Message-ID: <b6c7eaeb-34a3-4927-b4d6-70aab3ce1c46@nvidia.com>
Date: Fri, 21 Nov 2025 20:14:59 -0800
From: John Hubbard <jhubbard@...dia.com>
To: Boqun Feng <boqun.feng@...il.com>
Cc: Lyude Paul <lyude@...hat.com>, rust-for-linux@...r.kernel.org,
 linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
 Daniel Almeida <daniel.almeida@...labora.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 <lossin@...nel.org>,
 Andreas Hindborg <a.hindborg@...nel.org>, Alice Ryhl <aliceryhl@...gle.com>,
 Trevor Gross <tmgross@...ch.edu>, Danilo Krummrich <dakr@...nel.org>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
 Will Deacon <will@...nel.org>, Waiman Long <longman@...hat.com>
Subject: Re: [PATCH v14 00/16] Refcounted interrupts, SpinLockIrq for rust

On 11/21/25 7:35 PM, Boqun Feng wrote:
> On Fri, Nov 21, 2025 at 06:56:28PM -0800, John Hubbard wrote:
>> On 11/21/25 6:38 PM, Boqun Feng wrote:
> [...]
>>>
>>> Last but not least, safe Rust is preferred, but it doesn't mean unsafe
>>> code should be avoided completely, if we establish some data that shows
>>
>> Perhaps we need to be slightly more precise. I'm not sure if you are
>> referring to the usual practice of creating an unsafe block, wrapped
>> within a safe Rust function, or something else?
>>
> 
> I was referring to providing an unsafe API for core kernel
> functionality, for example local_irq_disable(), and then teaching how to
> use it correctly.

Ack.

> 
>>> some unsafe code provides better performance and we have clear guideline
>>> for the particular scenarios, then it's definitely OK. Hence I don't
>>> fully agree your saying "Safe Rust is the whole point of this project",
>>> to me understanding how we can utilize the type system and other tools
>>> is more of a realistic goal.
>>>
>>>> Is 3.6x longer really something we are stuck with? Or is there some other
>>>> way forward that could potentially provide higher performance, for Safe
>>>> Rust?
>>>>
>>>
>>> Well by 3.6x longer, you mean ~1.3ns vs ~4.5ns, right? And in real world
>>> code, the code in the interrupt disabling critical section would be more
>>> than couples of nano seconds, hence the delta will probably be
>>> noise-out. But again, yes if 3ns turns out to be a bottleneck in the
>>> driver, we are happy to look into, but you need to show the data.
>>>
>>
>> So this is what I'm asking about: given that we *already know* that we
>> have a performance drop in the micro-benchmark, is there any reasonable
>> approach that avoids this? Or has a less noticeable impact?
>>
> 
> Lyude had tried another approach [1], which uses an unsafe public API,
> and doesn't work (easily) with CondVar or PREEMPT_RT And that eventually
> triggered more discussion about a better API design, and as Thomas
> pointed out [2]: "Stop worrying about mostly irrelevant low level
> details which are not relevant to the primary audience of rust adoption.
> We can worry about them when we replace the scheduler and the low level
> interrupt handling code ten years down the road." And I agreed. The
> current implementation is actually quite efficient and should even
> out-perform the existing API in some cases as I pointed out. More
> importantly, it utilizes Rust type system and make it easy to use (or
> hard to mis-use).
> 
> That being said, if anyone has a better idea, feel free to bring it up.
> 
>> I'm asking early (see above: I agree that this is "premature"), because
>> we have early data.
>>
>> It would be nice to explore now, rather than later, after someone shows
>> up with detailed perf data about their use case.
>>
>>
> 
> Not sure I fully agree with this, given it's to my knowledge the best
> solution at the moment, I feel it's hard to justify the cost of
> exploring a better solution without a real usage. But then again, if
> anyone has any better idea feel free to bring it up.
> 
> [1]: https://lore.kernel.org/rust-for-linux/20240916213025.477225-2-lyude@redhat.com/
> [2]: https://lore.kernel.org/rust-for-linux/87iktrahld.ffs@tglx/
> 

Thanks for this context, I hadn't followed the earlier discussions,
and when looking at this v14, it seemed to gloss over the performance
implications (they were linked to, but not discussed).

I won't further harass you all about this, let's see how it goes. :)

Optionally, it might be helpful to include some top-level notes
that justify the choices made so far.

thanks,
-- 
John Hubbard

Powered by blists - more mailing lists