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SubjectRe: [PATCH v2 3/5] powerpc/mm: Allow more than 16 low slices
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Le 19/01/2018 à 09:30, Aneesh Kumar K.V a écrit :
> Christophe Leroy <christophe.leroy@c-s.fr> writes:
>
>> While the implementation of the "slices" address space allows
>> a significant amount of high slices, it limits the number of
>> low slices to 16 due to the use of a single u64 low_slices_psize
>> element in struct mm_context_t
>>
>> On the 8xx, the minimum slice size is the size of the area
>> covered by a single PMD entry, ie 4M in 4K pages mode and 64M in
>> 16K pages mode. This means we could have resp. up to 1024 and 64
>> slices.
>>
>> In order to override this limitation, this patch switches the
>> handling of low_slices to BITMAPs as done already for high_slices.
>
> Does it have a performance impact. When we switched high_slices
> that was one of the question asked. Now with a topdown search we should
> mostly be using the high_slices. But it will good to get numbers for
> ppc64 for this change.

It should have almost no performance impact at all, because all bitmap
functions used a simplified way when the number of bits is small and
constant:

- ret->low_slices = 0;
+ slice_bitmap_zero(ret->low_slices, SLICE_NUM_LOW);


static inline void bitmap_zero(unsigned long *dst, unsigned int nbits)
{
if (small_const_nbits(nbits))
*dst = 0UL;
else {
unsigned int len = BITS_TO_LONGS(nbits) * sizeof(unsigned long);
memset(dst, 0, len);
}
}



- dst->low_slices |= src->low_slices;
+ slice_bitmap_or(dst->low_slices, dst->low_slices, src->low_slices,
+ SLICE_NUM_LOW);


static inline void bitmap_or(unsigned long *dst, const unsigned long *src1,
const unsigned long *src2, unsigned int nbits)
{
if (small_const_nbits(nbits))
*dst = *src1 | *src2;
else
__bitmap_or(dst, src1, src2, nbits);
}


>
>
>>
>> Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
>> ---
>> v2: Usign slice_bitmap_xxx() macros instead of bitmap_xxx() functions.
>>
>> arch/powerpc/include/asm/book3s/64/mmu.h | 2 +-
>> arch/powerpc/include/asm/mmu-8xx.h | 2 +-
>> arch/powerpc/include/asm/paca.h | 2 +-
>> arch/powerpc/kernel/paca.c | 3 +-
>> arch/powerpc/mm/hash_utils_64.c | 13 ++--
>> arch/powerpc/mm/slb_low.S | 8 ++-
>> arch/powerpc/mm/slice.c | 104 +++++++++++++++++--------------
>> 7 files changed, 74 insertions(+), 60 deletions(-)
>>
>> diff --git a/arch/powerpc/include/asm/book3s/64/mmu.h b/arch/powerpc/include/asm/book3s/64/mmu.h
>> index c9448e19847a..27e7e9732ea1 100644
>> --- a/arch/powerpc/include/asm/book3s/64/mmu.h
>> +++ b/arch/powerpc/include/asm/book3s/64/mmu.h
>> @@ -91,7 +91,7 @@ typedef struct {
>> struct npu_context *npu_context;
>>
>> #ifdef CONFIG_PPC_MM_SLICES
>> - u64 low_slices_psize; /* SLB page size encodings */
>> + unsigned char low_slices_psize[8]; /* SLB page size encodings */
>
> Can that 8 be a #define?

Sure

>
>
>> unsigned char high_slices_psize[SLICE_ARRAY_SIZE];
>> unsigned long slb_addr_limit;
>> #else
>
> -aneesh
>

Christophe

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