Re: [PATCH HOTFIX 6.17] mm/mremap: avoid expensive folio lookup on mremap folio pte batch

[David Hildenbrand <david@redhat.com>]

On 07.08.25 21:22, Lorenzo Stoakes wrote:
> On Thu, Aug 07, 2025 at 08:14:09PM +0100, Pedro Falcato wrote:
>> On Thu, Aug 07, 2025 at 07:58:19PM +0100, Lorenzo Stoakes wrote:
>>> It was discovered in the attached report that commit f822a9a81a31 ("mm:
>>> optimize mremap() by PTE batching") introduced a significant performance
>>> regression on a number of metrics on x86-64, most notably
>>> stress-ng.bigheap.realloc_calls_per_sec - indicating a 37.3% regression in
>>> number of mremap() calls per second.
>>>
>>> I was able to reproduce this locally on an intel x86-64 raptor lake system,
>>> noting an average of 143,857 realloc calls/sec (with a stddev of 4,531 or
>>> 3.1%) prior to this patch being applied, and 81,503 afterwards (stddev of
>>> 2,131 or 2.6%) - a 43.3% regression.
>>>
>>> During testing I was able to determine that there was no meaningful
>>> difference in efforts to optimise the folio_pte_batch() operation, nor
>>> checking folio_test_large().
>>>
>>> This is within expectation, as a regression this large is likely to
>>> indicate we are accessing memory that is not yet in a cache line (and
>>> perhaps may even cause a main memory fetch).
>>>
>>> The expectation by those discussing this from the start was that
>>> vm_normal_folio() (invoked by mremap_folio_pte_batch()) would likely be the
>>> culprit due to having to retrieve memory from the vmemmap (which mremap()
>>> page table moves does not otherwise do, meaning this is inevitably cold
>>> memory).
>>>
>>> I was able to definitively determine that this theory is indeed correct and
>>> the cause of the issue.
>>>
>>> The solution is to restore part of an approach previously discarded on
>>> review, that is to invoke pte_batch_hint() which explicitly determines,
>>> through reference to the PTE alone (thus no vmemmap lookup), what the PTE
>>> batch size may be.
>>>
>>> On platforms other than arm64 this is currently hardcoded to return 1, so
>>> this naturally resolves the issue for x86-64, and for arm64 introduces
>>> little to no overhead as the pte cache line will be hot.
>>>
>>> With this patch applied, we move from 81,503 realloc calls/sec to
>>> 138,701 (stddev of 496.1 or 0.4%), which is a -3.6% regression, however
>>> accounting for the variance in the original result, this is broadly
>>> restoring performance to its prior state.
>>>
>>
>> So, do we still have a regression then? If so, do we have any idea why?
> 
> It's within 1 stddev of the original results, so I'd say it's possibly
> noise.

It's very likely noise. And even if it's not, even a simple code layout 
change by the compiler can provoke something like that.

-- 
Cheers,

David / dhildenb