Subject: Re: [PATCH] secretmem: optimize page_is_secretmem()

From: Mike Rapoport <[email-masked]>
Date: Mon, 19 Apr 2021 13:14:19 +0300

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

-- 
Sincerely yours,
Mike.

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