Re: [PATCH v1] arch: Enable function alignment for arm64

[Will Deacon <will@kernel.org>]

On Wed, Dec 07, 2022 at 09:36:48PM -0800, Mina Almasry wrote:
> 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@infradead.org>
> ---
>  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.

This increases the size of .text for a defconfig build by ~2%, so I think it
would be nice to have some real numbers for the performance uplift. Are you
able to elaborate beyond "double-digit percentage hackbench regression"?

In general, however, I'm supportive of the patch (and it seems that x86
does the same thing) so:

Acked-by: Will Deacon <will@kernel.org>

Will