From 7a214b4ec1c967533439b4ef16941cabb52a88a8 Mon Sep 17 00:00:00 2001 From: Reinette Chatre Date: Fri, 3 Apr 2026 18:56:00 -0700 Subject: [PATCH] selftests/resctrl: Improve accuracy of cache occupancy test MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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 Link: https://lore.kernel.org/r/b160592179f88069cdc679563e152007998a0d76.1775266384.git.reinette.chatre@intel.com Reported-by: Dave Martin Signed-off-by: Reinette Chatre Tested-by: Chen Yu Reviewed-by: Ilpo Järvinen Link: https://lore.kernel.org/lkml/aO+7MeSMV29VdbQs@e133380.arm.com/ Signed-off-by: Shuah Khan --- 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; }