linux-kernel - Re: [External] Re: [PATCH RFC] riscv: use gp to save percpu offset

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Message-ID: <CAEEQ3w=PsM5T+yMrEGdWZ2nm7m7SX3vzscLtWpOPVu1zpfm3YQ@mail.gmail.com>
Date: Fri, 30 Aug 2024 18:01:31 +0800
From: yunhui cui <cuiyunhui@...edance.com>
To: "Christoph Lameter (Ampere)" <cl@...two.org>
Cc: punit.agrawal@...edance.com, paul.walmsley@...ive.com, palmer@...belt.com, 
	aou@...s.berkeley.edu, dennis@...nel.org, tj@...nel.org, 
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	charlie@...osinc.com, cleger@...osinc.com, puranjay@...nel.org, 
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	linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org
Subject: Re: [External] Re: [PATCH RFC] riscv: use gp to save percpu offset

Hi Christoph,


On Sat, Aug 24, 2024 at 9:57 AM Christoph Lameter (Ampere)
<cl@...two.org> wrote:
>
> On Sat, 24 Aug 2024, Yunhui Cui wrote:
>
> > Compared to directly fetching the per-CPU offset from memory (or cache),
> > using the global pointer (gp) to store the per-CPU offset can save one
> > memory access.
>
> Yes! That is a step in the right direction.
>
> Is there something like gp relative addressing so that we can do loads
> and stores relative to gp as well?
>
> Are there atomics that can do read modify write relative to GP? That would
> get  you to comparable per cpu efficiency to x86. x86 can do relative
> addressing and RMV in one instruction which allows one to drop the preempt
> enable/disable since one instruction cannot be interrupted.

Your suggestion is excellent. If conditions permit, we can indeed move
closer to the x86 architecture.

Thanks,
Yunhui