Re: [PATCH] secretmem: optimize page_is_secretmem()

[David Hildenbrand]

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@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().

I would have thought the fast path "(PageCompound(page) || 
!PageLRU(page))" would already avoid calling page_mapping() in many cases.


--
Thanks,

David / dhildenb