[PATCH V2 11/13] selftests/resctrl: Use cache size to determine "fill_buf" buffer size

[Reinette Chatre]

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.chatre@xxxxxxxxx>
---
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