Re: [PATCH] mm/vmalloc: request large order pages from buddy allocator

Dev Jain
2025-12-11T15:43:33Z

On 11/12/25 9:09 pm, Uladzislau Rezki wrote:
> On Thu, Dec 11, 2025 at 03:28:56PM +0000, Ryan Roberts wrote:
>> On 10/12/2025 22:28, Vishal Moola (Oracle) wrote:
>>> On Wed, Dec 10, 2025 at 01:21:22PM +0000, Ryan Roberts wrote:
>>>> Hi Vishal,
>>>>
>>>>
>>>> On 21/10/2025 20:44, Vishal Moola (Oracle) wrote:
>>>>> Sometimes, vm_area_alloc_pages() will want many pages from the buddy
>>>>> allocator. Rather than making requests to the buddy allocator for at
>>>>> most 100 pages at a time, we can eagerly request large order pages a
>>>>> smaller number of times.
>>>>>
>>>>> We still split the large order pages down to order-0 as the rest of the
>>>>> vmalloc code (and some callers) depend on it. We still defer to the bulk
>>>>> allocator and fallback path in case of order-0 pages or failure.
>>>>>
>>>>> Running 1000 iterations of allocations on a small 4GB system finds:
>>>>>
>>>>> 1000 2mb allocations:
>>>>> [Baseline] [This patch]
>>>>> real 46.310s real 0m34.582
>>>>> user 0.001s user 0.006s
>>>>> sys 46.058s sys 0m34.365s
>>>>>
>>>>> 10000 200kb allocations:
>>>>> [Baseline] [This patch]
>>>>> real 56.104s real 0m43.696
>>>>> user 0.001s user 0.003s
>>>>> sys 55.375s sys 0m42.995s
>>>> I'm seeing some big vmalloc micro benchmark regressions on arm64, for which
>>>> bisect is pointing to this patch.
>>> Ulad had similar findings/concerns[1]. Tldr: The numbers you are seeing
>>> are expected for how the test module is currently written.
>> Hmm... simplistically, I'd say that either the tests are bad, in which case they
>> should be deleted, or they are good, in which case we shouldn't ignore the
>> regressions. Having tests that we learn to ignore is the worst of both worlds.
>>
> Uh.. Tests are for measure vmalloc performance and stressing. They can not be just
> removed :) In some sense they are synthetic, from the other hand they allow to find
> problems and bottle-necks + measure perf. You have identified regression with it :)
>
> I think, the problem is in the
>
> + 14.05% 0.11% [kernel] [k] remove_vm_area
> + 11.85% 1.82% [kernel] [k] __alloc_frozen_pages_noprof
> + 10.91% 0.36% [kernel] [k] __get_vm_area_node
> + 10.60% 7.58% [kernel] [k] insert_vmap_area
> + 10.02% 4.67% [kernel] [k] get_page_from_freelist
>
>
> get_page_from_freelist() call. With a patch it adds 10% of cycles on
> top whereas without patch i do not see the symbol at all, i.e. pages
> are obtained really fast from the pcp list, not from the body.
>
> The question is, why high-order pages are not end-up in the pcp-cache?
> I think it is due to the fact, that we split such pages and freeing them
> as order-0 one.

Please take a look at my RFC:

https://lore.kernel.org/all/20251112110807.69958-1-dev.jain@arm.com/

You are right, we allocate large folios but then split them up and free
them as basepages. In patch 2 I have proved (not rigorously) that pcp
draining is one of the issues.

>
> Any thoughts?
>
> --
> Uladzislau Rezki
>