On Fri, Jul 22, 2016 at 10:57:28PM +1000, Nicholas Piggin wrote:
> Calculating the slice mask can become a signifcant overhead for
> get_unmapped_area. The mask is relatively small and does not change
> frequently, so we can cache it in the mm context.
>
> This saves about 30% kernel time on a 4K user address allocation
> in a microbenchmark.
>
> Comments on the approach taken? I think there is the option for fixed
> allocations to avoid some of the slice calculation entirely, but first
> I think it will be good to have a general speedup that covers all
> mmaps.
>
> Cc: Benjamin Herrenschmidt <b...@kernel.crashing.org>
> Cc: Anton Blanchard <an...@samba.org>
> ---
> arch/powerpc/include/asm/book3s/64/mmu.h | 8 +++++++
> arch/powerpc/mm/slice.c | 39
> ++++++++++++++++++++++++++++++--
> 2 files changed, 45 insertions(+), 2 deletions(-)
>
> diff --git a/arch/powerpc/include/asm/book3s/64/mmu.h
> b/arch/powerpc/include/asm/book3s/64/mmu.h
> index 5854263..0d15af4 100644
> --- a/arch/powerpc/include/asm/book3s/64/mmu.h
> +++ b/arch/powerpc/include/asm/book3s/64/mmu.h
> @@ -71,6 +71,14 @@ typedef struct {
> #ifdef CONFIG_PPC_MM_SLICES
> u64 low_slices_psize; /* SLB page size encodings */
> unsigned char high_slices_psize[SLICE_ARRAY_SIZE];
> + struct slice_mask mask_4k;
> +# ifdef CONFIG_PPC_64K_PAGES
> + struct slice_mask mask_64k;
> +# endif
> +# ifdef CONFIG_HUGETLB_PAGE
> + struct slice_mask mask_16m;
> + struct slice_mask mask_16g;
> +# endif
Should we cache these in mmu_psize_defs? I am not 100% sure
if want to overload that structure, but it provides a convient
way of saying mmu_psize_defs[psize].mask instead of all
the if checks
> #else
> u16 sllp; /* SLB page size encoding */
> #endif
> diff --git a/arch/powerpc/mm/slice.c b/arch/powerpc/mm/slice.c
> index 2b27458..559ea5f 100644
> --- a/arch/powerpc/mm/slice.c
> +++ b/arch/powerpc/mm/slice.c
> @@ -147,7 +147,7 @@ static struct slice_mask slice_mask_for_free(struct
> mm_struct *mm)
> return ret;
> }
>
> -static struct slice_mask slice_mask_for_size(struct mm_struct *mm, int psize)
> +static struct slice_mask calc_slice_mask_for_size(struct mm_struct *mm, int
> psize)
> {
> unsigned char *hpsizes;
> int index, mask_index;
> @@ -171,6 +171,36 @@ static struct slice_mask slice_mask_for_size(struct
> mm_struct *mm, int psize)
> return ret;
> }
>
> +static void recalc_slice_mask_cache(struct mm_struct *mm)
> +{
> + mm->context.mask_4k = calc_slice_mask_for_size(mm, MMU_PAGE_4K);
> +#ifdef CONFIG_PPC_64K_PAGES
> + mm->context.mask_64k = calc_slice_mask_for_size(mm, MMU_PAGE_64K);
> +#endif
> +# ifdef CONFIG_HUGETLB_PAGE
> + /* Radix does not come here */
> + mm->context.mask_16m = calc_slice_mask_for_size(mm, MMU_PAGE_16M);
> + mm->context.mask_16g = calc_slice_mask_for_size(mm, MMU_PAGE_16G);
> +# endif
> +}
Should the function above be called under slice_convert_lock?
> +
> +static struct slice_mask slice_mask_for_size(struct mm_struct *mm, int psize)
> +{
> + if (psize == MMU_PAGE_4K)
> + return mm->context.mask_4k;
> +#ifdef CONFIG_PPC_64K_PAGES
> + if (psize == MMU_PAGE_64K)
> + return mm->context.mask_64k;
> +#endif
> +# ifdef CONFIG_HUGETLB_PAGE
> + if (psize == MMU_PAGE_16M)
> + return mm->context.mask_16m;
> + if (psize == MMU_PAGE_16G)
> + return mm->context.mask_16g;
> +# endif
> + BUG();
> +}
> +
> static int slice_check_fit(struct slice_mask mask, struct slice_mask
> available)
> {
> return (mask.low_slices & available.low_slices) == mask.low_slices &&
> @@ -233,6 +263,8 @@ static void slice_convert(struct mm_struct *mm, struct
> slice_mask mask, int psiz
>
> spin_unlock_irqrestore(&slice_convert_lock, flags);
>
> + recalc_slice_mask_cache(mm);
> +
> copro_flush_all_slbs(mm);
> }
>
> @@ -625,7 +657,7 @@ void slice_set_user_psize(struct mm_struct *mm, unsigned
> int psize)
> goto bail;
>
> mm->context.user_psize = psize;
> - wmb();
> + wmb(); /* Why? */
>
> lpsizes = mm->context.low_slices_psize;
> for (i = 0; i < SLICE_NUM_LOW; i++)
> @@ -652,6 +684,9 @@ void slice_set_user_psize(struct mm_struct *mm, unsigned
> int psize)
> mm->context.low_slices_psize,
> mm->context.high_slices_psize);
>
> + spin_unlock_irqrestore(&slice_convert_lock, flags);
> + recalc_slice_mask_cache(mm);
> + return;
> bail:
> spin_unlock_irqrestore(&slice_convert_lock, flags);
> }
> --
> 2.8.1
>
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