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

[Mina Almasry]

On Wed, Jan 25, 2023 at 3:16 AM David Laight <David.Laight@xxxxxxxxxx> wrote:
>
> From: Will Deacon <will@xxxxxxxxxx>
> > Sent: 24 January 2023 12:09
> >
> > 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.
> > >
> ...
> >
> > 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:
>
> I bet it just changes the alignment of the code so that more
> functions are using different cache lines.
>
> All sorts of other random changes are likely to have a similar effect.
>
> Cache-line aligning the start of a function probably reduces the
> number of cache lines the functions needs - but that isn't guaranteed.
> It also slightly reduces the delay on a cache miss - but they are so
> slow it probably makes almost no difference.
>

David, my understanding is similar to yours. I.e. without explicit alignment:

1. Random changes to the code can cause critical path functions to
become aligned or unaligned which will cause perf
regressions/improvements.
2. Random changes to the code can cause critical path functions to be
placed near a cache line boundary, causing one more cache line to be
loaded when they are run, which will cause perf regressions.

So for these very reasons function alignment is a good thing.

>         David
>
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