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

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Message-ID: <CACRpkdZoiA3hQN2Pgz8g01T9TdapRwiQLWc1_MttSE-FWUy7bg@mail.gmail.com>
Date:   Thu, 23 Nov 2023 08:59:48 +0100
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Huang Shijie <shijie@...amperecomputing.com>
Cc:     catalin.marinas@....com, will@...nel.org, mark.rutland@....com,
        suzuki.poulose@....com, broonie@...nel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        anshuman.khandual@....com, robh@...nel.org, oliver.upton@...ux.dev,
        maz@...nel.org, patches@...erecomputing.com,
        Kohei Tarumizu <tarumizu.kohei@...itsu.com>
Subject: Re: [PATCH 0/4] arm64: an optimization for AmpereOne

On Wed, Nov 22, 2023 at 10:29 AM Huang Shijie
<shijie@...amperecomputing.com> 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@mail.gmail.com/#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