Messages in this thread | | | Date | Tue, 24 Jan 2023 12:09:15 +0000 | From | Will Deacon <> | Subject | Re: [PATCH v1] arch: Enable function alignment for arm64 |
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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
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