Re: [PATCH v2 2/6] slub: Use alloc_pages_node() in alloc_slab_page()

[Vlastimil Babka <vbabka@suse.cz>]

On 7/11/24 8:04 PM, Christoph Lameter (Ampere) wrote:
> On Thu, 11 Jul 2024, Vlastimil Babka wrote:
> 
>>> There are also the cpuset/cgroup restrictions via the zonelists that are
>>> bypassed by removing alloc_pages()
>>
>> AFAICS cpusets are handled on a level that's reached by both paths, i.e.
>> prepare_alloc_pages(), and I see nothing that would make switching to
>> alloc_pages_node() bypass it. Am I missing something?
> 
> You are correct. cpuset/cgroup restrictions also apply to 
> alloc_pages_node().
> 
>>> We have some internal patches now that implement memory policies on a per
>>> object basis for SLUB here.
>>>
>>> This is a 10-15% regression on various benchmarks when objects like the
>>> scheduler statistics structures are misplaced.
>>
>> I believe it would be best if you submitted a patch with with all that
>> reasoning. Thanks!

I still believe that :)

> Turns out those performance issues are related to the issue that NUMA 
> locality is only considered at the folio level for slab allocation. 
> Individual object allocations are not subject to it.
> 
> The performance issue comes about in the following way:
> 
> Two kernel threads run on the same cpu using the same slab cache. One of 
> them keeps on allocating from a different node via kmalloc_node() and the 
> other is using kmalloc(). Then the kmalloc_node() thread will always 
> ensure that the per cpu slab is from the other node.
> 
> The other thread will use kmalloc() which does not check which node the 
> per cpu slab is from. Therefore the kmallc thread can continually be 
> served objects that are not local. That is not good and causes 
> misplacement of objects.
> 
> But that issue is something separate from this commit here and we see the 
> same regression before this commit.
> 
> This patch still needs to be reverted since the rationale for the patch 
> is not right and it disables memory policy support. Results in the 
> strange situation that memory policies are used in get_any_partial() in 
> slub but not during allocation anymore.
>