By default the MBM and MBA tests use the "fill_buf" benchmark to
read from a buffer with the goal to measure the memory bandwidth
generated by this buffer access.

Care should be taken when sizing the buffer used by the "fill_buf"
benchmark. If the buffer is small enough to fit in the cache then
it cannot be expected that the benchmark will generate much memory
bandwidth. For example, on a system with 320MB L3 cache the existing
hardcoded default of 250MB is insufficient.

Use the measured cache size to determine a buffer size that can be
expected to trigger memory access while keeping the existing default
as minimum that has been appropriate for testing so far.

Signed-off-by: Reinette Chatre <reinette.cha...@intel.com>
---
Changes since V1:
- Ensure buffer is at least double L3 cache size. (Ilpo)
- Support user override of default buffer size. (Ilpo)
---
 tools/testing/selftests/resctrl/mba_test.c | 8 +++++++-
 tools/testing/selftests/resctrl/mbm_test.c | 8 +++++++-
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/resctrl/mba_test.c 
b/tools/testing/selftests/resctrl/mba_test.c
index 7e43056c8737..d8d9637c1951 100644
--- a/tools/testing/selftests/resctrl/mba_test.c
+++ b/tools/testing/selftests/resctrl/mba_test.c
@@ -182,7 +182,13 @@ static int mba_run_test(const struct resctrl_test *test, 
const struct user_param
                fill_buf.memflush = uparams->fill_buf->memflush;
                param.fill_buf = &fill_buf;
        } else if (!uparams->benchmark_cmd[0]) {
-               fill_buf.buf_size = DEFAULT_SPAN;
+               unsigned long cache_total_size = 0;
+
+               ret = get_cache_size(uparams->cpu, "L3", &cache_total_size);
+               if (ret)
+                       return ret;
+               fill_buf.buf_size = cache_total_size * 2 > DEFAULT_SPAN ?
+                                   cache_total_size * 2 : DEFAULT_SPAN;
                fill_buf.memflush = 1;
                param.fill_buf = &fill_buf;
        }
diff --git a/tools/testing/selftests/resctrl/mbm_test.c 
b/tools/testing/selftests/resctrl/mbm_test.c
index b1f03a73333f..7635ee6b9339 100644
--- a/tools/testing/selftests/resctrl/mbm_test.c
+++ b/tools/testing/selftests/resctrl/mbm_test.c
@@ -149,7 +149,13 @@ static int mbm_run_test(const struct resctrl_test *test, 
const struct user_param
                fill_buf.memflush = uparams->fill_buf->memflush;
                param.fill_buf = &fill_buf;
        } else if (!uparams->benchmark_cmd[0]) {
-               fill_buf.buf_size = DEFAULT_SPAN;
+               unsigned long cache_total_size = 0;
+
+               ret = get_cache_size(uparams->cpu, "L3", &cache_total_size);
+               if (ret)
+                       return ret;
+               fill_buf.buf_size = cache_total_size * 2 > DEFAULT_SPAN ?
+                                   cache_total_size * 2 : DEFAULT_SPAN;
                fill_buf.memflush = 1;
                param.fill_buf = &fill_buf;
        }
-- 
2.46.0


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