Re: [PATCH] lib/find_bit_benchmark: avoid clearing randomly filled bitmap in test_find_first_bit()

[Yury Norov]

On Wed, Mar 11, 2026 at 12:21:26AM +0900, Akinobu Mita wrote:
> test_find_first_bit() searches for a set bit from the beginning of the
> test bitmap and clears it repeatedly, eventually clearing the entire
> bitmap.
> 
> After test_find_first_bit() is executed, test_find_first_and_bit() and
> test_find_next_and_bit() are executed without randomly reinitializing the
> cleared bitmap.
> 
> In the first phase (testing find_bit() with a random-filled bitmap),
> test_find_first_bit() only operates on 1/10 of the entire size of the
> testing bitmap, so this isn't a big problem.
> 
> However, in the second phase (testing find_bit() with a sparse bitmap),
> test_find_first_bit() clears the entire test bitmap, so the subsequent
> test_find_first_and_bit() and test_find_next_and_bit() will not find any
> set bits. This is probably not the intended benchmark.
> 
> To fix this issue, test_find_first_bit() operates on a duplicated bitmap
> and does not clear the original test bitmap.
> The same is already done in test_find_first_and_bit().
> 
> While we're at it, add const qualifiers to the bitmap pointer arguments
> in the test functions.
> 
> Signed-off-by: Akinobu Mita <akinobu.mita@xxxxxxxxx>
> Cc: Yury Norov <yury.norov@xxxxxxxxx>
> Cc: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>

Applied, thanks!

> ---
>  lib/find_bit_benchmark.c | 15 +++++++++------
>  1 file changed, 9 insertions(+), 6 deletions(-)
> 
> diff --git a/lib/find_bit_benchmark.c b/lib/find_bit_benchmark.c
> index 402e160e7186..00d9dc61cd46 100644
> --- a/lib/find_bit_benchmark.c
> +++ b/lib/find_bit_benchmark.c
> @@ -30,18 +30,20 @@ static DECLARE_BITMAP(bitmap, BITMAP_LEN) __initdata;
>  static DECLARE_BITMAP(bitmap2, BITMAP_LEN) __initdata;
>  
>  /*
> - * This is Schlemiel the Painter's algorithm. It should be called after
> - * all other tests for the same bitmap because it sets all bits of bitmap to 1.
> + * This is Schlemiel the Painter's algorithm.
>   */
> -static int __init test_find_first_bit(void *bitmap, unsigned long len)
> +static int __init test_find_first_bit(const void *bitmap, unsigned long len)
>  {
> +	static DECLARE_BITMAP(cp, BITMAP_LEN) __initdata;
>  	unsigned long i, cnt;
>  	ktime_t time;
>  
> +	bitmap_copy(cp, bitmap, BITMAP_LEN);
> +
>  	time = ktime_get();
>  	for (cnt = i = 0; i < len; cnt++) {
> -		i = find_first_bit(bitmap, len);
> -		__clear_bit(i, bitmap);
> +		i = find_first_bit(cp, len);
> +		__clear_bit(i, cp);
>  	}
>  	time = ktime_get() - time;
>  	pr_err("find_first_bit:     %18llu ns, %6ld iterations\n", time, cnt);
> @@ -49,7 +51,8 @@ static int __init test_find_first_bit(void *bitmap, unsigned long len)
>  	return 0;
>  }
>  
> -static int __init test_find_first_and_bit(void *bitmap, const void *bitmap2, unsigned long len)
> +static int __init test_find_first_and_bit(const void *bitmap, const void *bitmap2,
> +		unsigned long len)
>  {
>  	static DECLARE_BITMAP(cp, BITMAP_LEN) __initdata;
>  	unsigned long i, cnt;
> -- 
> 2.43.0