On Fri, 13 Mar 2026, Reinette Chatre wrote: > Errata for Sierra Forest [1] (SRF42) and Granite Rapids [2] (GNR12) > describe the problem that MBM on Intel RDT may overcount memory bandwidth > measurements. The resctrl tests compare memory bandwidth reported by iMC > PMU to that reported by MBM causing the tests to fail on these systems > depending on the settings of the platform related to the errata. > > Since the resctrl tests need to run under various conditions it is not > possible to ensure system settings are such that MBM will not overcount. > It has been observed that the overcounting can be controlled via the > buffer size used in the MBM and MBA tests that rely on comparisons > between iMC PMU and MBM measurements. > > Running the MBM test on affected platforms with different buffer sizes it > can be observed that the difference between iMC PMU and MBM counts reduce > as the buffer size increases. After increasing the buffer size to more > than 4X the differences between iMC PMU and MBM become insignificant. > > Increase the buffer size used in MBM and MBA tests to 4X L3 size to reduce > possibility of tests failing due to difference in counts reported by iMC > PMU and MBM. > > Signed-off-by: Reinette Chatre <[email protected]> > Tested-by: Chen Yu <[email protected]> > Link: > https://edc.intel.com/content/www/us/en/design/products-and-solutions/processors-and-chipsets/sierra-forest/xeon-6700-series-processor-with-e-cores-specification-update/errata-details/ > # [1] > Link: > https://edc.intel.com/content/www/us/en/design/products-and-solutions/processors-and-chipsets/birch-stream/xeon-6900-6700-6500-series-processors-with-p-cores-specification-update/011US/errata-details/ > # [2] > --- > Changes since v2: > - Add Chen Yu's tag. > --- > tools/testing/selftests/resctrl/fill_buf.c | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/tools/testing/selftests/resctrl/fill_buf.c > b/tools/testing/selftests/resctrl/fill_buf.c > index 19a01a52dc1a..b9fa7968cd6e 100644 > --- a/tools/testing/selftests/resctrl/fill_buf.c > +++ b/tools/testing/selftests/resctrl/fill_buf.c > @@ -139,6 +139,6 @@ ssize_t get_fill_buf_size(int cpu_no, const char > *cache_type) > if (ret) > return ret; > > - return cache_total_size * 2 > MINIMUM_SPAN ? > - cache_total_size * 2 : MINIMUM_SPAN; > + return cache_total_size * 4 > MINIMUM_SPAN ? > + cache_total_size * 4 : MINIMUM_SPAN; > } >
Reviewed-by: Ilpo Järvinen <[email protected]> -- i.
