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

Ilpo Järvinen
2024-10-04T14:20:35Z

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On Thu, 12 Sep 2024, Reinette Chatre wrote:

> 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@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;
> }
>

It has a bit of code duplication feel in it so I'd consider adding
something like ssize_t get_default_span() (perhaps there exists a better
name for it that is not "span" based). Also DEFAULT_SPAN is no longer
truly the default span.

But neither is the end of the world as is...

Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>

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i.