Re: [mm PATCH v2 1/6] mm: Use mm_zero_struct_page from SPARC on all 64b architectures

[Pavel Tatashin]

Still, lets get some real data, please provide Intel data before vs
after. I could test on an ARM processor.

Pavel

On Sat, Oct 13, 2018 at 1:18 PM Alexander Duyck
<alexander.duyck@xxxxxxxxx> wrote:
>
> Well in the case of x86 the call to memset is expensive as well. In
> most cases it is 16 cycles plus 1 cycle per 16 bytes if I recall
> correctly. So for example in the case of skbuff which was a little
> over 192 bytes I know Jesper Brouer and myself were going back and
> forth with the idea of if we should try to do something similar.
>
> I'm suspecting for the 64b architectures impacted by this change there
> should be little to no negative impact. The main reason for that being
> the fact that the compiler can actually drop some of the writes by
> merging them with the later assignments.
>
> Thanks.
>
> - Alex
>
> On Sat, Oct 13, 2018 at 9:58 AM Pavel Tatashin <pasha.tatashin@xxxxxxxxx> wrote:
> >
> > I am worried about this change. I added SPARC optimized
> > mm_zero_struct_page() specifically to SPARC because it has a poor
> > performance with small memset()s, since it uses STBI instructions.
> > However, other architectures might not suffer with small memset()s,
> > and have hardware optimized memset variants for small sizes. Don't
> > forget, this is a leaf routine on most arches, so the function call
> > should be cheap. Also, the macro itself is not very flexible: when
> > size of struct page is changed, it also must be modified (we could add
> > fall throughs though), I would add this macro only to those arches
> > that benefit from this change, in other words, I would like to see
> > performance data.
> >
> > I will review the rest of the patches in this series on Monday.
> >
> > Thank you,
> > Pavel
> > On Thu, Oct 11, 2018 at 6:17 PM Alexander Duyck
> > <alexander.h.duyck@xxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx>
> > > ---
> > >  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 273d4dbd3883..dee407998366 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>.
> > >   */
> > >  #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
> > >
> >