Re: [PATCH] secretmem: optimize page_is_secretmem()

[David Hildenbrand <david@redhat.com>]

On 19.04.21 12:14, Mike Rapoport wrote:
> On Mon, Apr 19, 2021 at 11:40:56AM +0200, David Hildenbrand wrote:
>> 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_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@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)
> 
> Not sure. Replacing page_mapping() with page_file_mapping() at the
> call sites at fs/ and mm/ increased the defconfig image by nearly 2k
> and page_file_mapping() is way simpler than page_mapping()
> 
> add/remove: 1/0 grow/shrink: 35/0 up/down: 1960/0 (1960)
> Function                                     old     new   delta
> shrink_page_list                            3414    3670    +256
> __set_page_dirty_nobuffers                   242     349    +107
> check_move_unevictable_pages                 904     987     +83
> move_to_new_page                             591     671     +80
> shrink_active_list                           912     970     +58
> move_pages_to_lru                            911     965     +54
> migrate_pages                               2500    2554     +54
> shmem_swapin_page                           1145    1197     +52
> shmem_undo_range                            1669    1719     +50
> __test_set_page_writeback                    620     670     +50
> __set_page_dirty_buffers                     187     237     +50
> __pagevec_lru_add                            757     807     +50
> __munlock_pagevec                           1155    1205     +50
> __dump_page                                 1101    1151     +50
> __cancel_dirty_page                          182     232     +50
> __remove_mapping                             461     510     +49
> rmap_walk_file                               402     449     +47
> isolate_movable_page                         240     287     +47
> test_clear_page_writeback                    668     714     +46
> page_cache_pipe_buf_try_steal                171     217     +46
> page_endio                                   246     290     +44
> page_file_mapping                              -      43     +43
> __isolate_lru_page_prepare                   254     297     +43
> hugetlb_page_mapping_lock_write               39      81     +42
> iomap_set_page_dirty                         110     151     +41
> clear_page_dirty_for_io                      324     364     +40
> wait_on_page_writeback_killable              118     157     +39
> wait_on_page_writeback                       105     144     +39
> set_page_dirty                               159     198     +39
> putback_movable_page                          32      71     +39
> page_mkclean                                 172     211     +39
> mark_buffer_dirty                            176     215     +39
> invalidate_inode_page                        122     161     +39
> delete_from_page_cache                       139     178     +39
> PageMovable                                   49      86     +37
> isolate_migratepages_block                  2843    2872     +29
> Total: Before=17068648, After=17070608, chg +0.01%
>   
>> Willy can most probably give the best advise here :)
> 
> I think that's what folios are for :)

Exactly my thought. :)


-- 
Thanks,

David / dhildenb
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