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

Uladzislau Rezki
2025-12-11T15:39:50Z

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.

Any thoughts?

--
Uladzislau Rezki