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@linux.ibm.com
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_secretmemFurther 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@intel.com Signed-off-by: Mike Rapoport rppt@linux.ibm.com
@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 :)