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

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Message-ID: <CAHS8izNiM=ckfN1Bvpz5ARSv5OTFi6WLAA-Q9Gnzavp6p4zM_w@mail.gmail.com>
Date:   Wed, 7 Dec 2022 22:47:47 -0800
From:   Mina Almasry <almasrymina@...gle.com>
To:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] arch: Enable function alignment for arm64

On Wed, Dec 7, 2022 at 9:36 PM Mina Almasry <almasrymina@...gle.com> 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@...radead.org>

I missed the Signed-off-by: Mina Almasry <almasrymina@...gle.com> tag
here. I can add with the next version.

> ---
>  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