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

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Message-ID: <CACQBu=XaOohewMnLj9PvgR5rYBxzYSXf2OAjCzUY=GFTJ9L=+Q@mail.gmail.com>
Date: Thu, 24 Apr 2025 18:45:33 +0200
From: Burak Emir <bqe@...gle.com>
To: Yury Norov <yury.norov@...il.com>
Cc: Rasmus Villemoes <linux@...musvillemoes.dk>, Viresh Kumar <viresh.kumar@...aro.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>, 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 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.

> 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.