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

[Glauber Costa]

On 08/15/2012 08:38 PM, Greg Thelen wrote:
> On Wed, Aug 15 2012, Glauber Costa wrote:
> 
>> 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.
> 
> Fair point.  But this doesn't seems like a feature.  It's probably not
> needed initially, but what do you think about creating a
> memcg_kernel_context structure which is allocated when memcg is
> allocated?  Kernel pages charged to a memcg would have
> page_cgroup->mem_cgroup=memcg_kernel_context rather than memcg.  This
> would allow the mem_cgroup and its css_id to be deleted when the cgroup
> is unlinked from cgroupfs while allowing for the active kernel pages to
> continue pointing to a valid memcg_kernel_context.  This would be a
> reference counted structure much like you are doing with memcg.  When a
> memcg is deleted the memcg_kernel_context would be linked into its
> surviving parent memcg.  This would avoid needing to visit each kernel
> page.

You need more, you need at the res_counters to stay around as well. And
probably other fields.

So my fear here is that as you add fields to that structure, you can
defeat a bit the goal of reducing memory consumption. Still leaves the
css space, yes. But by doing this we can introduce some subtle bugs by
having a field in the wrong structure.

Did you observe that to be a big problem in your systems?