Re: [PATCH] slub: limit number of slabs to scan in count_partial()

[Jianfeng Wang <jianfeng.w.wang@oracle.com>]

On 4/12/24 1:20 PM, Vlastimil Babka wrote:
> On 4/12/24 7:29 PM, Jianfeng Wang wrote:
>>
>>
>> On 4/12/24 12:48 AM, Vlastimil Babka wrote:
>>> On 4/11/24 7:02 PM, Christoph Lameter (Ampere) wrote:
>>>> On Thu, 11 Apr 2024, Jianfeng Wang wrote:
>>>>
>>>>> So, the fix is to limit the number of slabs to scan in
>>>>> count_partial(), and output an approximated result if the list is too
>>>>> long. Default to 10000 which should be enough for most sane cases.
>>>>
>>>>
>>>> That is a creative approach. The problem though is that objects on the 
>>>> partial lists are kind of sorted. The partial slabs with only a few 
>>>> objects available are at the start of the list so that allocations cause 
>>>> them to be removed from the partial list fast. Full slabs do not need to 
>>>> be tracked on any list.
>>>>
>>>> The partial slabs with few objects are put at the end of the partial list 
>>>> in the hope that the few objects remaining will also be freed which would 
>>>> allow the freeing of the slab folio.
>>>>
>>>> So the object density may be higher at the beginning of the list.
>>>>
>>>> kmem_cache_shrink() will explicitly sort the partial lists to put the 
>>>> partial pages in that order.
>>>>
>>>> Can you run some tests showing the difference between the estimation and 
>>>> the real count?
>>
>> Yes.
>> On a server with one NUMA node, I create a case that uses many dentry objects.
> 
> Could you describe in more detail how do you make dentry cache to grow such
> a large partial slabs list? Thanks.
> 

I utilized the fact that creating a folder will create a new dentry object;
deleting a folder will delete all its sub-folder's dentry objects.

Then, I started to create N folders, while each folder has M empty sub-folders.
Assuming that these operations would consume a large number of dentry
objects in the sequential order. Their slabs were very likely to be full slabs.
After all folders were created, I deleted a subset of the N folders (i.e.,
one out of every two folders). This would create many holes, which turned a
subset of full slabs into partial slabs.