[Devel] Re: [PATCH v4 23/25] memcg: propagate kmem limiting information to children

[Kamezawa Hiroyuki]

(2012/06/20 17:59), Glauber Costa wrote:
> On 06/19/2012 12:54 PM, Glauber Costa wrote:
>> On 06/19/2012 12:35 PM, Glauber Costa wrote:
>>> On 06/19/2012 04:16 AM, Kamezawa Hiroyuki wrote:
>>>> (2012/06/18 21:43), Glauber Costa wrote:
>>>>> On 06/18/2012 04:37 PM, Kamezawa Hiroyuki wrote:
>>>>>> (2012/06/18 19:28), Glauber Costa wrote:
>>>>>>> The current memcg slab cache management fails to present satisfatory hierarchical
>>>>>>> behavior in the following scenario:
>>>>>>>
>>>>>>> ->   /cgroups/memory/A/B/C
>>>>>>>
>>>>>>> * kmem limit set at A
>>>>>>> * A and B empty taskwise
>>>>>>> * bash in C does find /
>>>>>>>
>>>>>>> Because kmem_accounted is a boolean that was not set for C, no accounting
>>>>>>> would be done. This is, however, not what we expect.
>>>>>>>
>>>>>>
>>>>>> Hmm....do we need this new routines even while we have mem_cgroup_iter() ?
>>>>>>
>>>>>> Doesn't this work ?
>>>>>>
>>>>>> 	struct mem_cgroup {
>>>>>> 		.....
>>>>>> 		bool kmem_accounted_this;
>>>>>> 		atomic_t kmem_accounted;
>>>>>> 		....
>>>>>> 	}
>>>>>>
>>>>>> at set limit
>>>>>>
>>>>>> 	....set_limit(memcg) {
>>>>>>
>>>>>> 		if (newly accounted) {
>>>>>> 			mem_cgroup_iter() {
>>>>>> 				atomic_inc(&iter->kmem_accounted)
>>>>>> 			}
>>>>>> 		} else {
>>>>>> 			mem_cgroup_iter() {
>>>>>> 				atomic_dec(&iter->kmem_accounted);
>>>>>> 			}
>>>>>> 	}
>>>>>>
>>>>>>
>>>>>> hm ? Then, you can see kmem is accounted or not by atomic_read(&memcg->kmem_accounted);
>>>>>>
>>>>>
>>>>> Accounted by itself / parent is still useful, and I see no reason to use
>>>>> an atomic + bool if we can use a pair of bits.
>>>>>
>>>>> As for the routine, I guess mem_cgroup_iter will work... It does a lot
>>>>> more than I need, but for the sake of using what's already in there, I
>>>>> can switch to it with no problems.
>>>>>
>>>>
>>>> Hmm. please start from reusing existing routines.
>>>> If it's not enough, some enhancement for generic cgroup  will be welcomed
>>>> rather than completely new one only for memcg.
>>>>
>>>
>>> And now that I am trying to adapt the code to the new function, I
>>> remember clearly why I done this way. Sorry for my failed memory.
>>>
>>> That has to do with the order of the walk. I need to enforce hierarchy,
>>> which means whenever a cgroup has !use_hierarchy, I need to cut out that
>>> branch, but continue scanning the tree for other branches.
>>>
>>> That is a lot easier to do with depth-search tree walks like the one
>>> proposed in this patch. for_each_mem_cgroup() seems to walk the tree in
>>> css-creation order. Which means we need to keep track of parents that
>>> has hierarchy disabled at all times ( can be many ), and always test for
>>> ancestorship - which is expensive, but I don't particularly care.
>>>
>>> But I'll give another shot with this one.
>>>
>>
>> Humm, silly me. I was believing the hierarchical settings to be more
>> flexible than they really are.
>>
>> I thought that it could be possible for a children of a parent with
>> use_hierarchy = 1 to have use_hierarchy = 0.
>>
>> It seems not to be the case. This makes my life a lot easier.
>>
> 
> How about the following patch?
> 
> It is still expensive in the clear_bit case, because I can't just walk
> the whole tree flipping the bit down: I need to stop whenever I see a
> branch whose root is itself accounted - and the ordering of iter forces
> me to always check the tree up (So we got O(n*h) h being height instead
> of O(n)).
> 
> for flipping the bit up, it is easy enough.
> 
> 
Yes. It seems much nicer.

Thanks,
-Kame