linux-kernel - Re: [PATCH RFC 3/3] rust: percpu: add a rust per-CPU variable test

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Message-ID: <Z3LoZNsWpwvyT1FB@boqun-archlinux>
Date: Mon, 30 Dec 2024 10:37:24 -0800
From: Boqun Feng <boqun.feng@...il.com>
To: "Christoph Lameter (Ampere)" <cl@...two.org>
Cc: Mitchell Levy <levymitchell0@...il.com>,
	Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>, Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <benno.lossin@...ton.me>,
	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>,
	linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH RFC 3/3] rust: percpu: add a rust per-CPU variable test

On Fri, Dec 20, 2024 at 09:56:54AM -0800, Christoph Lameter (Ampere) wrote:
> On Thu, 19 Dec 2024, Mitchell Levy wrote:
> 
> > +        let mut native: i64 = 0;
> > +        let mut pcpu: PerCpuRef<i64> = unsafe { unsafe_get_per_cpu_ref!(PERCPU, CpuGuard::new()) };
> 
> A bit complex.
> 
> > +        native += -1;
> > +        *pcpu += -1;
> > +        assert!(native == *pcpu && native == -1);
> > +
> > +        native += 1;
> > +        *pcpu += 1;
> > +        assert!(native == *pcpu && native == 0);
> > +
> 
> That's pretty straightforward..... But is there no symbolic access to the
> per cpu namespace? How would you access the kernel per cpu variables
> defined in C?
> 

FWIW, there is a declare_extern_per_cpu!() marco in patch #1, and
Mitchell used it to access this_cpu_off (defined in C).

Maybe we want to see an example here?

Regards,
Boqun

> How do you go about using per cpu atomics like
> 
> this_cpu_inc(nr_dentry_unused);
> 
>