Re: [PATCH v10 4/5] rust: add find_bit_benchmark_rust module.

[Yury Norov <yury.norov@gmail.com>]

On Mon, Jun 02, 2025 at 01:36:45PM +0000, Burak Emir 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@google.com>
> Suggested-by: Yury Norov <yury.norov@gmail.com>
> Signed-off-by: Burak Emir <bqe@google.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@google.com>
>  M:	Burak Emir <bqe@google.com>
>  R:	Yury Norov <yury.norov@gmail.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..d8ed53f35495 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 RUST
> +	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/lib/Makefile b/lib/Makefile
> index f07b24ce1b3f..99e49a8f5bf8 100644
> --- a/lib/Makefile
> +++ b/lib/Makefile
> @@ -62,6 +62,7 @@ obj-y += hexdump.o
>  obj-$(CONFIG_TEST_HEXDUMP) += test_hexdump.o
>  obj-y += kstrtox.o
>  obj-$(CONFIG_FIND_BIT_BENCHMARK) += find_bit_benchmark.o
> +obj-$(CONFIG_FIND_BIT_BENCHMARK_RUST) += find_bit_benchmark_rust.o
>  obj-$(CONFIG_TEST_BPF) += test_bpf.o
>  test_dhry-objs := dhry_1.o dhry_2.o dhry_run.o
>  obj-$(CONFIG_TEST_DHRY) += test_dhry.o
> diff --git a/lib/find_bit_benchmark_rust.rs b/lib/find_bit_benchmark_rust.rs
> new file mode 100644
> index 000000000000..468a2087f68c
> --- /dev/null
> +++ b/lib/find_bit_benchmark_rust.rs
> @@ -0,0 +1,95 @@
> +// SPDX-License-Identifier: GPL-2.0
> +//! Benchmark for find_bit-like methods in Bitmap Rust API.
> +
> +use kernel::alloc::flags::GFP_KERNEL;
> +use kernel::bindings;
> +use kernel::bitmap::Bitmap;
> +use kernel::error::{code, Result};
> +use kernel::prelude::module;
> +use kernel::time::Ktime;
> +use kernel::ThisModule;
> +use kernel::{pr_cont, pr_err};
> +
> +const BITMAP_LEN: usize = 4096 * 8 * 10;
> +// Reciprocal of the fraction of bits that are set in sparse bitmap.
> +const SPARSENESS: usize = 500;

Is there any simple mechanism to keep C and rust sizes synced? (If no,
not a big deal to redefine them.)

> +
> +/// Test module that benchmarks performance of traversing bitmaps.
> +struct FindBitBenchmarkModule();
> +
> +fn test_next_bit(bitmap: &Bitmap) {
> +    let mut time = Ktime::ktime_get();
> +    let mut cnt = 0;
> +    let mut i = 0;
> +
> +    while let Some(index) = bitmap.next_bit(i) {
> +        cnt += 1;
> +        i = index + 1;
> +    }
> +
> +    time = Ktime::ktime_get() - time;
> +    pr_cont!(
> +        "next_bit:           {:18} ns, {:6} iterations\n",
> +        time.to_ns(),
> +        cnt
> +    );
> +}
> +
> +fn test_next_zero_bit(bitmap: &Bitmap) {
> +    let mut time = Ktime::ktime_get();
> +    let mut cnt = 0;
> +    let mut i = 0;
> +
> +    while let Some(index) = bitmap.next_zero_bit(i) {
> +        cnt += 1;
> +        i = index + 1;
> +    }
> +
> +    time = Ktime::ktime_get() - time;
> +    pr_cont!(
> +        "next_zero_bit:      {:18} ns, {:6} iterations\n",
> +        time.to_ns(),
> +        cnt
> +    );
> +}
> +
> +fn find_bit_test() {
> +    pr_err!("\n");
> +    pr_cont!("Start testing find_bit() Rust with random-filled bitmap\n");
> +
> +    let mut bitmap = Bitmap::new(BITMAP_LEN, GFP_KERNEL).expect("alloc bitmap failed");
> +    bitmap.fill_random();
> +
> +    test_next_bit(&bitmap);
> +    test_next_zero_bit(&bitmap);
> +
> +    pr_cont!("Start testing find_bit() Rust with sparse bitmap\n");
> +
> +    let mut bitmap = Bitmap::new(BITMAP_LEN, GFP_KERNEL).expect("alloc sparse bitmap failed");
> +    let nbits = BITMAP_LEN / SPARSENESS;
> +    for _i in 0..nbits {
> +        // SAFETY: BITMAP_LEN fits in 32 bits.
> +        let bit: usize =
> +            unsafe { bindings::__get_random_u32_below(BITMAP_LEN.try_into().unwrap()) as _ };
> +        bitmap.set_bit(bit);
> +    }
> +
> +    test_next_bit(&bitmap);
> +    test_next_zero_bit(&bitmap);
> +}
> +
> +impl kernel::Module for FindBitBenchmarkModule {
> +    fn init(_module: &'static ThisModule) -> Result<Self> {
> +        find_bit_test();
> +        // Return error so test module can be inserted again without rmmod.
> +        Err(code::EINVAL)
> +    }
> +}
> +
> +module! {
> +    type: FindBitBenchmarkModule,

I think we agreed to have the type something less unique, like:

        Benchmark.

> +    name: "find_bit_benchmark_rust_module",

What is the name policy for rust? Maybe a more human-readable name
would work better here?

All the above are nits. Please have my

Reviewed-by: Yury Norov [NVIDIA] <yury.norov@gmail.com>

Thanks,
Yury

> +    authors: ["Burak Emir <bqe@google.com>"],
> +    description: "Module with benchmark for bitmap Rust API",
> +    license: "GPL v2",
> +}
> diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
> index b6bf3b039c1b..f6ca7f1dd08b 100644
> --- a/rust/bindings/bindings_helper.h
> +++ b/rust/bindings/bindings_helper.h
> @@ -31,6 +31,7 @@
>  #include <linux/platform_device.h>
>  #include <linux/poll.h>
>  #include <linux/property.h>
> +#include <linux/random.h>
>  #include <linux/refcount.h>
>  #include <linux/sched.h>
>  #include <linux/security.h>
> 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,
> +            );
> +        }
> +    }
>  }
>  
>  impl CBitmap {
> -- 
> 2.49.0.1204.g71687c7c1d-goog