selftests/resctrl: Raise threshold at which MBM and PMU values are compared

Commit 501cfdb ("selftests/resctrl: Do not compare performance
counters and resctrl at low bandwidth") introduced a threshold under which
memory bandwidth values from MBM and performance counters are not compared.
This is needed because MBM and the PMUs do not have an identical view of
memory bandwidth since PMUs can count all memory traffic while MBM does not
count "overhead" (for example RAS) traffic that cannot be attributed to an
RMID. As a ratio this difference in view of memory bandwidth is pronounced
at low memory bandwidths.

The 750MiB threshold was chosen arbitrarily after comparisons on different
platforms. Exposed to more platforms after introduction this threshold has
proven to be inadequate.

Having accurate comparison between performance counters and MBM requires
careful management of system load as well as control of features that
introduce extra memory traffic, for example, patrol scrub. This is not
appropriate for the resctrl selftests that are intended to run on a
variety of systems with various configurations.

Increase the memory bandwidth threshold under which no comparison is made
between performance counters and MBM. Add additional leniency by increasing
the percentage of difference that will be tolerated between these counts.

There is no impact to the validity of the resctrl selftests results as a
measure of resctrl subsystem health.

Link: https://lore.kernel.org/r/b374c33ddd324130d6255cbb91c3dd500e8277e7.1775266384.git.reinette.chatre@intel.com
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Tested-by: Chen Yu <yu.c.chen@intel.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

Author: rchatre

tools/testing/selftests/resctrl/mba_test.c

@@ -12,7 +12,7 @@
 
 #define RESULT_FILE_NAME	"result_mba"
 #define NUM_OF_RUNS		5
-#define MAX_DIFF_PERCENT	8
+#define MAX_DIFF_PERCENT	15
 #define ALLOCATION_MAX		100
 #define ALLOCATION_MIN		10
 #define ALLOCATION_STEP		10

tools/testing/selftests/resctrl/mbm_test.c

@@ -11,7 +11,7 @@
 #include "resctrl.h"
 
 #define RESULT_FILE_NAME	"result_mbm"
-#define MAX_DIFF_PERCENT	8
+#define MAX_DIFF_PERCENT	15
 #define NUM_OF_RUNS		5
 
 static int

tools/testing/selftests/resctrl/resctrl.h

@@ -55,7 +55,7 @@
  * and MBM respectively, for instance generating "overhead" traffic which
  * is not counted against any specific RMID.
  */
-#define THROTTLE_THRESHOLD	750
+#define THROTTLE_THRESHOLD	2500
 
 /*
  * fill_buf_param:	"fill_buf" benchmark parameters