Re: [PATCH 02/10] mm: Maintain memcg-aware shrinkers in mcg_shrinkers array

[Kirill Tkhai <ktkhai@virtuozzo.com>]

On 27.03.2018 12:18, Vladimir Davydov wrote:
> On Mon, Mar 26, 2018 at 06:20:55PM +0300, Kirill Tkhai wrote:
>> On 24.03.2018 21:45, Vladimir Davydov wrote:
>>> On Wed, Mar 21, 2018 at 04:21:29PM +0300, Kirill Tkhai wrote:
>>>> The patch introduces mcg_shrinkers array to keep memcg-aware
>>>> shrinkers in order of their shrinker::id.
>>>>
>>>> This allows to access the shrinkers dirrectly by the id,
>>>> without iteration over shrinker_list list.
>>>
>>> Why don't you simply use idr instead of ida? With idr you wouldn't need
>>> the array mapping shrinker id to shrinker ptr. AFAIU you need this
>>> mapping to look up the shrinker by id in shrink_slab. The latter doesn't
>>> seem to be a hot path so using idr there should be acceptable. Since we
>>> already have shrinker_rwsem, which is taken for reading by shrink_slab,
>>> we wouldn't even need any additional locking for it.
>>
>> The reason is ida may allocate memory, and since list_lru_add() can't fail,
>> we can't do that there. If we allocate all the ida memory at the time of
>> memcg creation (i.e., preallocate it), this is not different to the way
>> the bitmap makes.
>>
>> While bitmap has the agvantage, since it's simplest data structure (while
>> ida has some radix tree overhead).
>>
>> Also, bitmap does not require a lock, there is single atomic operation
>> to set or clear a bit, and it scales better, when anything.
> 
> I didn't mean the per-memcg bitmaps - I think it's OK to use plain
> arrays for them and reallocate them with the aid of RCU.
> 
> What I actually mean is the mapping shrink_id => shrinker. AFAIU it
> isn't accessed from list_lru, it is only needed to look up a shrinker
> by id from shrink_slab(). The latter is rather a slow path so I think
> we can use an IDR for this mapping instead of IDA + plain array.

This is good idea.

Thanks,
Kirill