Re: [WIP 0/3] Memory model and atomic API in Rust

From: Boqun Feng <boqun.feng@gmail.com>
To: Mark Rutland <mark.rutland@arm.com>
Cc: rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-arch@vger.kernel.org, llvm@lists.linux.dev,
	Miguel Ojeda <ojeda@kernel.org>,
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Subject: Re: [WIP 0/3] Memory model and atomic API in Rust
Date: Mon, 25 Mar 2024 13:59:55 -0700	[thread overview]
Message-ID: <ZgHly_fioG7X4wGE@boqun-archlinux> (raw)
In-Reply-To: <ZgFVnar3nS4F8eIX@FVFF77S0Q05N>

On Mon, Mar 25, 2024 at 10:44:45AM +0000, Mark Rutland wrote:
[...]
> > 
> > * I choose to re-implement atomics in Rust `asm` because we are still
> >   figuring out how we can make it easy and maintainable for Rust to call
> >   a C function _inlinely_ (Gary makes some progress [2]). Otherwise,
> >   atomic primitives would be function calls, and that can be performance
> >   bottleneck in a few cases.
> 
> I don't think we want to maintain two copies of each architecture's atomics.
> This gets painful very quickly (e.g. as arm64's atomics get patched between
> LL/SC and LSE forms).
> 

No argument here ;-)

> Can we start off with out-of-line atomics, and see where the bottlenecks are?
> 
> It's relatively easy to do that today, at least for the atomic*_*() APIs:
> 
>   https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/commit/?h=atomics/outlined&id=e0a77bfa63e7416d610769aa4ab62bc06993ce56
> 
> ... which IIUC covers the "AtomicI32, AtomicI64 and AtomicUsize" cases you
> mention above.
> 

Thanks! Yes, I know I should check with you before I finalize the
implementation ;-) I will try to integrate that but things to notice:

* For module usage, we need to EXPORT_SYMBOL_GPL() all the atomics, I'm
  OK with that, but I don't know how others feel about it.

* Alice reported performance gap between inline and out-of-line refcount
  operations in Rust binder driver:

	https://github.com/Darksonn/linux/commit/b4be1bd6c44225bf7276a4666fd30b8da9cba517	

  I don't know how much worse since I don't have the data, but that's
  one of the reasons I started with inline asm.

That being said, I totally agree that we could start with out-of-line
atomics, and maybe provide inline version for performance critical
paths. Hoping is we can figure out how Rust could inline a C function
eventually.

Regards,
Boqun

> Mark.