Re: [PATCH] slub: limit count of partial slabs scanned to gather statistics

Vlastimil Babka
2020-05-07T14:12:33Z

On 5/7/20 7:25 AM, Konstantin Khlebnikov wrote:
> On 06/05/2020 14.56, Vlastimil Babka wrote:
>> On 5/4/20 6:07 PM, Konstantin Khlebnikov wrote:
>>> To get exact count of free and used objects slub have to scan list of
>>> partial slabs. This may take at long time. Scanning holds spinlock and
>>> blocks allocations which move partial slabs to per-cpu lists and back.
>>>
>>> Example found in the wild:
>>>
>>> # cat /sys/kernel/slab/dentry/partial
>>> 14478538 N0=7329569 N1=7148969
>>> # time cat /sys/kernel/slab/dentry/objects
>>> 286225471 N0=136967768 N1=149257703
>>>
>>> real 0m1.722s
>>> user 0m0.001s
>>> sys 0m1.721s
>>>
>>> The same problem in slab was addressed in commit f728b0a5d72a ("mm, slab:
>>> faster active and free stats") by adding more kmem cache statistics.
>>> For slub same approach requires atomic op on fast path when object frees.
>>
>> In general yeah, but are you sure about this one? AFAICS this is about pages in
>> the n->partial list, where manipulations happen under n->list_lock and shouldn't
>> be fast path. It should be feasible to add a counter under the same lock, so it
>> wouldn't even need to be atomic?
>
> SLUB allocates objects from prepared per-cpu slabs, they could be subtracted from
> count of free object under this lock in advance when slab moved out of this list.
>
> But at freeing path object might belong to any slab, including global partials.

Right, freeing can indeed modify a global partial without taking the lock.
Nevermind then.