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

[Mateusz Guzik <mjguzik@gmail.com>]

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.