Messages in this thread | ![/](/images/icornerl.gif) | | From | David Laight <> | Subject | RE: [PATCH v1] arch: Enable function alignment for arm64 | Date | Wed, 25 Jan 2023 11:16:19 +0000 |
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From: Will Deacon <will@kernel.org> > 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
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