On 12.08.24 10:18, Mateusz Guzik wrote:
On Mon, Aug 12, 2024 at 10:12 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
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);
Well one may need to justify it with bloat-o-meter which is why I did
not just straight up inline the entire thing.
But if you are down to fight opposition of the sort I agree this is
the patch to benchmark. :)
I spotted that we already to that for
PageHead()/PageTail()/page_is_fake_head(). So we effectively
force-inline it everywhere except into _compound_head() I think.
But yeah, measuring the bloat would be a necessary exercise.
--
Cheers,
David / dhildenb
