Re: [mm PATCH v3 1/6] mm: Use mm_zero_struct_page from SPARC on all 64b architectures

[Mike Rapoport <rppt@linux.ibm.com>]

On Tue, Oct 16, 2018 at 03:01:11PM -0400, Pavel Tatashin wrote:
> 
> 
> On 10/15/18 4:26 PM, Alexander Duyck wrote:
> > This change makes it so that we use the same approach that was already in
> > use on Sparc on all the archtectures that support a 64b long.
> > 
> > This is mostly motivated by the fact that 8 to 10 store/move instructions
> > are likely always going to be faster than having to call into a function
> > that is not specialized for handling page init.
> > 
> > An added advantage to doing it this way is that the compiler can get away
> > with combining writes in the __init_single_page call. As a result the
> > memset call will be reduced to only about 4 write operations, or at least
> > that is what I am seeing with GCC 6.2 as the flags, LRU poitners, and
> > count/mapcount seem to be cancelling out at least 4 of the 8 assignments on
> > my system.
> > 
> > One change I had to make to the function was to reduce the minimum page
> > size to 56 to support some powerpc64 configurations.
> > 
> > Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> 
> 
> I have tested on Broadcom's Stingray cpu with 48G RAM:
> __init_single_page() takes 19.30ns / 64-byte struct page
> Wit the change it takes 17.33ns / 64-byte struct page
 
I gave it a run on an OpenPower (S812LC 8348-21C) with Power8 processor and
with 128G of RAM. My results for 64-byte struct page were:

before: 4.6788ns
after: 4.5882ns

My two cents :)

> Please add this data and also the data from Intel to the description.
> 
> Thank you,
> Pavel
> 
> > ---
> >  arch/sparc/include/asm/pgtable_64.h |   30 ------------------------------
> >  include/linux/mm.h                  |   34 ++++++++++++++++++++++++++++++++++
> >  2 files changed, 34 insertions(+), 30 deletions(-)
> > 
> > diff --git a/arch/sparc/include/asm/pgtable_64.h b/arch/sparc/include/asm/pgtable_64.h
> > index 1393a8ac596b..22500c3be7a9 100644
> > --- a/arch/sparc/include/asm/pgtable_64.h
> > +++ b/arch/sparc/include/asm/pgtable_64.h
> > @@ -231,36 +231,6 @@
> >  extern struct page *mem_map_zero;
> >  #define ZERO_PAGE(vaddr)	(mem_map_zero)
> >  
> > -/* This macro must be updated when the size of struct page grows above 80
> > - * or reduces below 64.
> > - * The idea that compiler optimizes out switch() statement, and only
> > - * leaves clrx instructions
> > - */
> > -#define	mm_zero_struct_page(pp) do {					\
> > -	unsigned long *_pp = (void *)(pp);				\
> > -									\
> > -	 /* Check that struct page is either 64, 72, or 80 bytes */	\
> > -	BUILD_BUG_ON(sizeof(struct page) & 7);				\
> > -	BUILD_BUG_ON(sizeof(struct page) < 64);				\
> > -	BUILD_BUG_ON(sizeof(struct page) > 80);				\
> > -									\
> > -	switch (sizeof(struct page)) {					\
> > -	case 80:							\
> > -		_pp[9] = 0;	/* fallthrough */			\
> > -	case 72:							\
> > -		_pp[8] = 0;	/* fallthrough */			\
> > -	default:							\
> > -		_pp[7] = 0;						\
> > -		_pp[6] = 0;						\
> > -		_pp[5] = 0;						\
> > -		_pp[4] = 0;						\
> > -		_pp[3] = 0;						\
> > -		_pp[2] = 0;						\
> > -		_pp[1] = 0;						\
> > -		_pp[0] = 0;						\
> > -	}								\
> > -} while (0)
> > -
> >  /* PFNs are real physical page numbers.  However, mem_map only begins to record
> >   * per-page information starting at pfn_base.  This is to handle systems where
> >   * the first physical page in the machine is at some huge physical address,
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index bb0de406f8e7..ec6e57a0c14e 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -102,8 +102,42 @@ static inline void set_max_mapnr(unsigned long limit) { }
> >   * zeroing by defining this macro in <asm/pgtable.h>.
> >   */
> 
> The comment above becomes outdated. Please change, we use optimized
> mm_zero_struct_page on every 64-bit platform.
> 
> >  #ifndef mm_zero_struct_page
> > +#if BITS_PER_LONG == 64
> > +/* This function must be updated when the size of struct page grows above 80
> > + * or reduces below 64. The idea that compiler optimizes out switch()
> > + * statement, and only leaves move/store instructions
> > + */
> > +#define	mm_zero_struct_page(pp) __mm_zero_struct_page(pp)
> > +static inline void __mm_zero_struct_page(struct page *page)
> > +{
> > +	unsigned long *_pp = (void *)page;
> > +
> > +	 /* Check that struct page is either 56, 64, 72, or 80 bytes */
> > +	BUILD_BUG_ON(sizeof(struct page) & 7);
> > +	BUILD_BUG_ON(sizeof(struct page) < 56);
> > +	BUILD_BUG_ON(sizeof(struct page) > 80);
> > +
> > +	switch (sizeof(struct page)) {
> > +	case 80:
> > +		_pp[9] = 0;	/* fallthrough */
> > +	case 72:
> > +		_pp[8] = 0;	/* fallthrough */
> > +	default:
> > +		_pp[7] = 0;	/* fallthrough */
> > +	case 56:
> > +		_pp[6] = 0;
> > +		_pp[5] = 0;
> > +		_pp[4] = 0;
> > +		_pp[3] = 0;
> > +		_pp[2] = 0;
> > +		_pp[1] = 0;
> > +		_pp[0] = 0;
> > +	}
> > +}
> > +#else
> >  #define mm_zero_struct_page(pp)  ((void)memset((pp), 0, sizeof(struct page)))
> >  #endif
> > +#endif
> >  
> >  /*
> >   * Default maximum number of active map areas, this limits the number of vmas
> > 
> 

-- 
Sincerely yours,
Mike.