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

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
>

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