Re: [PATCH v3 01/10] selftests/resctrl: Improve accuracy of cache occupancy test

[Ilpo Järvinen]

On Fri, 13 Mar 2026, Reinette Chatre wrote:

> Dave Martin reported inconsistent CMT test failures. In one experiment
> the first run of the CMT test failed because of too large (24%) difference
> between measured and achievable cache occupancy while the second run passed
> with an acceptable 4% difference.
> 
> The CMT test is susceptible to interference from the rest of the system.
> This can be demonstrated with a utility like stress-ng by running the CMT
> test while introducing cache misses using:
> 
>    stress-ng --matrix-3d 0 --matrix-3d-zyx
> 
> Below shows an example of the CMT test failing because of a significant
> difference between measured and achievable cache occupancy when run with
> interference:
>     # Starting CMT test ...
>     # Mounting resctrl to "/sys/fs/resctrl"
>     # Cache size :335544320
>     # Writing benchmark parameters to resctrl FS
>     # Benchmark PID: 7011
>     # Checking for pass/fail
>     # Fail: Check cache miss rate within 15%
>     # Percent diff=99
>     # Number of bits: 5
>     # Average LLC val: 235929
>     # Cache span (bytes): 83886080
>     not ok 1 CMT: test
> 
> The CMT test creates a new control group that is also capable of monitoring
> and assigns the workload to it. The workload allocates a buffer that by
> default fills a portion of the L3 and keeps reading from the buffer,
> measuring the L3 occupancy at intervals. The test passes if the workload's
> L3 occupancy is within 15% of the buffer size.
> 
> By not adjusting any capacity bitmasks the workload shares the cache with
> the rest of the system. Any other task that may be running could evict
> the workload's data from the cache causing it to have low cache occupancy.
> 
> Reduce interference from the rest of the system by ensuring that the
> workload's control group uses the capacity bitmask found in the user
> parameters for L3 and that the rest of the system can only allocate into
> the inverse of the workload's L3 cache portion. Other tasks can thus no
> longer evict the workload's data from L3.
> 
> With the above adjustments the CMT test is more consistent. Repeating the
> CMT test while generating interference with stress-ng on a sample
> system after applying the fixes show significant improvement in test
> accuracy:
> 
>     # Starting CMT test ...
>     # Mounting resctrl to "/sys/fs/resctrl"
>     # Cache size :335544320
>     # Writing benchmark parameters to resctrl FS
>     # Write schema "L3:0=fffe0" to resctrl FS
>     # Write schema "L3:0=1f" to resctrl FS
>     # Benchmark PID: 7089
>     # Checking for pass/fail
>     # Pass: Check cache miss rate within 15%
>     # Percent diff=12
>     # Number of bits: 5
>     # Average LLC val: 73269248
>     # Cache span (bytes): 83886080
>     ok 1 CMT: test
> 
> Reported-by: Dave Martin <Dave.Martin@xxxxxxx>
> Signed-off-by: Reinette Chatre <reinette.chatre@xxxxxxxxx>
> Tested-by: Chen Yu <yu.c.chen@xxxxxxxxx>
> Link: https://lore.kernel.org/lkml/aO+7MeSMV29VdbQs@xxxxxxxxxxxxxxx/
> ---
> Changes since v1:
> - Fix typo in changelog: "data my be in L2" -> "data may be in L2".
> 
> Changes since v2:
> - Split patch to separate changes impacting L3 and L2 resource. (Ilpo)
> - Re-run tests after patch split to ensure test impact match patch
>   and update changelog with refreshed data.
> - Since fix is now split across two patches: "Closes:" -> "Link:"
> - Rename "long_mask" to "full_mask". (Ilpo)
> - Add Chen Yu's tag.
> ---
>  tools/testing/selftests/resctrl/cmt_test.c    | 26 +++++++++++++++++--
>  tools/testing/selftests/resctrl/mba_test.c    |  4 ++-
>  tools/testing/selftests/resctrl/mbm_test.c    |  4 ++-
>  tools/testing/selftests/resctrl/resctrl.h     |  4 ++-
>  tools/testing/selftests/resctrl/resctrl_val.c |  2 +-
>  5 files changed, 34 insertions(+), 6 deletions(-)
> 
> diff --git a/tools/testing/selftests/resctrl/cmt_test.c b/tools/testing/selftests/resctrl/cmt_test.c
> index d09e693dc739..7bc6cf49c1c5 100644
> --- a/tools/testing/selftests/resctrl/cmt_test.c
> +++ b/tools/testing/selftests/resctrl/cmt_test.c
> @@ -19,12 +19,34 @@
>  #define CON_MON_LCC_OCCUP_PATH		\
>  	"%s/%s/mon_data/mon_L3_%02d/llc_occupancy"
>  
> -static int cmt_init(const struct resctrl_val_param *param, int domain_id)
> +/*
> + * Initialize capacity bitmasks (CBMs) of:
> + * - control group being tested per test parameters,
> + * - default resource group as inverse of control group being tested to prevent
> + *   other tasks from interfering with test.
> + */
> +static int cmt_init(const struct resctrl_test *test,
> +		    const struct user_params *uparams,
> +		    const struct resctrl_val_param *param, int domain_id)
>  {
> +	unsigned long full_mask;
> +	char schemata[64];
> +	int ret;
> +
>  	sprintf(llc_occup_path, CON_MON_LCC_OCCUP_PATH, RESCTRL_PATH,
>  		param->ctrlgrp, domain_id);
>  
> -	return 0;
> +	ret = get_full_cbm(test->resource, &full_mask);
> +	if (ret)
> +		return ret;
> +
> +	snprintf(schemata, sizeof(schemata), "%lx", ~param->mask & full_mask);
> +	ret = write_schemata("", schemata, uparams->cpu, test->resource);
> +	if (ret)
> +		return ret;
> +
> +	snprintf(schemata, sizeof(schemata), "%lx", param->mask);
> +	return write_schemata(param->ctrlgrp, schemata, uparams->cpu, test->resource);
>  }
>  
>  static int cmt_setup(const struct resctrl_test *test,
> diff --git a/tools/testing/selftests/resctrl/mba_test.c b/tools/testing/selftests/resctrl/mba_test.c
> index c7e9adc0368f..cd4c715b7ffd 100644
> --- a/tools/testing/selftests/resctrl/mba_test.c
> +++ b/tools/testing/selftests/resctrl/mba_test.c
> @@ -17,7 +17,9 @@
>  #define ALLOCATION_MIN		10
>  #define ALLOCATION_STEP		10
>  
> -static int mba_init(const struct resctrl_val_param *param, int domain_id)
> +static int mba_init(const struct resctrl_test *test,
> +		    const struct user_params *uparams,
> +		    const struct resctrl_val_param *param, int domain_id)
>  {
>  	int ret;
>  
> diff --git a/tools/testing/selftests/resctrl/mbm_test.c b/tools/testing/selftests/resctrl/mbm_test.c
> index 84d8bc250539..58201f844740 100644
> --- a/tools/testing/selftests/resctrl/mbm_test.c
> +++ b/tools/testing/selftests/resctrl/mbm_test.c
> @@ -83,7 +83,9 @@ static int check_results(size_t span)
>  	return ret;
>  }
>  
> -static int mbm_init(const struct resctrl_val_param *param, int domain_id)
> +static int mbm_init(const struct resctrl_test *test,
> +		    const struct user_params *uparams,
> +		    const struct resctrl_val_param *param, int domain_id)
>  {
>  	int ret;
>  
> diff --git a/tools/testing/selftests/resctrl/resctrl.h b/tools/testing/selftests/resctrl/resctrl.h
> index afe635b6e48d..c72045c74ac4 100644
> --- a/tools/testing/selftests/resctrl/resctrl.h
> +++ b/tools/testing/selftests/resctrl/resctrl.h
> @@ -135,7 +135,9 @@ struct resctrl_val_param {
>  	char			filename[64];
>  	unsigned long		mask;
>  	int			num_of_runs;
> -	int			(*init)(const struct resctrl_val_param *param,
> +	int			(*init)(const struct resctrl_test *test,
> +					const struct user_params *uparams,
> +					const struct resctrl_val_param *param,
>  					int domain_id);
>  	int			(*setup)(const struct resctrl_test *test,
>  					 const struct user_params *uparams,
> diff --git a/tools/testing/selftests/resctrl/resctrl_val.c b/tools/testing/selftests/resctrl/resctrl_val.c
> index 7c08e936572d..a5a8badb83d4 100644
> --- a/tools/testing/selftests/resctrl/resctrl_val.c
> +++ b/tools/testing/selftests/resctrl/resctrl_val.c
> @@ -569,7 +569,7 @@ int resctrl_val(const struct resctrl_test *test,
>  		goto reset_affinity;
>  
>  	if (param->init) {
> -		ret = param->init(param, domain_id);
> +		ret = param->init(test, uparams, param, domain_id);
>  		if (ret)
>  			goto reset_affinity;
>  	}
> 

Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@xxxxxxxxxxxxxxx>

-- 
 i.