Re: [PATCH v3 08/10] selftests/resctrl: Remove requirement on cache miss rate

["Ilpo Järvinen" <ilpo.jarvinen@linux.intel.com>]

On Fri, 13 Mar 2026, Reinette Chatre wrote:

> As the CAT test reads the same buffer into different sized cache portions
> it compares the number of cache misses against an expected percentage
> based on the size of the cache portion.
> 
> Systems and test conditions vary. The CAT test is a test of resctrl
> subsystem health and not a test of the hardware architecture so it is not
> required to place requirements on the size of the difference in cache
> misses, just that the number of cache misses when reading a buffer
> increase as the cache portion used for the buffer decreases.
> 
> Remove additional constraint on how big the difference between cache
> misses should be as the cache portion size changes. Only test that the
> cache misses increase as the cache portion size decreases. This remains
> a good sanity check of resctrl subsystem health while reducing impact
> of hardware architectural differences and the various conditions under
> which the test may run.
> 
> Increase the size difference between cache portions to additionally avoid
> any consequences resulting from smaller increments.
> 
> Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
> Tested-by: Chen Yu <yu.c.chen@intel.com>
> ---
> Changes since v2:
> - Add Chen Yu's tag.
> ---
>  tools/testing/selftests/resctrl/cat_test.c | 33 ++++------------------
>  1 file changed, 5 insertions(+), 28 deletions(-)
> 
> diff --git a/tools/testing/selftests/resctrl/cat_test.c b/tools/testing/selftests/resctrl/cat_test.c
> index f00b622c1460..8bc47f06679a 100644
> --- a/tools/testing/selftests/resctrl/cat_test.c
> +++ b/tools/testing/selftests/resctrl/cat_test.c
> @@ -14,42 +14,20 @@
>  #define RESULT_FILE_NAME	"result_cat"
>  #define NUM_OF_RUNS		5
>  
> -/*
> - * Minimum difference in LLC misses between a test with n+1 bits CBM to the
> - * test with n bits is MIN_DIFF_PERCENT_PER_BIT * (n - 1). With e.g. 5 vs 4
> - * bits in the CBM mask, the minimum difference must be at least
> - * MIN_DIFF_PERCENT_PER_BIT * (4 - 1) = 3 percent.
> - *
> - * The relationship between number of used CBM bits and difference in LLC
> - * misses is not expected to be linear. With a small number of bits, the
> - * margin is smaller than with larger number of bits. For selftest purposes,
> - * however, linear approach is enough because ultimately only pass/fail
> - * decision has to be made and distinction between strong and stronger
> - * signal is irrelevant.
> - */
> -#define MIN_DIFF_PERCENT_PER_BIT	1UL
> -
>  static int show_results_info(__u64 sum_llc_val, int no_of_bits,
>  			     unsigned long cache_span,
> -			     unsigned long min_diff_percent,
>  			     unsigned long num_of_runs, bool platform,
>  			     __s64 *prev_avg_llc_val)
>  {
>  	__u64 avg_llc_val = 0;
> -	float avg_diff;
>  	int ret = 0;
>  
>  	avg_llc_val = sum_llc_val / num_of_runs;
>  	if (*prev_avg_llc_val) {
> -		float delta = (__s64)(avg_llc_val - *prev_avg_llc_val);
> -
> -		avg_diff = delta / *prev_avg_llc_val;
> -		ret = platform && (avg_diff * 100) < (float)min_diff_percent;
> -
> -		ksft_print_msg("%s Check cache miss rate changed more than %.1f%%\n",
> -			       ret ? "Fail:" : "Pass:", (float)min_diff_percent);
> +		ret = platform && (avg_llc_val < *prev_avg_llc_val);
>  
> -		ksft_print_msg("Percent diff=%.1f\n", avg_diff * 100);
> +		ksft_print_msg("%s Check cache miss rate increased\n",
> +			       ret ? "Fail:" : "Pass:");

While I'm fine with removing the amount of change check, this no longer 
shows any numbers which would be a bit annoying if/when there's a failure.

-- 
 i.

>  	}
>  	*prev_avg_llc_val = avg_llc_val;
>  
> @@ -58,10 +36,10 @@ static int show_results_info(__u64 sum_llc_val, int no_of_bits,
>  	return ret;
>  }
>  
> -/* Remove the highest bit from CBM */
> +/* Remove the highest bits from CBM */
>  static unsigned long next_mask(unsigned long current_mask)
>  {
> -	return current_mask & (current_mask >> 1);
> +	return current_mask & (current_mask >> 2);
>  }
>  
>  static int check_results(struct resctrl_val_param *param, const char *cache_type,
> @@ -112,7 +90,6 @@ static int check_results(struct resctrl_val_param *param, const char *cache_type
>  
>  		ret = show_results_info(sum_llc_perf_miss, bits,
>  					alloc_size / 64,
> -					MIN_DIFF_PERCENT_PER_BIT * (bits - 1),
>  					runs, get_vendor() == ARCH_INTEL,
>  					&prev_avg_llc_val);
>  		if (ret)
>