Re: [PATCH] secretmem: optimize page_is_secretmem()
From: David Hildenbrand
Date: Mon Apr 19 2021 - 05:41:16 EST
On 19.04.21 11:38, David Hildenbrand wrote:
On 19.04.21 11:36, Mike Rapoport wrote:
On Mon, Apr 19, 2021 at 11:15:02AM +0200, David Hildenbrand wrote:
On 19.04.21 10:42, Mike Rapoport wrote:
From: Mike Rapoport <rppt@xxxxxxxxxxxxx>
Kernel test robot reported -4.2% regression of will-it-scale.per_thread_ops
due to commit "mm: introduce memfd_secret system call to create "secret"
memory areas".
The perf profile of the test indicated that the regression is caused by
page_is_secretmem() called from gup_pte_range() (inlined by gup_pgd_range):
27.76 +2.5 30.23 perf-profile.children.cycles-pp.gup_pgd_range
0.00 +3.2 3.19 ± 2% perf-profile.children.cycles-pp.page_mapping
0.00 +3.7 3.66 ± 2% perf-profile.children.cycles-pp.page_is_secretmem
Further analysis showed that the slow down happens because neither
page_is_secretmem() nor page_mapping() are not inline and moreover,
multiple page flags checks in page_mapping() involve calling
compound_head() several times for the same page.
Make page_is_secretmem() inline and replace page_mapping() with page flag
checks that do not imply page-to-head conversion.
Reported-by: kernel test robot <oliver.sang@xxxxxxxxx>
Signed-off-by: Mike Rapoport <rppt@xxxxxxxxxxxxx>
---
@Andrew,
The patch is vs v5.12-rc7-mmots-2021-04-15-16-28, I'd appreciate if it would
be added as a fixup to the memfd_secret series.
include/linux/secretmem.h | 26 +++++++++++++++++++++++++-
mm/secretmem.c | 12 +-----------
2 files changed, 26 insertions(+), 12 deletions(-)
diff --git a/include/linux/secretmem.h b/include/linux/secretmem.h
index 907a6734059c..b842b38cbeb1 100644
--- a/include/linux/secretmem.h
+++ b/include/linux/secretmem.h
@@ -4,8 +4,32 @@
#ifdef CONFIG_SECRETMEM
+extern const struct address_space_operations secretmem_aops;
+
+static inline bool page_is_secretmem(struct page *page)
+{
+ struct address_space *mapping;
+
+ /*
+ * Using page_mapping() is quite slow because of the actual call
+ * instruction and repeated compound_head(page) inside the
+ * page_mapping() function.
+ * We know that secretmem pages are not compound and LRU so we can
+ * save a couple of cycles here.
+ */
+ if (PageCompound(page) || !PageLRU(page))
+ return false;
I'd assume secretmem pages are rare in basically every setup out there. So
maybe throwing in a couple of likely()/unlikely() might make sense.
I'd say we could do unlikely(page_is_secretmem()) at call sites. Here I can
hardly estimate which pages are going to be checked.
+
+ mapping = (struct address_space *)
+ ((unsigned long)page->mapping & ~PAGE_MAPPING_FLAGS);
+
Not sure if open-coding page_mapping is really a good idea here -- or even
necessary after the fast path above is in place. Anyhow, just my 2 cents.
Well, most if the -4.2% of the performance regression kbuild reported were
due to repeated compount_head(page) in page_mapping(). So the whole point
of this patch is to avoid calling page_mapping().
I would have thought the fast path "(PageCompound(page) ||
!PageLRU(page))" would already avoid calling page_mapping() in many cases.
(and I do wonder if a generic page_mapping() optimization would make
sense instead)
Willy can most probably give the best advise here :)
--
Thanks,
David / dhildenb