Re: [linus:master] [mm] c0bff412e6: stress-ng.clone.ops_per_sec -2.9% regression
From: Yin Fengwei
Date: Tue Aug 13 2024 - 03:10:15 EST
On 8/12/24 00:49, 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.
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 5769fe6e4950..2d5d61ab385b 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -194,34 +194,13 @@ enum pageflags {
#ifdef CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP
DECLARE_STATIC_KEY_FALSE(hugetlb_optimize_vmemmap_key);
-/*
- * Return the real head page struct iff the @page is a fake head page, otherwise
- * return the @page itself. See Documentation/mm/vmemmap_dedup.rst.
- */
+const struct page *_page_fixed_fake_head(const struct page *page);
+
static __always_inline const struct page *page_fixed_fake_head(const struct page *page)
{
if (!static_branch_unlikely(&hugetlb_optimize_vmemmap_key))
return page;
-
- /*
- * Only addresses aligned with PAGE_SIZE of struct page may be fake head
- * struct page. The alignment check aims to avoid access the fields (
- * e.g. compound_head) of the @page[1]. It can avoid touch a (possibly)
- * cold cacheline in some cases.
- */
- if (IS_ALIGNED((unsigned long)page, PAGE_SIZE) &&
- test_bit(PG_head, &page->flags)) {
- /*
- * We can safely access the field of the @page[1] with PG_head
- * because the @page is a compound page composed with at least
- * two contiguous pages.
- */
- unsigned long head = READ_ONCE(page[1].compound_head);
-
- if (likely(head & 1))
- return (const struct page *)(head - 1);
- }
- return page;
+ return _page_fixed_fake_head(page);
}
#else
static inline const struct page *page_fixed_fake_head(const struct page *page)
@@ -235,7 +214,7 @@ 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);
diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index 829112b0a914..3fbc00db607a 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -19,6 +19,33 @@
#include <asm/tlbflush.h>
#include "hugetlb_vmemmap.h"
+/*
+ * Return the real head page struct iff the @page is a fake head page, otherwise
+ * return the @page itself. See Documentation/mm/vmemmap_dedup.rst.
+ */
+const struct page *_page_fixed_fake_head(const struct page *page)
+{
+ /*
+ * Only addresses aligned with PAGE_SIZE of struct page may be fake head
+ * struct page. The alignment check aims to avoid access the fields (
+ * e.g. compound_head) of the @page[1]. It can avoid touch a (possibly)
+ * cold cacheline in some cases.
+ */
+ if (IS_ALIGNED((unsigned long)page, PAGE_SIZE) &&
+ test_bit(PG_head, &page->flags)) {
+ /*
+ * We can safely access the field of the @page[1] with PG_head
+ * because the @page is a compound page composed with at least
+ * two contiguous pages.
+ */
+ unsigned long head = READ_ONCE(page[1].compound_head);
+
+ if (likely(head & 1))
+ return (const struct page *)(head - 1);
+ }
+ return page;
+}
+
/**
* struct vmemmap_remap_walk - walk vmemmap page table
*
The change can resolve the regression (from -3% to 0.5%):
Please note:
9cb28da54643ad464c47585cd5866c30b0218e67 is the parent commit
3f16e4b516ef02d9461b7e0b6c50e05ba0811886 is the commit with above
patch
c0bff412e67b781d761e330ff9578aa9ed2be79e is the commit which
introduced regression
=========================================================================================
tbox_group/testcase/rootfs/kconfig/compiler/nr_threads/testtime/test/cpufreq_governor/debug-setup:
lkp-icl-2sp8/stress-ng/debian-12-x86_64-20240206.cgz/x86_64-rhel-8.3/gcc-12/100%/60s/clone/performance/yfw_test2
commit:
9cb28da54643ad464c47585cd5866c30b0218e67
3f16e4b516ef02d9461b7e0b6c50e05ba0811886
c0bff412e67b781d761e330ff9578aa9ed2be79e
9cb28da54643ad46 3f16e4b516ef02d9461b7e0b6c5 c0bff412e67b781d761e330ff95
---------------- --------------------------- ---------------------------
fail:runs %reproduction fail:runs %reproduction fail:runs
| | | | |
3:3 0% 3:3 0% 3:3
stress-ng.clone.microsecs_per_clone.pass
3:3 0% 3:3 0% 3:3
stress-ng.clone.pass
%stddev %change %stddev %change %stddev
\ | \ | \
2904 -0.6% 2886 +3.7% 3011
stress-ng.clone.microsecs_per_clone
563520 +0.5% 566296 -3.1% 546122
stress-ng.clone.ops
9306 +0.5% 9356 -3.0% 9024
stress-ng.clone.ops_per_sec
BTW, the change needs to export symbol _page_fixed_fake_head otherwise
some modules hit build error.
Regards
Yin, Fengwei