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

[Kirill Tkhai]

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@xxxxxxxxxxxxx>
>> ---
>>  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.

>> +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?
 
>> +	if (!ret) {
>> +		shrinker->id = id;
>> +		bitmap_id_start = shrinker->id + 1;
>> +	}
>> +	up_write(&bitmap_rwsem);
>> +	if (ret == -EAGAIN)
>> +		goto retry;
>> +
>> +	return ret;
>> +}

Thanks,
Kirill