved to a different cpuset.  
> > > Your oom_victim_name task may exist globally, but will always be 
> > > identified for oom kill even when the oom exists exclusively in a disjoint 
> > > cpuset.  That does _not_ lead to future memory freeing that current can 
> > > use, and if the parent of the killed task decides to immediately fork 
> > > another instance, this cpuset will be completely livelocked. 
> > 
> > Please check the patch first. It selects process according to the
> > badness, check memory group first and fallbacks to scan other processes
> > if process with the given name was not found or name is null.
> > 
> 
> Again, your patch _completely_ breaks cpuset oom killing.  That is a 
> completely separate issue than the memory controller, and it's 
> disappointing you still don't see it.
> 
> In a cpuset constrained oom condition, we do not explicitly exclude all 
> tasks that are in a disjoint, exclusive cpuset since it's quite possible 
> that a task has allocated memory outside its cpuset (either because its 
> cpuset assignment has changed or because its cpuset's mems has changed) 
> and killing it would free memory in current's cpuset.  We do, however, 
> prefer to kill a task within the same cpuset; that preference is 
> implemented in the badness() scoring.
> 
> If a task exists on the system in a disjoint, exclusive cpuset that 
> matches oom_victim_name, your patch will cause it to be killed even though 
> badness() has penalized it for not sharing a cpuset (dividing its score by 
> eight).  That probably needlessly killed oom_victim_name since it won't 
> allow for future memory freeing in the oom-triggering cpuset and the 
> original oom condition persists.

It is exactly the purpose of the patch: to kill what is requested to be
killed.

I wonder how do you expect users to guess via libastral that even
adjusted score does not work, since it happens that task is so special,
that it can not be killed :)

> Now if the parent of that task or another system task forks 
> oom_victim_name again, the same thing will happen on the next iteration of 
> the oom killer.  This will not free any memory in current's cpuset and it 
> will effectively be livelocked.

My knowledge about cpusets is somewhat between zero and void, even more
I opened mm/kill.c the first time when created a patch (oom-killer is
not that interesting actually, but it is a matter of taste of course).

I can create exactly the reverse situation when task is supposed to be
killed, but because of the cpuset/group/whatever else you pointed above,
its score will be decreased and rogue task will continue to live :)
This game can be played by both, but let's leave that for others.

The purpose of the patch is to create an ability to kill what is
needed by the user. Exactly what user wants. User can kill the system by
millions of ways, and we allow him to do so, but we do not really allow
him what to kill when system breaks into oom condition. With my patch it
is possible. And it is exactly what is expected by the user who does not
know anything else except the names of the applications he starts.
And please let's not start again with oom_adj if arguments are still the
same: it does not work in the case I showed.

Piece? :)

Please provide a way to fix the problem I described without my patch,
and everything will be immediately resolved :)

-- 
	Evgeniy Polyakov
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