Re: [RFC PATCH v3 0/7] slub: Delay freezing of CPU partial slabs

[Chengming Zhou <chengming.zhou@linux.dev>]

On 2023/10/31 00:19, Vlastimil Babka wrote:
> On 10/28/23 04:36, Chengming Zhou wrote:
>>>
>>>
>>> After this patch the PG_workingset indicates the state of being on the partial lists.
>>>
>>> What does "frozen slab" then mean? The slab is being allocated from? Is that information useful or can we drop the frozen flag?
>>
>> Right, frozen slab is the cpu slab, which is being allocated from by the cpu that froze it.
>>
>> IMHO, the "frozen" bit is useful because:
>>
>> 1. PG_workingset is only useful on partial slab, which indicates the slab is on the node
>>    partial list, so we can manipulate its list in the __slab_free() path.
>>
>> 2. But for full slab (slab->freelist == NULL), PG_workingset is not much useful, we don't
>>    safely know whether it's used as the cpu slab or not just from this flag. So __slab_free()
>>    still rely on the "frozen" bit to know it.
> 
> Well, we could extend the meaning of PG_workingset to mean "not a cpu slab
> or pecpu partial slab" i.e. both on node partial list and full. However it
> would increase the number of cases where __slab_free() has to lock the
> list_lock and check the PG_working set. "slab->freelist == NULL" might
> happen often exactly because the freelist became cpu freelist.

Ah, right, it's possible to do like this.

> 
>> 3. And the maintaining of "frozen" has no extra cost now, since it's changed together with "freelist"
>>    and other counter using cmpxchg, we already have the cmpxchg when start to use a slab as the cpu slab.
> 
> And together with this point, I don't see a reason to drop the frozen bit.
> It's still useful for cpu slabs. It just wasn't the best possible solution
> for percpu partial slabs.
> 
>> Maybe I missed something, I don't know how to drop the frozen flag.
> 
> Should be possible, but not worth it IMHO.

Agree, we'll just keep "frozen" for the cpu slabs.

Thanks!