Re: [RFC PATCH] memcg: export knobs for the defaul cgroup hierarchy

[Vladimir Davydov]

On Mon, Jul 21, 2014 at 11:07:24AM +0200, Michal Hocko wrote:
> On Fri 18-07-14 19:44:43, Vladimir Davydov wrote:
> > On Wed, Jul 16, 2014 at 11:58:14AM -0400, Johannes Weiner wrote:
> > > On Wed, Jul 16, 2014 at 04:39:38PM +0200, Michal Hocko wrote:
> > > > +#ifdef CONFIG_MEMCG_KMEM
> > > > +	{
> > > > +		.name = "kmem.limit_in_bytes",
> > > > +		.private = MEMFILE_PRIVATE(_KMEM, RES_LIMIT),
> > > > +		.write = mem_cgroup_write,
> > > > +		.read_u64 = mem_cgroup_read_u64,
> > > > +	},
> > > 
> > > Does it really make sense to have a separate limit for kmem only?
> > > IIRC, the reason we introduced this was that this memory is not
> > > reclaimable and so we need to limit it.
> > > 
> > > But the opposite effect happened: because it's not reclaimable, the
> > > separate kmem limit is actually unusable for any values smaller than
> > > the overall memory limit: because there is no reclaim mechanism for
> > > that limit, once you hit it, it's over, there is nothing you can do
> > > anymore.  The problem isn't so much unreclaimable memory, the problem
> > > is unreclaimable limits.
> > > 
> > > If the global case produces memory pressure through kernel memory
> > > allocations, we reclaim page cache, anonymous pages, inodes, dentries
> > > etc.  I think the same should happen for kmem: kmem should just be
> > > accounted and limited in the overall memory limit of a group, and when
> > > pressure arises, we go after anything that's reclaimable.
> > 
> > Personally, I don't think there's much sense in having a separate knob
> > for kmem limit either. Until we have a user with a sane use case for it,
> > let's not propagate it to the new interface.
> 
> What about fork-bomb forks protection? I thought that was the primary usecase
> for K < U? Or how can we handle that use case with a single limit? A
> special gfp flag to not trigger OOM path when called from some kmem
> charge paths?

Hmm, for a moment I thought that putting a fork-bomb inside a memory
cgroup with kmem accounting enabled and K=U will isolate it from the
rest of the system and therefore there's no need in K<U, but now I
realize it's not quite right.

In contrast to user memory, thread stack allocations have costly order,
they cannot be swapped out, and on 32-bit systems they will consume a
limited resource of low mem. Although the latter two doesn't look like
being of much concern, costly order of stack pages certainly does I
think.

Is this what you mean by saying we have to disable OOM from some kmem
charge paths? To prevent OOM on the global level that might trigger due
to lack of high order pages for task stack?

> What about task_count or what was the name of the controller which was
> dropped and suggested to be replaced by kmem accounting? I can imagine
> that to be implemented by a separate K limit which would be roughtly
> stack_size * task_count + pillow for slab.

I wonder how big this pillow for slab should be...

Thanks.
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