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

From: Michal Hocko
Date: Mon Jun 03 2013 - 15:31:58 EST

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
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