selftests/resctrl: Make wraparound handling obvious

Within mba_setup() the programmed bandwidth delay value starts
at the maximum (100, or rather ALLOCATION_MAX) and progresses
towards ALLOCATION_MIN by decrementing with ALLOCATION_STEP.

The programmed bandwidth delay should never be negative, so
representing it with an unsigned int is most appropriate. This
may introduce confusion because of the "allocation > ALLOCATION_MAX"
check used to check wraparound of the subtraction.

Modify the mba_setup() flow to start at the minimum, ALLOCATION_MIN,
and incrementally, with ALLOCATION_STEP steps, adjust the
bandwidth delay value. This avoids wraparound while making the purpose
of "allocation > ALLOCATION_MAX" clear and eliminates the
need for the "allocation < ALLOCATION_MIN" check.

Reported-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Closes: https://lore.kernel.org/lkml/1903ac13-5c9c-ef8d-78e0-417ac34a971b@linux.intel.com/
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

diff --git a/tools/testing/selftests/resctrl/mba_test.c b/tools/testing/selftests/resctrl/mba_test.c
index ab8496a4925b461a7e5ec3ec354c7200a2da7396..da40a8ed44138f4964beaaf8e7d4e05b0a7fc24f 100644 (file)
--- a/tools/testing/selftests/resctrl/mba_test.c
+++ b/tools/testing/selftests/resctrl/mba_test.c
@@ -39,7 +39,8 @@ static int mba_setup(const struct resctrl_test *test,
                     const struct user_params *uparams,
                     struct resctrl_val_param *p)
 {
-       static int runs_per_allocation, allocation = 100;
+       static unsigned int allocation = ALLOCATION_MIN;
+       static int runs_per_allocation;
        char allocation_str[64];
        int ret;
 
@@ -50,7 +51,7 @@ static int mba_setup(const struct resctrl_test *test,
        if (runs_per_allocation++ != 0)
                return 0;
 
-       if (allocation < ALLOCATION_MIN || allocation > ALLOCATION_MAX)
+       if (allocation > ALLOCATION_MAX)
                return END_OF_TESTS;
 
        sprintf(allocation_str, "%d", allocation);
@@ -59,7 +60,7 @@ static int mba_setup(const struct resctrl_test *test,
        if (ret < 0)
                return ret;
 
-       allocation -= ALLOCATION_STEP;
+       allocation += ALLOCATION_STEP;
 
        return 0;
 }
@@ -72,8 +73,9 @@ static int mba_measure(const struct user_params *uparams,
 
 static bool show_mba_info(unsigned long *bw_imc, unsigned long *bw_resc)
 {
-       int allocation, runs;
+       unsigned int allocation;
        bool ret = false;
+       int runs;
 
        ksft_print_msg("Results are displayed in (MB)\n");
        /* Memory bandwidth from 100% down to 10% */
@@ -103,7 +105,7 @@ static bool show_mba_info(unsigned long *bw_imc, unsigned long *bw_resc)
                               avg_diff_per > MAX_DIFF_PERCENT ?
                               "Fail:" : "Pass:",
                               MAX_DIFF_PERCENT,
-                              ALLOCATION_MAX - ALLOCATION_STEP * allocation);
+                              ALLOCATION_MIN + ALLOCATION_STEP * allocation);
 
                ksft_print_msg("avg_diff_per: %d%%\n", avg_diff_per);
                ksft_print_msg("avg_bw_imc: %lu\n", avg_bw_imc);