Re: [PATCH v2 3/5] powerpc/mm: Allow more than 16 low slices

[Christophe LEROY]

Le 19/01/2018 Ã 09:30, Aneesh Kumar K.V a ÃcritÂ:
> Christophe Leroy <christophe.leroy@xxxxxx> 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@xxxxxx>
> > ---
> >   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