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

[Aneesh Kumar K.V]

On 01/19/2018 02:29 PM, Christophe LEROY wrote:
> 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:
>
> -ÂÂÂ ret->low_slices = 0;
> +ÂÂÂ slice_bitmap_zero(ret->low_slices, SLICE_NUM_LOW);
>
>
> static inline void bitmap_zero(unsigned long *dst, unsigned int nbits)
> {
>  ÂÂÂÂif (small_const_nbits(nbits))
>  ÂÂÂÂÂÂÂ *dst = 0UL;
>  ÂÂÂÂelse {
>  ÂÂÂÂÂÂÂ unsigned int len = BITS_TO_LONGS(nbits) * sizeof(unsigned long);
>  ÂÂÂÂÂÂÂ memset(dst, 0, len);
>  ÂÂÂÂ}
> }
>
>
>
> -ÂÂÂ dst->low_slices |= src->low_slices;
> +ÂÂÂ slice_bitmap_or(dst->low_slices, dst->low_slices, src->low_slices,
> +ÂÂÂÂÂÂÂÂÂÂÂ SLICE_NUM_LOW);
>
>
> static inline void bitmap_or(unsigned long *dst, const unsigned long *src1,
>  ÂÂÂÂÂÂÂÂÂÂÂ const unsigned long *src2, unsigned int nbits)
> {
>  ÂÂÂÂif (small_const_nbits(nbits))
>  ÂÂÂÂÂÂÂ *dst = *src1 | *src2;
>  ÂÂÂÂelse
>  ÂÂÂÂÂÂÂ __bitmap_or(dst, src1, src2, nbits);
> }

may be capture that in commit message saying since we are 64 bit on 
ppc64 there is no impact there?

-aneesh