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

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Message-ID: <ZgHly_fioG7X4wGE@boqun-archlinux>
Date: Mon, 25 Mar 2024 13:59:55 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Mark Rutland <mark.rutland@....com>
Cc: rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, llvm@...ts.linux.dev,
	Miguel Ojeda <ojeda@...nel.org>,	Alex Gaynor <alex.gaynor@...il.com>,
	Wedson Almeida Filho <wedsonaf@...il.com>,	Gary Guo <gary@...yguo.net>,
	"Bj\"orn Roy Baron" <bjorn3_gh@...tonmail.com>,
	Benno Lossin <benno.lossin@...ton.me>,
	Andreas Hindborg <a.hindborg@...sung.com>,
	Alice Ryhl <aliceryhl@...gle.com>,
	Alan Stern <stern@...land.harvard.edu>,
	Andrea Parri <parri.andrea@...il.com>,	Will Deacon <will@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Nicholas Piggin <npiggin@...il.com>,	David Howells <dhowells@...hat.com>,
	Jade Alglave <j.alglave@....ac.uk>,	Luc Maranget <luc.maranget@...ia.fr>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Akira Yokosawa <akiyks@...il.com>,	Daniel Lustig <dlustig@...dia.com>,
	Joel Fernandes <joel@...lfernandes.org>,
	Nathan Chancellor <nathan@...nel.org>,
	Nick Desaulniers <ndesaulniers@...gle.com>,	kent.overstreet@...il.com,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, elver@...gle.com,
	Thomas Gleixner <tglx@...utronix.de>,	Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Catalin Marinas <catalin.marinas@....com>,	torvalds@...ux-foundation.org,
 linux-arm-kernel@...ts.infradead.org,	linux-fsdevel@...r.kernel.org
Subject: Re: [WIP 0/3] Memory model and atomic API in Rust

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.