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

[Mateusz Guzik <mjguzik@gmail.com>]

On Thu, Aug 1, 2024 at 3:34 PM David Hildenbrand <david@redhat.com> wrote:
>
> 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.
>

why not massage _compound_head back to an inlineable form instead? for
all i know you may even register a small win in total

-- 
Mateusz Guzik <mjguzik gmail.com>