selftests/resctrl: Do not compare performance counters and resctrl at low bandwidth

The MBA test incrementally throttles memory bandwidth, each time
followed by a comparison between the memory bandwidth observed
by the performance counters and resctrl respectively.

While a comparison between performance counters and resctrl is
generally appropriate, they do not have an identical view of
memory bandwidth. For example RAS features or memory performance
features that generate memory traffic may drive accesses that are
counted differently by performance counters and MBM respectively,
for instance generating "overhead" traffic which is not counted
against any specific RMID. As a ratio, this different view of memory
bandwidth becomes more apparent at low memory bandwidths.

It is not practical to enable/disable the various features that
may generate memory bandwidth to give performance counters and
resctrl an identical view. Instead, do not compare performance
counters and resctrl view of memory bandwidth when the memory
bandwidth is low.

Bandwidth throttling behaves differently across platforms
so it is not appropriate to drop measurement data simply based
on the throttling level. Instead, use a threshold of 750MiB
that has been observed to support adequate comparison between
performance counters and resctrl.

Signed-off-by: Reinette Chatre <[email protected]>
Reviewed-by: Ilpo Järvinen <[email protected]>
Signed-off-by: Shuah Khan <[email protected]>

diff --git a/tools/testing/selftests/resctrl/mba_test.c b/tools/testing/selftests/resctrl/mba_test.c
index bf37f35556607644687661ffac3010d392fc10e8..5b4f0aa7a3a47847e5c57e125e516aacc44836b0 100644 (file)
--- a/tools/testing/selftests/resctrl/mba_test.c
+++ b/tools/testing/selftests/resctrl/mba_test.c
@@ -98,6 +98,13 @@ static bool show_mba_info(unsigned long *bw_imc, unsigned long *bw_resc)
 
                avg_bw_imc = sum_bw_imc / (NUM_OF_RUNS - 1);
                avg_bw_resc = sum_bw_resc / (NUM_OF_RUNS - 1);
+               if (avg_bw_imc < THROTTLE_THRESHOLD || avg_bw_resc < THROTTLE_THRESHOLD) {
+                       ksft_print_msg("Bandwidth below threshold (%d MiB). Dropping results from MBA schemata %u.\n",
+                                      THROTTLE_THRESHOLD,
+                                      ALLOCATION_MIN + ALLOCATION_STEP * allocation);
+                       continue;
+               }
+
                avg_diff = (float)labs(avg_bw_resc - avg_bw_imc) / avg_bw_imc;
                avg_diff_per = (int)(avg_diff * 100);
 

diff --git a/tools/testing/selftests/resctrl/resctrl.h b/tools/testing/selftests/resctrl/resctrl.h
index a553fe975938e67f1c36c031748b907843440a60..dab1953fc7a066c1ab794fa97673083bee1e4902 100644 (file)
--- a/tools/testing/selftests/resctrl/resctrl.h
+++ b/tools/testing/selftests/resctrl/resctrl.h
@@ -43,6 +43,16 @@
 
 #define MINIMUM_SPAN           (250 * MB)
 
+/*
+ * Memory bandwidth (in MiB) below which the bandwidth comparisons
+ * between iMC and resctrl are considered unreliable. For example RAS
+ * features or memory performance features that generate memory traffic
+ * may drive accesses that are counted differently by performance counters
+ * and MBM respectively, for instance generating "overhead" traffic which
+ * is not counted against any specific RMID.
+ */
+#define THROTTLE_THRESHOLD     750
+
 /*
  * fill_buf_param:     "fill_buf" benchmark parameters
  * @buf_size:          Size (in bytes) of buffer used in benchmark.