s390/mm: have 16 byte aligned struct pages

Select HAVE_ALIGNED_STRUCT_PAGE on s390, so that the slub allocator can make
use of compare and swap double for lockless updates. This increases the size
of struct page to 64 bytes (instead of 56 bytes), however the performance gain
justifies the increased size:

- now excactly four struct pages fit into a single cache line; the
  case that accessing a struct page causes two cache line loads
  does not exist anymore.
- calculating the offset of a struct page within the memmap array
  is only a simple shift instead of a more expensive multiplication.

A "hackbench 200 process 200" run on a 32 cpu system did show an 8% runtime
improvement.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
index 5dba755a43e60911ae2a5b922bde0c9f4a70b6f1..d385f396dfee90172e5a984702cd9ded8a76387d 100644 (file)
--- a/arch/s390/Kconfig
+++ b/arch/s390/Kconfig
@@ -96,6 +96,7 @@ config S390
        select HAVE_MEMBLOCK_NODE_MAP
        select HAVE_CMPXCHG_LOCAL
        select HAVE_CMPXCHG_DOUBLE
+       select HAVE_ALIGNED_STRUCT_PAGE if SLUB
        select HAVE_VIRT_CPU_ACCOUNTING
        select VIRT_CPU_ACCOUNTING
        select ARCH_DISCARD_MEMBLOCK