linux-kernel - Re: [PATCH 4/5] rust: percpu: Add pin-hole optimizations for numerics

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Message-ID: <685d9677.170a0220.3b599c.7713@mx.google.com>
Date: Thu, 26 Jun 2025 11:50:31 -0700
From: Mitchell Levy <levymitchell0@...il.com>
To: "Christoph Lameter (Ampere)" <cl@...two.org>
Cc: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>,
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dennis Zhou <dennis@...nel.org>, Tejun Heo <tj@...nel.org>,
	Danilo Krummrich <dakr@...nel.org>,
	Benno Lossin <lossin@...nel.org>, linux-kernel@...r.kernel.org,
	rust-for-linux@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 4/5] rust: percpu: Add pin-hole optimizations for numerics

On Wed, Jun 25, 2025 at 10:21:17AM -0700, Christoph Lameter (Ampere) wrote:
> On Tue, 24 Jun 2025, Mitchell Levy wrote:
> 
> > The C implementations of `this_cpu_add`, `this_cpu_sub`, etc., are
> > optimized to save an instruction by avoiding having to compute
> > `this_cpu_ptr(&x)` for some per-CPU variable `x`. For example, rather
> > than
> 
> Cool. Great progress for Rust support. Maybe we can switch the SLUB
> allocator over or come up with SLRB for the Slab Rust allocator ;-)

Thank you!

I'm certainly very excited about the prospect of more Rust :)

> 
> > +        impl PerCpuNumeric<'_, $ty> {
> > +            /// Adds `rhs` to the per-CPU variable.
> > +            pub fn add(&mut self, rhs: $ty) {
> > +                // SAFETY: `self.ptr.0` is a valid offset into the per-CPU area (i.e., valid as a
> > +                // pointer relative to the `gs` segment register) by the invariants of PerCpu.
> > +                unsafe {
> > +                    asm!(
> > +                        concat!("add gs:[{off}], {val}"),
> > +                        off = in(reg) self.ptr.0 as *mut $ty,
> > +                        val = in(reg_byte) rhs,
> 
> That looks arch specific to x86? What about ARM and other platforms?

Yes; pretty much everything added by this series is x86_64 specific. In
`rust/kernel/lib.rs` the whole percpu module is gated behind
`#[cfg(CONFIG_X86_64)]`. 

I'm certainly interested in adding support for ARM and other
architectures. That said, x86 is where I started, and since it's in a
workable state, I wanted to get some input from the list.