[PATCH v1] arch: Enable function alignment for arm64

[Mina Almasry]

We recently ran into a double-digit percentage hackbench regression
when backporting commit 12df140f0bdf ("mm,hugetlb: take hugetlb_lock
before decrementing h->resv_huge_pages") to an older kernel. This was
surprising since hackbench does use hugetlb pages at all and the
modified code is not invoked. After some debugging we found that the
regression can be fixed by back-porting commit d49a0626216b ("arch:
Introduce CONFIG_FUNCTION_ALIGNMENT") and enabling function alignment
for arm64. I suggest enabling it by default for arm64 if possible.

Tested by examing function alignment on a compiled object file
before/after:

After this patch:

$ ~/is-aligned.sh mm/hugetlb.o 16
file=mm/hugetlb.o, alignment=16
total number of functions: 146
total number of unaligned: 0

Before this patch:

$ ~/is-aligned.sh mm/hugetlb.o 16
file=mm/hugetlb.o, alignment=16
total number of functions: 146
total number of unaligned: 94

Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
---
 arch/arm64/Kconfig | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index cf6d1cd8b6dc..bcc9e1578937 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -235,6 +235,7 @@ config ARM64
 	select TRACE_IRQFLAGS_SUPPORT
 	select TRACE_IRQFLAGS_NMI_SUPPORT
 	select HAVE_SOFTIRQ_ON_OWN_STACK
+	select FUNCTION_ALIGNMENT_16B
 	help
 	  ARM 64-bit (AArch64) Linux support.

--
2.39.0.rc0.267.gcb52ba06e7-goog