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

[David Hildenbrand]

On 12.08.24 06:49, Mateusz Guzik wrote:
> On Mon, Aug 12, 2024 at 12:43:08PM +0800, Yin Fengwei wrote:
> > Hi David,
> >
> > On 8/1/24 09:44, David Hildenbrand wrote:
> > > On 01.08.24 15:37, Mateusz Guzik wrote:
> > > > On Thu, Aug 1, 2024 at 3:34 PM David Hildenbrand <david@xxxxxxxxxx>
> > > > 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:
> > Do you want us to test this change? Or you have further optimization
> > ongoing? Thanks.
>
> I verified the thing below boots, I have no idea about performance. If
> it helps it can be massaged later from style perspective.

As quite a lot of setups already run with the vmemmap optimization enabled, I
wonder how effective this would be (might need more fine tuning, did not look
at the generated code):


diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 085dd8dcbea2..7ddcdbd712ec 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -233,7 +233,7 @@ static __always_inline int page_is_fake_head(const struct page *page)
        return page_fixed_fake_head(page) != page;
 }
 
-static inline unsigned long _compound_head(const struct page *page)
+static __always_inline unsigned long _compound_head(const struct page *page)
 {
        unsigned long head = READ_ONCE(page->compound_head);
 


--
Cheers,

David / dhildenb