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.
