On 19.04.21 11:36, Mike Rapoport wrote:

On Mon, Apr 19, 2021 at 11:15:02AM +0200, David Hildenbrand wrote:

quoted

On 19.04.21 10:42, Mike Rapoport wrote:

quoted

From: Mike Rapoport <redacted>

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 <redacted>
Signed-off-by: Mike Rapoport <redacted>
---

@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.
  

quoted

quoted

+
+	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.


-- 
Thanks,

David / dhildenb


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