linux-kernel - Re: [patch] mm, memcg: add oom killer delay

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Date:	Mon, 3 Jun 2013 21:31:47 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	cgroups@...r.kernel.org
Subject: Re: [patch] mm, memcg: add oom killer delay

On Mon 03-06-13 11:18:09, David Rientjes wrote:
> On Sat, 1 Jun 2013, Michal Hocko wrote:
[...]
> > I still do not see why you cannot simply read tasks file into a
> > preallocated buffer. This would be few pages even for thousands of pids.
> > You do not have to track processes as they come and go.
> > 
> 
> What do you suggest when you read the "tasks" file and it returns -ENOMEM 
> because kmalloc() fails because the userspace oom handler's memcg is also 
> oom? 

That would require that you track kernel allocations which is currently
done only for explicit caches.

> Obviously it's not a situation we want to get into, but unless you 
> know that handler's exact memory usage across multiple versions, nothing 
> else is sharing that memcg, and it's a perfect implementation, you can't 
> guarantee it.  We need to address real world problems that occur in 
> practice.

If you really need to have such a guarantee then you can have a _global_
watchdog observing oom_control of all groups that provide such a vague
requirements for oom user handlers.

> > As I said before. oom_delay_millisecs is actually really easy to be done
> > from userspace. If you really need a safety break then you can register
> > such a handler as a fallback. I am not familiar with eventfd internals
> > much but I guess that multiple handlers are possible. The fallback might
> > be enforeced by the admin (when a new group is created) or by the
> > container itself. Would something like this work for your use case?
> > 
> 
> You're suggesting another userspace process that solely waits for a set 
> duration and then reenables the oom killer?

Yes which kicks the oom killer.

> It faces all the same problems as the true userspace oom handler: it's
> own perfect implementation and it's own memcg constraints.

But that solution might be implemented as a global policy living in a
group with some reservations.

[...]
-- 
Michal Hocko
SUSE Labs
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