[PATCH] secretmem: optimize page_is_secretmem()
David Hildenbrand
david at redhat.com
Mon Apr 19 10:38:40 BST 2021
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 at 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_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 at intel.com>
>>> Signed-off-by: Mike Rapoport <rppt at 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.
--
Thanks,
David / dhildenb
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