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Subject: Re: [PATCH v2 04/12] mm: Assign memcg-aware shrinkers bitmap to memcg
To:     Vladimir Davydov <vdavydov.dev@gmail.com>
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From:   Kirill Tkhai <ktkhai@virtuozzo.com>
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Date:   Thu, 3 May 2018 14:15:59 +0300
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On 28.04.2018 18:08, Vladimir Davydov wrote:
> On Tue, Apr 24, 2018 at 03:24:53PM +0300, Kirill Tkhai wrote:
>>>>>>>> +int expand_shrinker_maps(int old_nr, int nr)
>>>>>>>> +{
>>>>>>>> +	int id, size, old_size, node, ret;
>>>>>>>> +	struct mem_cgroup *memcg;
>>>>>>>> +
>>>>>>>> +	old_size = old_nr / BITS_PER_BYTE;
>>>>>>>> +	size = nr / BITS_PER_BYTE;
>>>>>>>> +
>>>>>>>> +	down_write(&shrinkers_max_nr_rwsem);
>>>>>>>> +	for_each_node(node) {
>>>>>>>
>>>>>>> Iterating over cgroups first, numa nodes second seems like a better idea
>>>>>>> to me. I think you should fold for_each_node in memcg_expand_maps.
>>>>>>>
>>>>>>>> +		idr_for_each_entry(&mem_cgroup_idr, memcg, id) {
>>>>>>>
>>>>>>> Iterating over mem_cgroup_idr looks strange. Why don't you use
>>>>>>> for_each_mem_cgroup?
>>>>>>
>>>>>> We want to allocate shrinkers maps in mem_cgroup_css_alloc(), since
>>>>>> mem_cgroup_css_online() mustn't fail (it's a requirement of currently
>>>>>> existing design of memcg_cgroup::id).
>>>>>>
>>>>>> A new memcg is added to parent's list between two of these calls:
>>>>>>
>>>>>> css_create()
>>>>>>   ss->css_alloc()
>>>>>>   list_add_tail_rcu(&css->sibling, &parent_css->children)
>>>>>>   ss->css_online()
>>>>>>
>>>>>> for_each_mem_cgroup() does not see allocated, but not linked children.
>>>>>
>>>>> Why don't we move shrinker map allocation to css_online then?
>>>>
>>>> Because the design of memcg_cgroup::id prohibits mem_cgroup_css_online() to fail.
>>>> This function can't fail.
>>>
>>> I fail to understand why it is so. Could you please elaborate?
>>
>> mem_cgroup::id is freed not in mem_cgroup_css_free(), but earlier. It's freed
>> between mem_cgroup_css_offline() and mem_cgroup_free(), after the last reference
>> is put.
>>
>> In case of sometimes we want to free it in mem_cgroup_css_free(), this will
>> introduce assymmetric in the logic, which makes it more difficult. There is
>> already a bug, which I fixed in
>>
>> "memcg: remove memcg_cgroup::id from IDR on mem_cgroup_css_alloc() failure"
>>
>> new change will make this code completely not-modular and unreadable.
> 
> How is that? AFAIU all we need to do to handle css_online failure
> properly is call mem_cgroup_id_remove() from mem_cgroup_css_free().
> That's it, as mem_cgroup_id_remove() is already safe to call more
> than once for the same cgroup - the first call will free the id
> while all subsequent calls will do nothing.

I seemed confusing a reader for me, but now I'll agree with you, since
it's OK for you as for a reader.
 
>>  
>>>>
>>>> I don't think it will be good to dive into reworking of this stuff for this patchset,
>>>> which is really already big. Also, it will be assymmetric to allocate one part of
>>>> data in css_alloc(), while another data in css_free(). This breaks cgroup design,
>>>> which specially introduces this two function to differ allocation and onlining.
>>>> Also, I've just move the allocation to alloc_mem_cgroup_per_node_info() like it was
>>>> suggested in comments to v1...
>>>
>>> Yeah, but (ab)using mem_cgroup_idr for iterating over all allocated
>>> memory cgroups looks rather dubious to me...
>>
>> But we have to iterate over all allocated memory cgroups in any way,
>> as all of them must have expanded maps. What is the problem?
>> It's rather simple method, and it faster then for_each_mem_cgroup()
>> cycle, since it does not have to play with get and put of refcounters.
> 
> I don't like this, because mem_cgroup_idr was initially introduced to
> avoid depletion of css ids by offline cgroups. We could fix that problem
> by extending swap_cgroup to UINT_MAX, in which case mem_cgroup_idr
> wouldn't be needed at all. Reusing mem_cgroup_idr for iterating over
> allocated cgroups deprives us of the ability to reconsider that design
> decision in future, neither does it look natural IMO. Besides, in order
> to use mem_cgroup_idr for your purpose, you have to reshuffle the code
> of mem_cgroup_css_alloc in a rather contrived way IMO.
> 
> I agree that allocating parts of struct mem_cgroup in css_online may
> look dubious, but IMHO it's better than inventing a new way to iterate
> over cgroups instead of using the iterator provided by cgroup core.
> May be, you should ask Tejun which way he thinks is better.
> 
> Thanks,
> Vladimir
>