Re: [PATCH 01/10] mm: Assign id to every memcg-aware shrinker

[Kirill Tkhai <ktkhai@virtuozzo.com>]

On 27.03.2018 12:15, Vladimir Davydov wrote:
> On Mon, Mar 26, 2018 at 06:09:35PM +0300, Kirill Tkhai wrote:
>> Hi, Vladimir,
>>
>> thanks for your review!
>>
>> On 24.03.2018 21:40, Vladimir Davydov wrote:
>>> Hello Kirill,
>>>
>>> I don't have any objections to the idea behind this patch set.
>>> Well, at least I don't know how to better tackle the problem you
>>> describe in the cover letter. Please, see below for my comments
>>> regarding implementation details.
>>>
>>> On Wed, Mar 21, 2018 at 04:21:17PM +0300, Kirill Tkhai wrote:
>>>> The patch introduces shrinker::id number, which is used to enumerate
>>>> memcg-aware shrinkers. The number start from 0, and the code tries
>>>> to maintain it as small as possible.
>>>>
>>>> This will be used as to represent a memcg-aware shrinkers in memcg
>>>> shrinkers map.
>>>>
>>>> Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
>>>> ---
>>>>  include/linux/shrinker.h |    1 +
>>>>  mm/vmscan.c              |   59 ++++++++++++++++++++++++++++++++++++++++++++++
>>>>  2 files changed, 60 insertions(+)
>>>>
>>>> diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
>>>> index a3894918a436..738de8ef5246 100644
>>>> --- a/include/linux/shrinker.h
>>>> +++ b/include/linux/shrinker.h
>>>> @@ -66,6 +66,7 @@ struct shrinker {
>>>>  
>>>>  	/* These are for internal use */
>>>>  	struct list_head list;
>>>> +	int id;
>>>
>>> This definition could definitely use a comment.
>>>
>>> BTW shouldn't we ifdef it?
>>
>> Ok
>>
>>>>  	/* objs pending delete, per node */
>>>>  	atomic_long_t *nr_deferred;
>>>>  };
>>>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>>>> index 8fcd9f8d7390..91b5120b924f 100644
>>>> --- a/mm/vmscan.c
>>>> +++ b/mm/vmscan.c
>>>> @@ -159,6 +159,56 @@ unsigned long vm_total_pages;
>>>>  static LIST_HEAD(shrinker_list);
>>>>  static DECLARE_RWSEM(shrinker_rwsem);
>>>>  
>>>> +#if defined(CONFIG_MEMCG) && !defined(CONFIG_SLOB)
>>>> +static DEFINE_IDA(bitmap_id_ida);
>>>> +static DECLARE_RWSEM(bitmap_rwsem);
>>>
>>> Can't we reuse shrinker_rwsem for protecting the ida?
>>
>> I think it won't be better, since we allocate memory under this semaphore.
>> After we use shrinker_rwsem, we'll have to allocate the memory with GFP_ATOMIC,
>> which does not seems good. Currently, the patchset makes shrinker_rwsem be taken
>> for a small time, just to assign already allocated memory to maps.
> 
> AFAIR it's OK to sleep under an rwsem so GFP_ATOMIC wouldn't be
> necessary. Anyway, we only need to allocate memory when we extend
> shrinker bitmaps, which is rare. In fact, there can only be a limited
> number of such calls, as we never shrink these bitmaps (which is fine
> by me).

We take bitmap_rwsem for writing to expand shrinkers maps. If we replace
it with shrinker_rwsem and the memory allocation get into reclaim, there
will be deadlock.

>>
>>>> +static int bitmap_id_start;
>>>> +
>>>> +static int alloc_shrinker_id(struct shrinker *shrinker)
>>>> +{
>>>> +	int id, ret;
>>>> +
>>>> +	if (!(shrinker->flags & SHRINKER_MEMCG_AWARE))
>>>> +		return 0;
>>>> +retry:
>>>> +	ida_pre_get(&bitmap_id_ida, GFP_KERNEL);
>>>> +	down_write(&bitmap_rwsem);
>>>> +	ret = ida_get_new_above(&bitmap_id_ida, bitmap_id_start, &id);
>>>
>>> AFAIK ida always allocates the smallest available id so you don't need
>>> to keep track of bitmap_id_start.
>>
>> I saw mnt_alloc_group_id() does the same, so this was the reason, the additional
>> variable was used. Doesn't this gives a good advise to ida and makes it find
>> a free id faster?
> 
> As Matthew pointed out, this is rather pointless.

Kirill