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Message-ID: <pXt1ZkjyScL3fnnuyw12MHGyxxDunKpUjhGl_tL4kg8FpobamrUohmzQyqMxtCPfgbUmmPl4ZE6wco8Yqs6aLD5jLTD5ZL-zeNo8wk0uKUM=@protonmail.com>
Date: Thu, 05 Jun 2025 21:37:00 +0000
From: Pekka Ristola <pekkarr@...tonmail.com>
To: Burak Emir <bqe@...gle.com>
Cc: Yury Norov <yury.norov@...il.com>, Kees Cook <kees@...nel.org>, 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>, "Gustavo A . R . Silva" <gustavoars@...nel.org>, Carlos LLama <cmllamas@...gle.com>, rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v11 4/5] rust: add find_bit_benchmark_rust module.
On Monday, June 2nd, 2025 at 16.53, Burak Emir <bqe@...gle.com> wrote:
> Microbenchmark protected by a config FIND_BIT_BENCHMARK_RUST,
> following `find_bit_benchmark.c` but testing the Rust Bitmap API.
>
> We add a fill_random() method protected by the config in order to
> maintain the abstraction.
>
> The sample output from the benchmark, both C and Rust version:
>
> find_bit_benchmark.c output:
> ```
> Start testing find_bit() with random-filled bitmap
> [ 438.101937] find_next_bit: 860188 ns, 163419 iterations
> [ 438.109471] find_next_zero_bit: 912342 ns, 164262 iterations
> [ 438.116820] find_last_bit: 726003 ns, 163419 iterations
> [ 438.130509] find_nth_bit: 7056993 ns, 16269 iterations
> [ 438.139099] find_first_bit: 1963272 ns, 16270 iterations
> [ 438.173043] find_first_and_bit: 27314224 ns, 32654 iterations
> [ 438.180065] find_next_and_bit: 398752 ns, 73705 iterations
> [ 438.186689]
> Start testing find_bit() with sparse bitmap
> [ 438.193375] find_next_bit: 9675 ns, 656 iterations
> [ 438.201765] find_next_zero_bit: 1766136 ns, 327025 iterations
> [ 438.208429] find_last_bit: 9017 ns, 656 iterations
> [ 438.217816] find_nth_bit: 2749742 ns, 655 iterations
> [ 438.225168] find_first_bit: 721799 ns, 656 iterations
> [ 438.231797] find_first_and_bit: 2819 ns, 1 iterations
> [ 438.238441] find_next_and_bit: 3159 ns, 1 iterations
> ```
>
> find_bit_benchmark_rust.rs output:
> ```
> [ 451.182459] find_bit_benchmark_rust_module:
> [ 451.186688] Start testing find_bit() Rust with random-filled bitmap
> [ 451.194450] next_bit: 777950 ns, 163644 iterations
> [ 451.201997] next_zero_bit: 918889 ns, 164036 iterations
> [ 451.208642] Start testing find_bit() Rust with sparse bitmap
> [ 451.214300] next_bit: 9181 ns, 654 iterations
> [ 451.222806] next_zero_bit: 1855504 ns, 327026 iterations
> ```
>
> Here are the results from 32 samples, with 95% confidence interval.
> The microbenchmark was built with RUST_BITMAP_HARDENED=n and run on a
> machine that did not execute other processes.
>
> Random-filled bitmap:
> +-----------+-------+-----------+--------------+-----------+-----------+
> | Benchmark | Lang | Mean (ms) | Std Dev (ms) | 95% CI Lo | 95% CI Hi |
> +-----------+-------+-----------+--------------+-----------+-----------+
> | find_bit/ | C | 825.07 | 53.89 | 806.40 | 843.74 |
> | next_bit | Rust | 870.91 | 46.29 | 854.88 | 886.95 |
> +-----------+-------+-----------+--------------+-----------+-----------+
> | find_zero/| C | 933.56 | 56.34 | 914.04 | 953.08 |
> | next_zero | Rust | 945.85 | 60.44 | 924.91 | 966.79 |
> +-----------+-------+-----------+--------------+-----------+-----------+
>
> Rust appears 5.5% slower for next_bit, 1.3% slower for next_zero.
>
> Sparse bitmap:
> +-----------+-------+-----------+--------------+-----------+-----------+
> | Benchmark | Lang | Mean (ms) | Std Dev (ms) | 95% CI Lo | 95% CI Hi |
> +-----------+-------+-----------+--------------+-----------+-----------+
> | find_bit/ | C | 13.17 | 6.21 | 11.01 | 15.32 |
> | next_bit | Rust | 14.30 | 8.27 | 11.43 | 17.17 |
> +-----------+-------+-----------+--------------+-----------+-----------+
> | find_zero/| C | 1859.31 | 82.30 | 1830.80 | 1887.83 |
> | next_zero | Rust | 1908.09 | 139.82 | 1859.65 | 1956.54 |
> +-----------+-------+-----------+--------------+-----------+-----------+
>
> Rust appears 8.5% slower for next_bit, 2.6% slower for next_zero.
>
> In summary, taking the arithmetic mean of all slow-downs, we can say
> the Rust API has a 4.5% slowdown.
>
> Suggested-by: Alice Ryhl <aliceryhl@...gle.com>
> Suggested-by: Yury Norov <yury.norov@...il.com>
> Signed-off-by: Burak Emir <bqe@...gle.com>
> ---
> MAINTAINERS | 1 +
> lib/Kconfig.debug | 13 +++++
> lib/Makefile | 1 +
> lib/find_bit_benchmark_rust.rs | 95 +++++++++++++++++++++++++++++++++
> rust/bindings/bindings_helper.h | 1 +
> rust/kernel/bitmap.rs | 14 +++++
> 6 files changed, 125 insertions(+)
> create mode 100644 lib/find_bit_benchmark_rust.rs
>
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 565eaa015d9e..943d85ed1876 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -4132,6 +4132,7 @@ M: Alice Ryhl <aliceryhl@...gle.com>
> M: Burak Emir <bqe@...gle.com>
> R: Yury Norov <yury.norov@...il.com>
> S: Maintained
> +F: lib/find_bit_benchmark_rust.rs
> F: rust/kernel/bitmap.rs
>
> BITOPS API
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index f9051ab610d5..3f826a73bdbf 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -2605,6 +2605,19 @@ config FIND_BIT_BENCHMARK
>
> If unsure, say N.
>
> +config FIND_BIT_BENCHMARK_RUST
> + tristate "Test find_bit functions in Rust"
> + depends on CONFIG_RUST
The `CONFIG_` prefix should be removed.
> + help
> + This builds the "find_bit_benchmark_rust" module. It is a micro
> + benchmark that measures the performance of Rust functions that
> + correspond to the find_*_bit() operations in C. It follows the
> + FIND_BIT_BENCHMARK closely but will in general not yield same
> + numbers due to extra bounds checks and overhead of foreign
> + function calls.
> +
> + If unsure, say N.
> +
> config TEST_FIRMWARE
> tristate "Test firmware loading via userspace interface"
> depends on FW_LOADER
[...]
> diff --git a/rust/kernel/bitmap.rs b/rust/kernel/bitmap.rs
> index 28c11e400d1e..9fefb2473099 100644
> --- a/rust/kernel/bitmap.rs
> +++ b/rust/kernel/bitmap.rs
> @@ -252,6 +252,20 @@ pub fn new(nbits: usize, flags: Flags) -> Result<Self, AllocError> {
> pub fn len(&self) -> usize {
> self.nbits
> }
> +
> + /// Fills this `Bitmap` with random bits.
> + #[cfg(CONFIG_FIND_BIT_BENCHMARK_RUST)]
> + pub fn fill_random(&mut self) {
> + // SAFETY: `self.as_mut_ptr` points to either an array of the
> + // appropriate length or one usize.
> + unsafe {
> + bindings::get_random_bytes(
> + self.as_mut_ptr() as *mut ffi::c_void,
> + usize::div_ceil(self.nbits, bindings::BITS_PER_LONG as usize)
> + * bindings::BITS_PER_LONG as usize,
I think the second argument to `get_random_bytes` should be in bytes, not
bits. So it should be multiplied by "bytes per long", not "bits per long".
> + );
> + }
> + }
> }
>
> impl CBitmap {
> --
> 2.49.0.1204.g71687c7c1d-goog
Cheers,
Pekka
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