Re: [PATCH 0/4] arm64: an optimization for AmpereOne

[Linus Walleij]

On Wed, Nov 22, 2023 at 10:29 AM Huang Shijie
<shijie@xxxxxxxxxxxxxxxxxxxxxx> wrote:

> 0) Background:
>    We found that AmpereOne benefits from aggressive prefetches when
>    using 4K page size.
>
> 1) This patch:
>     1.1) adds new WORKAROUND_AMPERE_AC03_PREFETCH capability.
>     1.2) uses MIDR_AMPERE1 to filter the processor.
>     1.3) uses alternative_if to alternative the code
>          for AmpereOne.
>     1.4) adds software prefetches for the specific loop.
>          Also add a macro add_prefetch.
>
> 2) Test result:
>     In hugetlb or tmpfs, We can get big seqential read performance improvement
>     up to 1.3x ~ 1.4x.

In June 2022 Fujitsu tried to add a similar feature for A64FX, here is
the essence
of my feedback from back then, it applies here as well:
https://lore.kernel.org/linux-arm-kernel/CACRpkdbPLFOoPdX4L6ABV8GKpC8cQGP3s2aN2AvRHEK49U9VMg@xxxxxxxxxxxxxx/#t

TL;DR: this is a hack, if you want to accelerate the memory hierarchy,
then work with the MM developers to figure out how to do that in a
structured and scientific way that will work with any prefetching hardware
on any CPU.

Yours,
Linus Walleij