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

[Vlastimil Babka <vbabka@suse.cz>]

On 7/10/24 6:43 PM, Christoph Lameter (Ampere) wrote:
> On Wed, 10 Jul 2024, Vlastimil Babka wrote:
> 
>>> With this patch cgroup restrictions memory policies etc etc no longer work
>>> in the slab allocator.
>>
>> The only difference is memory policy from get_task_policy(), and the rest is
>> the same, right?
> 
> 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?

>> But this only affects new slab page allocation, while getting objects from
>> existing slabs isn't subject to memory policies, so now it's at least
>> consistent? Do you have some use case where it matters?
> 
> Yes this means you cannot redirect kmalloc based kernel metadata 
> allocation when creating f.e. cgroups for another NUMA node. This affects 
> all kernel metadata allocation during syscalls that used to be 
> controllable via numa methods.
> 
> SLAB implemented memory allocations policies per object. SLUB moved that 
> to implement these policies only when allocating a page frame. If this 
> patch is left in then there wont be any support for memory allocation 
> policies left in the slab allocators.
> 
> 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!