[PATCH] secretmem: optimize page_is_secretmem()
David Hildenbrand
david at redhat.com
Mon Apr 19 11:21:30 BST 2021
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 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.
>>
>> (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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