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

Ilpo Järvinen
2026-03-31T08:07:30Z

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On Fri, 27 Mar 2026, Reinette Chatre wrote:

> Hi Ilpo,
>
> On 3/27/26 10:45 AM, Ilpo Järvinen wrote:
> > On Fri, 13 Mar 2026, Reinette Chatre wrote:
> >> -/*
> >> - * 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.
> >
>
> This snippet only removes display of the number that is no longer computed ("avg_diff").
> The values that are compared now, avg_llc_val and it previous value, are printed
> in the call to show_cache_info() that follows this snippet but is not visible in the diff.
>
> Below is an example of what a user running the CAT test will see after these changes.
> Since show_cache_info() always prints avg_llc_val the user can obtain insight into failure
> by considering it and its previous measurement.
>
> # Starting L3_CAT test ...
> # Mounting resctrl to "/sys/fs/resctrl"
> # Cache size :117964800
> # Writing benchmark parameters to resctrl FS
> # Write schema "L2:1=0x1" to resctrl FS
> # Write schema "L3:0=1fc0" to resctrl FS
> # Write schema "L3:0=3f" to resctrl FS
> # Write schema "L3:0=1ff0" to resctrl FS
> # Write schema "L3:0=f" to resctrl FS
> # Write schema "L3:0=1ffc" to resctrl FS
> # Write schema "L3:0=3" to resctrl FS
> # Checking for pass/fail
> # Number of bits: 6
> # Average LLC val: 445092
> # Cache span (lines): 737280
> # Pass: Check cache miss rate increased
> # Number of bits: 4
> # Average LLC val: 724472
> # Cache span (lines): 491520
> # Pass: Check cache miss rate increased
> # Number of bits: 2
> # Average LLC val: 1085470
> # Cache span (lines): 245760
> ok 4 L3_CAT: test

Okay, I didn't remember there was another place printing the numbers.
No problem with this then,

Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>

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i.