Re: [PATCH] secretmem: optimize page_is_secretmem()

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 <[email protected]>
>>>
>>> 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 <[email protected]>
>>> Signed-off-by: Mike Rapoport <[email protected]>
>>> ---
>>>
>>> @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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