Re: [PATCH V3 12/15] selftests/resctrl: Use cache size to determine "fill_buf" buffer size

["Ilpo Järvinen" <ilpo.jarvinen@linux.intel.com>]

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On Thu, 17 Oct 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, now renamed to MINIMUM_SPAN, that has been appropriate for
> testing so far.
> 
> Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>

Look good, thanks.

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

--
 i.

> ---
> 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 88768f4d4961..790f4eb7871a 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]) {
>