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

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Message-ID: <677dbbb1.050a0220.39ba68.8825@mx.google.com>
Date: Tue, 7 Jan 2025 15:41:28 -0800
From: Mitchell Levy <levymitchell0@...il.com>
To: Charalampos Mitrodimas <charmitro@...teo.net>
Cc: "Christoph Lameter (Ampere)" <cl@...two.org>,
	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>,
	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 Sun, Jan 05, 2025 at 01:01:43PM +0000, Charalampos Mitrodimas wrote:
> "Christoph Lameter (Ampere)" <cl@...two.org> writes:
> 
> > 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.
> 
> I agree with this, maybe a helper function would suffise? Something in
> terms of,
>   unsafe fn get_per_cpu<T>(var: &PerCpuVariable<T>) -> PerCpuRef<T> {
> 	  unsafe_get_per_cpu_ref!(var, CpuGuard::new())
>   }

I'm certainly open to adding such a helper. Is the main concern here the
unwieldy name? Generally, I prefer to keep modifications to global state
(disabling preemption via CpuGuard::new()) as explicit as possible, but
if there's consensus to the contrary, I'm happy to roll it into the
macro/a helper function.

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