linux-kernel - Re: [PATCH v7 4/5] rust: add find_bit_benchmark

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Message-ID: <680a6b54.d40a0220.27afd9.5e84@mx.google.com>
Date: Thu, 24 Apr 2025 09:48:17 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Burak Emir <bqe@...gle.com>
Cc: Yury Norov <yury.norov@...il.com>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	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>,
	rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org,
	Rong Xu <xur@...gle.com>
Subject: Re: [PATCH v7 4/5] rust: add find_bit_benchmark_rust module.

On Thu, Apr 24, 2025 at 06:45:33PM +0200, Burak Emir wrote:
> On Wed, Apr 23, 2025 at 6:56 PM Yury Norov <yury.norov@...il.com> wrote:
> > So? Can you show your numbers?
> 
> For now, I only have numbers that may not be very interesting:
> 
> - for find_next_bit,  find_next_zero_bit and find_next_zero_bit (sparse):
>   22 ns/iteration in C, 32 ns/iteration in Rust.
> 
> - for sparse find_next_bit (sparse):
>   60 ns/iteration in C, 70 ns/iteration in Rust.
> 
> This is a VM running nested in a VM. More importantly: the C helper
> method is not inlined.
> So we are likely measuring the overhead (plus the extra bounds checking).
> 
> I would like to get cross-language inlining to work with thinLTO to
> have a more realistic comparison.
> However, that is not something that works out of the box.
> I am looking at Gary Guo's patch for this:
> https://lore.kernel.org/all/20250319205141.3528424-1-gary@garyguo.net/
> Currently, I get duplicate symbol errors.
> 

You will need to add __rust_helper attribute for the new rust helpers
introduce in your patches. See Gary's patch #2 for example.

Regards,
Boqun

> > Can you print the existing C test output back to back with the new one?
> > Can you also ask 0-day folks to enable your test in their rust config?
> 
> Will look into these. Rong (hi!) is working on LTO for kernel and will
> know a lot more than me how Rust will fit in eventually.
> IMHO, making cross-language inlining work out of the box will be a
> necessary baseline to get Rust performance for hot code.
> 
> > > We add a fill_random() method protected by the config in order to
> > > maintain the abstraction.
> > >
> > > Minor fix to the documentation of the corresponding C config
> > > FIND_BIT_BENCHMARK, it was mentioning the wrong module name.
> >
> > Indeed. Can you make it a separate patch, please?
> 
> Will do.