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

[Glauber Costa]

On 08/20/2012 05:36 PM, Kamezawa Hiroyuki wrote:
> (2012/08/16 2:00), Glauber Costa wrote:
>> 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.
>>
> 
> Hm, can't we free css_id and delete css structure from the css_id idr tree
> when a memcg goes zombie ?
> 
> Thanks,
> -Kame

Kame,

I wrote a patch that does exactly that. Can you take a look? (I posted
it already)
I actually need to go back to it, because greg seems to be right saying
that that will break things for memsw. But a simplified version may work.