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, now renamed to MINIMUM_SPAN, that has been appropriate for
testing so far.

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
---
Changes since V3:
- Add Ilpo's Reviewed-by tag.

Changes since V2:
- Move duplicate code into helper. (Ilpo)
- Rename DEFAULT_SPAN to MINIMUM_SPAN to reflect its new purpose. (Ilpo)
- Do _not_ add Ilpo's Reviewed-by tag ... the patch changed too much.

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/fill_buf.c      | 13 +++++++++++++
 tools/testing/selftests/resctrl/mba_test.c      |  7 ++++++-
 tools/testing/selftests/resctrl/mbm_test.c      |  7 ++++++-
 tools/testing/selftests/resctrl/resctrl.h       |  3 ++-
 tools/testing/selftests/resctrl/resctrl_tests.c |  2 +-
 5 files changed, 28 insertions(+), 4 deletions(-)

diff --git a/tools/testing/selftests/resctrl/fill_buf.c b/tools/testing/selftests/resctrl/fill_buf.c
index 380cc35f10c6..19a01a52dc1a 100644
--- a/tools/testing/selftests/resctrl/fill_buf.c
+++ b/tools/testing/selftests/resctrl/fill_buf.c
@@ -129,3 +129,16 @@ unsigned char *alloc_buffer(size_t buf_size, bool memflush)
 
 	return buf;
 }
+
+ssize_t get_fill_buf_size(int cpu_no, const char *cache_type)
+{
+	unsigned long cache_total_size = 0;
+	int ret;
+
+	ret = get_cache_size(cpu_no, cache_type, &cache_total_size);
+	if (ret)
+		return ret;
+
+	return cache_total_size * 2 > MINIMUM_SPAN ?
+			cache_total_size * 2 : MINIMUM_SPAN;
+}
diff --git a/tools/testing/selftests/resctrl/mba_test.c b/tools/testing/selftests/resctrl/mba_test.c
index 74d95c460bd0..bf37f3555660 100644
--- a/tools/testing/selftests/resctrl/mba_test.c
+++ b/tools/testing/selftests/resctrl/mba_test.c
@@ -182,7 +182,12 @@ 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;
+		ssize_t buf_size;
+
+		buf_size = get_fill_buf_size(uparams->cpu, "L3");
+		if (buf_size < 0)
+			return buf_size;
+		fill_buf.buf_size = buf_size;
 		fill_buf.memflush = true;
 		param.fill_buf = &fill_buf;
 	}
diff --git a/tools/testing/selftests/resctrl/mbm_test.c b/tools/testing/selftests/resctrl/mbm_test.c
index 72261413c868..4224f8ce3538 100644
--- a/tools/testing/selftests/resctrl/mbm_test.c
+++ b/tools/testing/selftests/resctrl/mbm_test.c
@@ -149,7 +149,12 @@ 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;
+		ssize_t buf_size;
+
+		buf_size = get_fill_buf_size(uparams->cpu, "L3");
+		if (buf_size < 0)
+			return buf_size;
+		fill_buf.buf_size = buf_size;
 		fill_buf.memflush = true;
 		param.fill_buf = &fill_buf;
 	}
diff --git a/tools/testing/selftests/resctrl/resctrl.h b/tools/testing/selftests/resctrl/resctrl.h
index 032cd9ebd761..a553fe975938 100644
--- a/tools/testing/selftests/resctrl/resctrl.h
+++ b/tools/testing/selftests/resctrl/resctrl.h
@@ -41,7 +41,7 @@
 
 #define BENCHMARK_ARGS		64
 
-#define DEFAULT_SPAN		(250 * MB)
+#define MINIMUM_SPAN		(250 * MB)
 
 /*
  * fill_buf_param:	"fill_buf" benchmark parameters
@@ -169,6 +169,7 @@ int perf_event_open(struct perf_event_attr *hw_event, pid_t pid, int cpu,
 unsigned char *alloc_buffer(size_t buf_size, bool memflush);
 void mem_flush(unsigned char *buf, size_t buf_size);
 void fill_cache_read(unsigned char *buf, size_t buf_size, bool once);
+ssize_t get_fill_buf_size(int cpu_no, const char *cache_type);
 int initialize_read_mem_bw_imc(void);
 int measure_read_mem_bw(const struct user_params *uparams,
 			struct resctrl_val_param *param, pid_t bm_pid);
diff --git a/tools/testing/selftests/resctrl/resctrl_tests.c b/tools/testing/selftests/resctrl/resctrl_tests.c
index 24daf76b4039..3335af815b21 100644
--- a/tools/testing/selftests/resctrl/resctrl_tests.c
+++ b/tools/testing/selftests/resctrl/resctrl_tests.c
@@ -189,7 +189,7 @@ static struct fill_buf_param *alloc_fill_buf_param(struct user_params *uparams)
 			ksft_exit_skip("Unable to parse benchmark buffer size.\n");
 		}
 	} else {
-		fill_param->buf_size = DEFAULT_SPAN;
+		fill_param->buf_size = MINIMUM_SPAN;
 	}
 
 	if (uparams->benchmark_cmd[2] && *uparams->benchmark_cmd[2] != '\0') {
-- 
2.47.0