Re: [PATCH] powerpc/64: implement a slice mask cache

[Nicholas Piggin <npiggin@gmail.com>]

On Sat, 23 Jul 2016 12:19:37 +1000
Balbir Singh <bsingharora@gmail.com> wrote:

> On Fri, Jul 22, 2016 at 10:57:28PM +1000, Nicholas Piggin wrote:
> > Calculating the slice mask can become a signifcant overhead for
> > get_unmapped_area. The mask is relatively small and does not change
> > frequently, so we can cache it in the mm context.
> > 
> > This saves about 30% kernel time on a 4K user address allocation
> > in a microbenchmark.
> > 
> > Comments on the approach taken? I think there is the option for
> > fixed allocations to avoid some of the slice calculation entirely,
> > but first I think it will be good to have a general speedup that
> > covers all mmaps.
> > 
> > Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> > Cc: Anton Blanchard <anton@samba.org>
> > ---
> >  arch/powerpc/include/asm/book3s/64/mmu.h |  8 +++++++
> >  arch/powerpc/mm/slice.c                  | 39
> > ++++++++++++++++++++++++++++++-- 2 files changed, 45 insertions(+),
> > 2 deletions(-)
> > 
> > diff --git a/arch/powerpc/include/asm/book3s/64/mmu.h
> > b/arch/powerpc/include/asm/book3s/64/mmu.h index 5854263..0d15af4
> > 100644 --- a/arch/powerpc/include/asm/book3s/64/mmu.h
> > +++ b/arch/powerpc/include/asm/book3s/64/mmu.h
> > @@ -71,6 +71,14 @@ typedef struct {
> >  #ifdef CONFIG_PPC_MM_SLICES
> >  	u64 low_slices_psize;	/* SLB page size encodings */
> >  	unsigned char high_slices_psize[SLICE_ARRAY_SIZE];
> > +	struct slice_mask mask_4k;
> > +# ifdef CONFIG_PPC_64K_PAGES
> > +	struct slice_mask mask_64k;
> > +# endif
> > +# ifdef CONFIG_HUGETLB_PAGE
> > +	struct slice_mask mask_16m;
> > +	struct slice_mask mask_16g;
> > +# endif  
> 
> Should we cache these in mmu_psize_defs? I am not 100% sure
> if want to overload that structure, but it provides a convient
> way of saying mmu_psize_defs[psize].mask instead of all
> the if checks

I'm not sure if we can, can we? mmu_psize_defs is global
whereas we need per-process structure.

The branches are a bit annoying, but we can't directly use an array
because it's too big. But see the comment at MMU_PAGE_* defines.
Perhaps we could change this structure to be sized at compile time to
only include possible page sizes, and would enable building a
structure like the above with simply

struct type blah[MMU_POSSIBLE_PAGE_COUNT];

Perhaps we can consider that as a follow on patch? It's probably a bit
more work to implement.


> >  #else
> >  	u16 sllp;		/* SLB page size encoding */
> >  #endif
> > diff --git a/arch/powerpc/mm/slice.c b/arch/powerpc/mm/slice.c
> > index 2b27458..559ea5f 100644
> > --- a/arch/powerpc/mm/slice.c
> > +++ b/arch/powerpc/mm/slice.c
> > @@ -147,7 +147,7 @@ static struct slice_mask
> > slice_mask_for_free(struct mm_struct *mm) return ret;
> >  }
> >  
> > -static struct slice_mask slice_mask_for_size(struct mm_struct *mm,
> > int psize) +static struct slice_mask
> > calc_slice_mask_for_size(struct mm_struct *mm, int psize) {
> >  	unsigned char *hpsizes;
> >  	int index, mask_index;
> > @@ -171,6 +171,36 @@ static struct slice_mask
> > slice_mask_for_size(struct mm_struct *mm, int psize) return ret;
> >  }
> >  
> > +static void recalc_slice_mask_cache(struct mm_struct *mm)
> > +{
> > +	mm->context.mask_4k = calc_slice_mask_for_size(mm,
> > MMU_PAGE_4K); +#ifdef CONFIG_PPC_64K_PAGES
> > +	mm->context.mask_64k = calc_slice_mask_for_size(mm,
> > MMU_PAGE_64K); +#endif
> > +# ifdef CONFIG_HUGETLB_PAGE
> > +	/* Radix does not come here */
> > +	mm->context.mask_16m = calc_slice_mask_for_size(mm,
> > MMU_PAGE_16M);
> > +	mm->context.mask_16g = calc_slice_mask_for_size(mm,
> > MMU_PAGE_16G); +# endif
> > +}  
> 
> Should the function above be called under slice_convert_lock?

Good question. The slice_convert_lock is... interesting. It only
protects the update-side of the slice page size arrays. I thought
this was okay last time I looked, but now you make me think again
maybe it is not. I need to check again what's providing exclusion
on the read side too.

I wanted to avoid doing more work under slice_convert_lock, but
we should just make that a per-mm lock anyway shouldn't we?

Thanks,
Nick