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@xxxxxxxxxxxxx>
> >
> > 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@xxxxxxxxx>
> > Signed-off-by: Mike Rapoport <rppt@xxxxxxxxxxxxx>
> > ---
> >
> > @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().
> The idea of the patch makes sense to me.
--
Sincerely yours,
Mike.
