Re: [PATCH 09/10] powerpc/mm/slice: use the dynamic high slice size to limit bitmap operations

[Christophe LEROY <christophe.leroy@c-s.fr>]

Le 06/03/2018 à 14:25, Nicholas Piggin a écrit :
> The number of high slices a process might use now depends on its
> address space size, and what allocation address it has requested.
> 
> This patch uses that limit throughout call chains where possible,
> rather than use the fixed SLICE_NUM_HIGH for bitmap operations.
> This saves some cost for processes that don't use very large address
> spaces.
> 
> Perormance numbers aren't changed significantly, this may change
> with larger address spaces or different mmap access patterns that
> require more slice mask building.
> 
> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
> ---
>   arch/powerpc/mm/slice.c | 75 +++++++++++++++++++++++++++++--------------------
>   1 file changed, 45 insertions(+), 30 deletions(-)
> 
> diff --git a/arch/powerpc/mm/slice.c b/arch/powerpc/mm/slice.c
> index 086c31b8b982..507d17e2cfcd 100644
> --- a/arch/powerpc/mm/slice.c
> +++ b/arch/powerpc/mm/slice.c
> @@ -61,14 +61,12 @@ static void slice_print_mask(const char *label, const struct slice_mask *mask) {
>   #endif
>   
>   static void slice_range_to_mask(unsigned long start, unsigned long len,
> -				struct slice_mask *ret)
> +				struct slice_mask *ret,
> +				unsigned long high_slices)
>   {
>   	unsigned long end = start + len - 1;
>   
>   	ret->low_slices = 0;
> -	if (SLICE_NUM_HIGH)
> -		bitmap_zero(ret->high_slices, SLICE_NUM_HIGH);
> -
>   	if (start < SLICE_LOW_TOP) {
>   		unsigned long mend = min(end,
>   					 (unsigned long)(SLICE_LOW_TOP - 1));
> @@ -77,6 +75,10 @@ static void slice_range_to_mask(unsigned long start, unsigned long len,
>   			- (1u << GET_LOW_SLICE_INDEX(start));
>   	}
>   
> +	if (!SLICE_NUM_HIGH)
> +		return;
> +
> +	bitmap_zero(ret->high_slices, high_slices);

In include/linux/bitmap.h, it is said:

  * Note that nbits should be always a compile time evaluable constant.
  * Otherwise many inlines will generate horrible code.

Not sure that's true, but it is written ...


>   	if ((start + len) > SLICE_LOW_TOP) {
>   		unsigned long start_index = GET_HIGH_SLICE_INDEX(start);
>   		unsigned long align_end = ALIGN(end, (1UL << SLICE_HIGH_SHIFT));
> @@ -120,22 +122,20 @@ static int slice_high_has_vma(struct mm_struct *mm, unsigned long slice)
>   }
>   
>   static void slice_mask_for_free(struct mm_struct *mm, struct slice_mask *ret,
> -				unsigned long high_limit)
> +				unsigned long high_slices)
>   {
>   	unsigned long i;
>   
>   	ret->low_slices = 0;
> -	if (SLICE_NUM_HIGH)
> -		bitmap_zero(ret->high_slices, SLICE_NUM_HIGH);
> -
>   	for (i = 0; i < SLICE_NUM_LOW; i++)
>   		if (!slice_low_has_vma(mm, i))
>   			ret->low_slices |= 1u << i;
>   
> -	if (high_limit <= SLICE_LOW_TOP)
> +	if (!SLICE_NUM_HIGH || !high_slices)
>   		return;
>   
> -	for (i = 0; i < GET_HIGH_SLICE_INDEX(high_limit); i++)
> +	bitmap_zero(ret->high_slices, high_slices);
> +	for (i = 0; i < high_slices; i++)
>   		if (!slice_high_has_vma(mm, i))
>   			__set_bit(i, ret->high_slices);
>   }
> @@ -232,6 +232,7 @@ static void slice_convert(struct mm_struct *mm,
>   {
>   	int index, mask_index;
>   	/* Write the new slice psize bits */
> +	unsigned long high_slices;
>   	unsigned char *hpsizes, *lpsizes;
>   	struct slice_mask *psize_mask, *old_mask;
>   	unsigned long i, flags;
> @@ -267,7 +268,8 @@ static void slice_convert(struct mm_struct *mm,
>   	}
>   
>   	hpsizes = mm->context.high_slices_psize;
> -	for (i = 0; i < GET_HIGH_SLICE_INDEX(mm->context.slb_addr_limit); i++) {
> +	high_slices = GET_HIGH_SLICE_INDEX(mm->context.slb_addr_limit);
> +	for (i = 0; SLICE_NUM_HIGH && i < high_slices; i++) {
>   		if (!test_bit(i, mask->high_slices))
>   			continue;
>   
> @@ -434,32 +436,37 @@ static unsigned long slice_find_area(struct mm_struct *mm, unsigned long len,
>   }
>   
>   static inline void slice_copy_mask(struct slice_mask *dst,
> -					const struct slice_mask *src)
> +					const struct slice_mask *src,
> +					unsigned long high_slices)
>   {
>   	dst->low_slices = src->low_slices;
>   	if (!SLICE_NUM_HIGH)
>   		return;
> -	bitmap_copy(dst->high_slices, src->high_slices, SLICE_NUM_HIGH);
> +	bitmap_copy(dst->high_slices, src->high_slices, high_slices);
>   }
>   
>   static inline void slice_or_mask(struct slice_mask *dst,
>   					const struct slice_mask *src1,
> -					const struct slice_mask *src2)
> +					const struct slice_mask *src2,
> +					unsigned long high_slices)
>   {
>   	dst->low_slices = src1->low_slices | src2->low_slices;
>   	if (!SLICE_NUM_HIGH)
>   		return;
> -	bitmap_or(dst->high_slices, src1->high_slices, src2->high_slices, SLICE_NUM_HIGH);
> +	bitmap_or(dst->high_slices, src1->high_slices, src2->high_slices,
> +			high_slices);

Why a new line here, this line is shorter than before.
Or that was forgotten in a previous patch ?

>   }
>   
>   static inline void slice_andnot_mask(struct slice_mask *dst,
>   					const struct slice_mask *src1,
> -					const struct slice_mask *src2)
> +					const struct slice_mask *src2,
> +					unsigned long high_slices)
>   {
>   	dst->low_slices = src1->low_slices & ~src2->low_slices;
>   	if (!SLICE_NUM_HIGH)
>   		return;
> -	bitmap_andnot(dst->high_slices, src1->high_slices, src2->high_slices, SLICE_NUM_HIGH);
> +	bitmap_andnot(dst->high_slices, src1->high_slices, src2->high_slices,
> +			high_slices);

Same comment.

>   }
>   
>   #ifdef CONFIG_PPC_64K_PAGES
> @@ -482,6 +489,7 @@ unsigned long slice_get_unmapped_area(unsigned long addr, unsigned long len,
>   	struct mm_struct *mm = current->mm;
>   	unsigned long newaddr;
>   	unsigned long high_limit;
> +	unsigned long high_slices;
>   
>   	high_limit = DEFAULT_MAP_WINDOW;
>   	if (addr >= high_limit || (fixed && (addr + len > high_limit)))
> @@ -498,6 +506,7 @@ unsigned long slice_get_unmapped_area(unsigned long addr, unsigned long len,
>   			return -ENOMEM;
>   	}
>   
> +	high_slices = GET_HIGH_SLICE_INDEX(high_limit);
>   	if (high_limit > mm->context.slb_addr_limit) {
>   		/*
>   		 * Increasing the slb_addr_limit does not require
> @@ -557,13 +566,13 @@ unsigned long slice_get_unmapped_area(unsigned long addr, unsigned long len,
>   	if (psize == MMU_PAGE_64K) {
>   		compat_maskp = slice_mask_for_size(mm, MMU_PAGE_4K);
>   		if (fixed)
> -			slice_or_mask(&good_mask, maskp, compat_maskp);
> +			slice_or_mask(&good_mask, maskp, compat_maskp, high_slices);
>   		else
> -			slice_copy_mask(&good_mask, maskp);
> +			slice_copy_mask(&good_mask, maskp, high_slices);
>   	} else
>   #endif
>   	{
> -		slice_copy_mask(&good_mask, maskp);
> +		slice_copy_mask(&good_mask, maskp, high_slices);
>   	}
>   	slice_print_mask(" good_mask", &good_mask);
>   	if (compat_maskp)
> @@ -596,8 +605,8 @@ unsigned long slice_get_unmapped_area(unsigned long addr, unsigned long len,
>   	 * We don't fit in the good mask, check what other slices are
>   	 * empty and thus can be converted
>   	 */
> -	slice_mask_for_free(mm, &potential_mask, high_limit);
> -	slice_or_mask(&potential_mask, &potential_mask, &good_mask);
> +	slice_mask_for_free(mm, &potential_mask, high_slices);
> +	slice_or_mask(&potential_mask, &potential_mask, &good_mask, high_slices);
>   	slice_print_mask(" potential", &potential_mask);
>   
>   	if (addr || fixed) {
> @@ -634,7 +643,7 @@ unsigned long slice_get_unmapped_area(unsigned long addr, unsigned long len,
>   #ifdef CONFIG_PPC_64K_PAGES
>   	if (addr == -ENOMEM && psize == MMU_PAGE_64K) {
>   		/* retry the search with 4k-page slices included */
> -		slice_or_mask(&potential_mask, &potential_mask, compat_maskp);
> +		slice_or_mask(&potential_mask, &potential_mask, compat_maskp, high_slices);
>   		addr = slice_find_area(mm, len, &potential_mask,
>   				       psize, topdown, high_limit);
>   	}
> @@ -643,17 +652,17 @@ unsigned long slice_get_unmapped_area(unsigned long addr, unsigned long len,
>   	if (addr == -ENOMEM)
>   		return -ENOMEM;
>   
> -	slice_range_to_mask(addr, len, &potential_mask);
> +	slice_range_to_mask(addr, len, &potential_mask, high_slices);
>   	slice_dbg(" found potential area at 0x%lx\n", addr);
>   	slice_print_mask(" mask", &potential_mask);
>   
>    convert:
> -	slice_andnot_mask(&potential_mask, &potential_mask, &good_mask);
> +	slice_andnot_mask(&potential_mask, &potential_mask, &good_mask, high_slices);
>   	if (compat_maskp && !fixed)
> -		slice_andnot_mask(&potential_mask, &potential_mask, compat_maskp);
> +		slice_andnot_mask(&potential_mask, &potential_mask, compat_maskp, high_slices);
>   	if (potential_mask.low_slices ||
>   		(SLICE_NUM_HIGH &&
> -		 !bitmap_empty(potential_mask.high_slices, SLICE_NUM_HIGH))) {
> +		 !bitmap_empty(potential_mask.high_slices, high_slices))) {

Are we sure high_slices is not nul here when SLICE_NUM_HIGH is not nul ?

Christophe

>   		slice_convert(mm, &potential_mask, psize);
>   		if (psize > MMU_PAGE_BASE)
>   			on_each_cpu(slice_flush_segments, mm, 1);
> @@ -727,7 +736,9 @@ void slice_init_new_context_exec(struct mm_struct *mm)
>   	mm->context.user_psize = psize;
>   
>   	/*
> -	 * Set all slice psizes to the default.
> +	 * Set all slice psizes to the default. High slices could
> +	 * be initialised up to slb_addr_limit if we ensure to
> +	 * initialise the rest of them as slb_addr_limit is expanded.
>   	 */
>   	lpsizes = mm->context.low_slices_psize;
>   	memset(lpsizes, (psize << 4) | psize, SLICE_NUM_LOW >> 1);
> @@ -748,10 +759,12 @@ void slice_set_range_psize(struct mm_struct *mm, unsigned long start,
>   			   unsigned long len, unsigned int psize)
>   {
>   	struct slice_mask mask;
> +	unsigned long high_slices;
>   
>   	VM_BUG_ON(radix_enabled());
>   
> -	slice_range_to_mask(start, len, &mask);
> +	high_slices = GET_HIGH_SLICE_INDEX(mm->context.slb_addr_limit);
> +	slice_range_to_mask(start, len, &mask, high_slices);
>   	slice_convert(mm, &mask, psize);
>   }
>   
> @@ -790,9 +803,11 @@ int is_hugepage_only_range(struct mm_struct *mm, unsigned long addr,
>   	if (psize == MMU_PAGE_64K) {
>   		const struct slice_mask *compat_maskp;
>   		struct slice_mask available;
> +		unsigned long high_slices;
>   
>   		compat_maskp = slice_mask_for_size(mm, MMU_PAGE_4K);
> -		slice_or_mask(&available, maskp, compat_maskp);
> +		high_slices = GET_HIGH_SLICE_INDEX(mm->context.slb_addr_limit);
> +		slice_or_mask(&available, maskp, compat_maskp, high_slices);
>   		return !slice_check_range_fits(mm, &available, addr, len);
>   	}
>   #endif
>