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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:   Tue, 24 Apr 2018 15:13:53 +0300
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Let's discuss on code with changes after your commits to v2 to have them made visible.
v3 is on the way

Kirill

On 24.04.2018 14:28, Vladimir Davydov wrote:
> On Mon, Apr 23, 2018 at 01:54:50PM +0300, Kirill Tkhai wrote:
>>>> @@ -1200,6 +1206,8 @@ extern int memcg_nr_cache_ids;
>>>>  void memcg_get_cache_ids(void);
>>>>  void memcg_put_cache_ids(void);
>>>>  
>>>> +extern int shrinkers_max_nr;
>>>> +
>>>
>>> memcg_shrinker_id_max?
>>
>> memcg_shrinker_id_max sounds like an includive value, doesn't it?
>> While shrinker->id < shrinker_max_nr.
>>
>> Let's better use memcg_shrinker_nr_max.
> 
> or memcg_nr_shrinker_ids (to match memcg_nr_cache_ids), not sure...
> 
> Come to think of it, this variable is kinda awkward: it is defined in
> vmscan.c but declared in memcontrol.h; it is used by vmscan.c for max
> shrinker id and by memcontrol.c for shrinker map capacity. Just a raw
> idea: what about splitting it in two: one is private to vmscan.c, used
> as max id, say we call it shrinker_id_max; the other is defined in
> memcontrol.c and is used for shrinker map capacity, say we call it
> memcg_shrinker_map_capacity. What do you think?
> 
>>>> +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?
> 
>>  
>>>> +			if (id == 1)
>>>> +				memcg = NULL;
>>>> +			ret = memcg_expand_maps(memcg, node, size, old_size);
>>>> +			if (ret)
>>>> +				goto unlock;
>>>> +		}
>>>> +
>>>> +		/* root_mem_cgroup is not initialized yet */
>>>> +		if (id == 0)
>>>> +			ret = memcg_expand_maps(NULL, node, size, old_size);
>>>> +	}
>>>> +unlock:
>>>> +	up_write(&shrinkers_max_nr_rwsem);
>>>> +	return ret;
>>>> +}