Subject: Re: [PATCH] powerpc/64: implement a slice mask cache

From: Balbir Singh <[email-masked]>
Date: Sat, 23 Jul 2016 12:19:37 +1000

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 <[email-masked]>
> Cc: Anton Blanchard <[email-masked]>
> ---
>  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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