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From:   Uros Bizjak <ubizjak@gmail.com>
Date:   Wed, 18 Oct 2023 19:07:45 +0200
Message-ID: <CAFULd4YHq7tKQBNk9bNYZkBo9C5XM+6=O7QZG3E-qKBVjSHb6g@mail.gmail.com>
Subject: Re: [PATCH v2 -tip] x86/percpu: Use C for arch_raw_cpu_ptr()
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Nadav Amit <namit@vmware.com>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Andy Lutomirski <luto@kernel.org>,
        Brian Gerst <brgerst@gmail.com>,
        Denys Vlasenko <dvlasenk@redhat.com>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Nick Desaulniers <ndesaulniers@google.com>
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On Wed, Oct 18, 2023 at 6:13=E2=80=AFPM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> On Wed, 18 Oct 2023 at 06:15, Uros Bizjak <ubizjak@gmail.com> wrote:
> >
> > Attached is the prototype patch that works for me (together with
> > Linus' FPU switching patch).
>
> That looks reasonable, but I think the separate compilation unit is
> unnecessary, and I still absolutely hate how that const_pcpu_hot thing
> is declared twice (your patch does it in both current.h and in
> percpu-hot.c).
>
> How about we just do the whole alias as a linker thing instead? So the
> same way that we just do
>
>     jiffies =3D jiffies_64;
>
> in our arch/x86/kernel/vmlinux.lds.S file, we could just do
>
>     const_pcpu_hot =3D pcpu_hot;
>
> in there.
>
> Then, as far as the compiler is concerned, we just have
>
>     DECLARE_PER_CPU_ALIGNED(
>         const struct pcpu_hot __percpu_seg_override,
>         const_pcpu_hot)
>
> and the compiler doesn't know that it's aliased to anything else.

Oh...

... this works, too! Please see the attached patch.

(Note to self: Leave linking stuff to linker) ;)

> And please do that declaration in just *one* place.

Sure. Now the patch looks quite slim, but works as expected, reducing
the number of current_task accesses from 3841 to 3220.

Thanks,
Uros.

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