Re: [linus:master] [mm] c0bff412e6: stress-ng.clone.ops_per_sec -2.9% regression

[David Hildenbrand <david@redhat.com>]

On 01.08.24 15:30, Mateusz Guzik wrote:
> On Thu, Aug 01, 2024 at 08:49:27AM +0200, David Hildenbrand wrote:
>> Yes indeed. fork() can be extremely sensitive to each added instruction.
>>
>> I even pointed out to Peter why I didn't add the PageHuge check in there
>> originally [1].
>>
>> "Well, and I didn't want to have runtime-hugetlb checks in
>> PageAnonExclusive code called on certainly-not-hugetlb code paths."
>>
>>
>> We now have to do a page_folio(page) and then test for hugetlb.
>>
>> 	return folio_test_hugetlb(page_folio(page));
>>
>> Nowadays, folio_test_hugetlb() will be faster than at c0bff412e6 times, so
>> maybe at least part of the overhead is gone.
>>
> 
> I'll note page_folio expands to a call to _compound_head.
> 
> While _compound_head is declared as an inline, it ends up being big
> enough that the compiler decides to emit a real function instead and
> real func calls are not particularly cheap.
> 
> I had a brief look with a profiler myself and for single-threaded usage
> the func is quite high up there, while it manages to get out with the
> first branch -- that is to say there is definitely performance lost for
> having a func call instead of an inlined branch.
> 
> The routine is deinlined because of a call to page_fixed_fake_head,
> which itself is annotated with always_inline.
> 
> This is of course patchable with minor shoveling.
> 
> I did not go for it because stress-ng results were too unstable for me
> to confidently state win/loss.
> 
> But should you want to whack the regression, this is what I would look
> into.
> 

This might improve it, at least for small folios I guess:

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 5769fe6e4950..7796ae116018 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -1086,7 +1086,7 @@ PAGE_TYPE_OPS(Zsmalloc, zsmalloc, zsmalloc)
   */
  static inline bool PageHuge(const struct page *page)
  {
-       return folio_test_hugetlb(page_folio(page));
+       return PageCompound(page) && folio_test_hugetlb(page_folio(page));
  }
  
  /*


We would avoid the function call for small folios.

-- 
Cheers,

David / dhildenb