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

On Sat, 23 Jul 2016 20:36:42 +1000
Balbir Singh [off-list ref] wrote:

On Sat, Jul 23, 2016 at 05:10:36PM +1000, Nicholas Piggin wrote:

quoted

On Sat, 23 Jul 2016 12:19:37 +1000
Balbir Singh [off-list ref] wrote:
  

quoted

On Fri, Jul 22, 2016 at 10:57:28PM +1000, Nicholas Piggin wrote:  

quoted

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 <redacted>
---
 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.
 

Oh! sorry, I meant a structure like mmu_psize_defs.

In that case, sure. Avoiding the branches might be worthwhile.

quoted

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.
  

Yeah.. good idea
MMU_PAGE_COUNT is 15, the size is going to be 15*8 bytes?

Unfortunately, slice_mask is 16 bytes. Only 10 are used, but it
seemed too ugly to try squashing things together.

quoted

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?
 

Yeah and Ben's comment in the reply suggest we already hold a
per mm lock on the read side.

Let's discuss this further in my reply to Ben.

Thanks,
Nick