Re: [PATCH 06/10] powerpc/mm/slice: implement slice_check_range_fits

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

Le 06/03/2018 à 14:25, Nicholas Piggin a écrit :
> Rather than build slice masks from a range then use that to check for
> fit in a candidate mask, implement slice_check_range_fits that checks
> if a range fits in a mask directly.
> 
> This allows several structures to be removed from stacks, and also we
> don't expect a huge range in a lot of these cases, so building and
> comparing a full mask is going to be more expensive than testing just
> one or two bits of the range.
> 
> On POWER8, this increases vfork+exec+exit performance by 0.3%
> and reduces time to mmap+munmap a 64kB page by 5%.
> 
> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
> ---
>   arch/powerpc/mm/slice.c | 63 ++++++++++++++++++++++++++-----------------------
>   1 file changed, 34 insertions(+), 29 deletions(-)
> 
> diff --git a/arch/powerpc/mm/slice.c b/arch/powerpc/mm/slice.c
> index 2115efe5e869..3841fca75006 100644
> --- a/arch/powerpc/mm/slice.c
> +++ b/arch/powerpc/mm/slice.c
> @@ -179,26 +179,35 @@ static struct slice_mask *slice_mask_for_size(struct mm_struct *mm, int psize)
>   #error "Must define the slice masks for page sizes supported by the platform"
>   #endif
>   
> -static int slice_check_fit(struct mm_struct *mm,
> -			   const struct slice_mask *mask,
> -			   const struct slice_mask *available)
> +static bool slice_check_range_fits(struct mm_struct *mm,
> +			   const struct slice_mask *available,
> +			   unsigned long start, unsigned long len)
>   {
> -	DECLARE_BITMAP(result, SLICE_NUM_HIGH);
> -	/*
> -	 * Make sure we just do bit compare only to the max
> -	 * addr limit and not the full bit map size.
> -	 */
> -	unsigned long slice_count = GET_HIGH_SLICE_INDEX(mm->context.slb_addr_limit);
> +	unsigned long end = start + len - 1;
> +	u64 low_slices = 0;
>   
> -	if (!SLICE_NUM_HIGH)
> -		return (mask->low_slices & available->low_slices) ==
> -		       mask->low_slices;
> +	if (start < SLICE_LOW_TOP) {
> +		unsigned long mend = min(end, (SLICE_LOW_TOP - 1));

See slice_range_to_mask()

You'll have an issue here with PPC32, you have to cast (SLICE_LOW_TOP - 
1) to unsigned long because SLICE_LOW_TOP is unsigned long long on PPC32

> +
> +		low_slices = (1u << (GET_LOW_SLICE_INDEX(mend) + 1))
> +				- (1u << GET_LOW_SLICE_INDEX(start));
> +	}
> +	if ((low_slices & available->low_slices) != low_slices)
> +		return false;
> +
> +	if (SLICE_NUM_HIGH && ((start + len) > SLICE_LOW_TOP)) {
> +		unsigned long start_index = GET_HIGH_SLICE_INDEX(start);
> +		unsigned long align_end = ALIGN(end, (1UL << SLICE_HIGH_SHIFT));
> +		unsigned long count = GET_HIGH_SLICE_INDEX(align_end) - start_index;
> +		unsigned long i;
>   
> -	bitmap_and(result, mask->high_slices,
> -		   available->high_slices, slice_count);
> +		for (i = start_index; i < start_index + count; i++) {
> +			if (!test_bit(i, available->high_slices))
> +				return false;
> +		}

What about using bitmap_find_next_zero_area()

> +	}
>   
> -	return (mask->low_slices & available->low_slices) == mask->low_slices &&
> -		bitmap_equal(result, mask->high_slices, slice_count);
> +	return true;
>   }
>   
>   static void slice_flush_segments(void *parm)
> @@ -562,15 +571,11 @@ unsigned long slice_get_unmapped_area(unsigned long addr, unsigned long len,
>   #endif
>   
>   	/* First check hint if it's valid or if we have MAP_FIXED */
> -	if (addr != 0 || fixed) {
> -		/* Build a mask for the requested range */
> -		slice_range_to_mask(addr, len, &mask);
> -		slice_print_mask(" mask", &mask);
> -
> +	if (addr || fixed) {

It is cleanup, should it really be part of this patch ?

>   		/* Check if we fit in the good mask. If we do, we just return,
>   		 * nothing else to do
>   		 */
> -		if (slice_check_fit(mm, &mask, &good_mask)) {
> +		if (slice_check_range_fits(mm, &good_mask, addr, len)) {
>   			slice_dbg(" fits good !\n");
>   			return addr;
>   		}
> @@ -596,10 +601,11 @@ unsigned long slice_get_unmapped_area(unsigned long addr, unsigned long len,
>   	slice_or_mask(&potential_mask, &good_mask);
>   	slice_print_mask(" potential", &potential_mask);
>   
> -	if ((addr != 0 || fixed) &&
> -			slice_check_fit(mm, &mask, &potential_mask)) {
> -		slice_dbg(" fits potential !\n");
> -		goto convert;
> +	if (addr || fixed) {
> +		if (slice_check_range_fits(mm, &potential_mask, addr, len)) {
> +			slice_dbg(" fits potential !\n");
> +			goto convert;
> +		}

Why not keep the original structure and just replacing slice_check_fit() 
by slice_check_range_fits() ?

I believe cleanups should not be mixed with real feature changes. If 
needed, you should have a cleanup patch up front the serie.

Christophe

>   	}
>   
>   	/* If we have MAP_FIXED and failed the above steps, then error out */
> @@ -772,13 +778,12 @@ void slice_set_range_psize(struct mm_struct *mm, unsigned long start,
>   int is_hugepage_only_range(struct mm_struct *mm, unsigned long addr,
>   			   unsigned long len)
>   {
> -	struct slice_mask mask, available;
> +	struct slice_mask available;
>   	unsigned int psize = mm->context.user_psize;
>   
>   	if (radix_enabled())
>   		return 0;
>   
> -	slice_range_to_mask(addr, len, &mask);
>   	available = *slice_mask_for_size(mm, psize);
>   #ifdef CONFIG_PPC_64K_PAGES
>   	/* We need to account for 4k slices too */
> @@ -795,6 +800,6 @@ int is_hugepage_only_range(struct mm_struct *mm, unsigned long addr,
>   	slice_print_mask(" mask", &mask);
>   	slice_print_mask(" available", &available);
>   #endif
> -	return !slice_check_fit(mm, &mask, &available);
> +	return !slice_check_range_fits(mm, &available, addr, len);
>   }
>   #endif
>