[PATCH 4/6] 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.cha...@intel.com>
---
 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 8ad433495f61..cad473b81a64 100644
--- a/tools/testing/selftests/resctrl/mba_test.c
+++ b/tools/testing/selftests/resctrl/mba_test.c
@@ -170,11 +170,17 @@ static int mba_run_test(const struct resctrl_test *test, 
const struct user_param
                .setup          = mba_setup,
                .measure        = mba_measure,
        };
+       unsigned long cache_total_size = 0;
        int ret;
 
        remove(RESULT_FILE_NAME);
 
-       param.fill_buf.buf_size = DEFAULT_SPAN;
+       ret = get_cache_size(uparams->cpu, "L3", &cache_total_size);
+       if (ret)
+               return ret;
+
+       param.fill_buf.buf_size = cache_total_size > DEFAULT_SPAN ?
+                                 cache_total_size * 2 : DEFAULT_SPAN;
        param.fill_buf.memflush = 1;
        param.fill_buf.operation = 0;
        param.fill_buf.once = false;
diff --git a/tools/testing/selftests/resctrl/mbm_test.c 
b/tools/testing/selftests/resctrl/mbm_test.c
index b6883f274c74..734bfa4f42b3 100644
--- a/tools/testing/selftests/resctrl/mbm_test.c
+++ b/tools/testing/selftests/resctrl/mbm_test.c
@@ -138,11 +138,17 @@ static int mbm_run_test(const struct resctrl_test *test, 
const struct user_param
                .setup          = mbm_setup,
                .measure        = mbm_measure,
        };
+       unsigned long cache_total_size = 0;
        int ret;
 
        remove(RESULT_FILE_NAME);
 
-       param.fill_buf.buf_size = DEFAULT_SPAN;
+       ret = get_cache_size(uparams->cpu, "L3", &cache_total_size);
+       if (ret)
+               return ret;
+
+       param.fill_buf.buf_size = cache_total_size > DEFAULT_SPAN ?
+                                 cache_total_size * 2 : DEFAULT_SPAN;
        param.fill_buf.memflush = 1;
        param.fill_buf.operation = 0;
        param.fill_buf.once = false;
-- 
2.46.0