[Devel] Re: [PATCH v2 06/11] memcg: kmem controller infrastructure

[Glauber Costa]

On 08/14/2012 10:58 PM, Greg Thelen wrote:
> On Mon, Aug 13 2012, Glauber Costa wrote:
> 
>>>>> +	WARN_ON(mem_cgroup_is_root(memcg));
>>>>> +	size = (1 << order) << PAGE_SHIFT;
>>>>> +	memcg_uncharge_kmem(memcg, size);
>>>>> +	mem_cgroup_put(memcg);
>>> Why do we need ref-counting here ? kmem res_counter cannot work as
>>> reference ?
>> This is of course the pair of the mem_cgroup_get() you commented on
>> earlier. If we need one, we need the other. If we don't need one, we
>> don't need the other =)
>>
>> The guarantee we're trying to give here is that the memcg structure will
>> stay around while there are dangling charges to kmem, that we decided
>> not to move (remember: moving it for the stack is simple, for the slab
>> is very complicated and ill-defined, and I believe it is better to treat
>> all kmem equally here)
> 
> By keeping memcg structures hanging around until the last referring kmem
> page is uncharged do such zombie memcg each consume a css_id and thus
> put pressure on the 64k css_id space?  I imagine in pathological cases
> this would prevent creation of new cgroups until these zombies are
> dereferenced.

Yes, but although this patch makes it more likely, it doesn't introduce
that. If the tasks, for instance, grab a reference to the cgroup dentry
in the filesystem (like their CWD, etc), they will also keep the cgroup
around.


> Is there any way to see how much kmem such zombie memcg are consuming?
> I think we could find these with
> for_each_mem_cgroup_tree(root_mem_cgroup).

Yes, just need an interface for that. But I think it is something that
can be addressed orthogonaly to this work, in a separate patch, not as
some fundamental limitation.

>  Basically, I'm wanting to
> know where kernel memory has been allocated.  For live memcg, an admin
> can cat memory.kmem.usage_in_bytes.  But for zombie memcg, I'm not sure
> how to get this info.  It looks like the root_mem_cgroup
> memory.kmem.usage_in_bytes is not hierarchically charged.
> 

Not sure what you mean by not being hierarchically charged. It should
be, when use_hierarchy = 1. As a matter of fact, I just tested it, and I
do see kmem being charged all the way to the root cgroup when hierarchy
is used. (we just can't limit it there)