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From: Huacai Chen <chenhuacai@kernel.org>
Date: Fri, 12 Apr 2024 12:00:53 +0800
Message-ID: <CAAhV-H5LMURye9syEasvbmm_gk5WrcJhbLZByXWXte5A5KaO8Q@mail.gmail.com>
Subject: Re: [PATCH v3 0/4] Give chance to build under !CONFIG_SMP for LoongArch
To: Arnd Bergmann <arnd@arndb.de>
Cc: Marc Zyngier <maz@kernel.org>, Tiezhu Yang <yangtiezhu@loongson.cn>, 
	Thomas Gleixner <tglx@linutronix.de>, loongarch@lists.linux.dev, 
	linux-kernel@vger.kernel.org, loongson-kernel@lists.loongnix.cn
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Hi, Arnd,

On Thu, Apr 11, 2024 at 2:07=E2=80=AFPM Arnd Bergmann <arnd@arndb.de> wrote=
:
>
> On Thu, Apr 11, 2024, at 06:26, Huacai Chen wrote:
> >
> > I remember that you both suggested not introducing NOSMP support for a
> > modern architecture which brings additional complexity. I wonder if
> > you still have the same attitude now. I will merge this series only if
> > you think it is worthy to introduce NOSMP now.
>
> It's an interesting question, as we have recently discussed two
> opposite ideas and may end up doing both (or possible neither)
> in the future:
>
> - On x86, there is no real reason to need non-SMP kernels as the
>   memory savings are fairly small, and it tends to break because
>   of lack of users testing it.
>
> - On arm64, we have never supported non-SMP kernels, but I have
>   looked at possibly adding this because there are still popular
>   ARM based system-in-package products with less than 128MB of
>   built-in RAM and only a single CPU. As these are moving from
>   32-bit to 64-bit cores, it becomes more interesting to build
>   a 64-bit UP kernel and save multiple megabytes.
>
> I think loongarch64 is in the same place as arm64 and riscv64
> (which does allow non-SMP builds) here, and there are good
> reasons to allow it on all of them now, even if we previously
> never had a need for it.
OK, thanks. This means you agree to support non-SMP on LoongArch now,
then I will review this series carefully.

Huacai
>
> That said, both the 9% observed performance improvements that
> Tiezhu Yang reported, and the memory savings that I saw are
> probably higher than they should be, so we may also want to
> investigate that further to see if we can improve the SMP
> kernel to better support UP runs.
>
>     Arnd
>