Re: [PATCH v2 02/13] selftests/resctrl: Calculate resctrl FS derived mem bw over sleep(1) only

[Reinette Chatre]

Hi Ilpo,

On 3/11/2024 6:52 AM, Ilpo Järvinen wrote:
> For MBM/MBA tests, measure_vals() calls get_mem_bw_imc() that performs
> the measurement over a duration of sleep(1) call. The memory bandwidth
> numbers from IMC are derived over this duration. The resctrl FS derived
> memory bandwidth, however, is calculated inside measure_vals() and only
> takes delta between the previous value and the current one which
> besides the actual test, also samples inter-test noise.
> 
> Rework the logic in measure_vals() and get_mem_bw_imc() such that the
> resctrl FS memory bandwidth section covers much shorter duration
> closely matching that of the IMC perf counters to improve measurement
> accuracy.
> 

Thank you very much for doing this.

> Suggested-by: Reinette Chatre <reinette.chatre@xxxxxxxxx>
> Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@xxxxxxxxxxxxxxx>
> ---
>  tools/testing/selftests/resctrl/resctrl_val.c | 72 +++++++++++++------
>  1 file changed, 50 insertions(+), 22 deletions(-)
> 
> diff --git a/tools/testing/selftests/resctrl/resctrl_val.c b/tools/testing/selftests/resctrl/resctrl_val.c
> index 36139cba7be8..4df2cd738f88 100644
> --- a/tools/testing/selftests/resctrl/resctrl_val.c
> +++ b/tools/testing/selftests/resctrl/resctrl_val.c
> @@ -293,28 +293,35 @@ static int initialize_mem_bw_imc(void)
>  }
>  
>  /*
> - * get_mem_bw_imc:	Memory band width as reported by iMC counters
> + * perf_open_imc_mem_bw - Open perf fds for IMCs
>   * @cpu_no:		CPU number that the benchmark PID is binded to
> - * @bw_report:		Bandwidth report type (reads, writes)
> - *
> - * Memory B/W utilized by a process on a socket can be calculated using
> - * iMC counters. Perf events are used to read these counters.
> - *
> - * Return: = 0 on success. < 0 on failure.

This "Return" still seems relevant.

>   */
> -static int get_mem_bw_imc(int cpu_no, char *bw_report, float *bw_imc)
> +static int perf_open_imc_mem_bw(int cpu_no)
>  {
> -	float reads, writes, of_mul_read, of_mul_write;
>  	int imc, j, ret;
>  
> -	/* Start all iMC counters to log values (both read and write) */
> -	reads = 0, writes = 0, of_mul_read = 1, of_mul_write = 1;
>  	for (imc = 0; imc < imcs; imc++) {
>  		for (j = 0; j < 2; j++) {
>  			ret = open_perf_event(imc, cpu_no, j);
>  			if (ret)
>  				return -1;
>  		}

I'm feeling more strongly that this inner loop makes the code harder to
understand and unwinding it would make it easier to understand.

> +	}
> +
> +	return 0;
> +}
> +
> +/*
> + * do_mem_bw_test - Perform memory bandwidth test
> + *
> + * Runs memory bandwidth test over one second period. Also, handles starting
> + * and stopping of the IMC perf counters around the test.
> + */
> +static void do_imc_mem_bw_test(void)
> +{
> +	int imc, j;
> +
> +	for (imc = 0; imc < imcs; imc++) {
>  		for (j = 0; j < 2; j++)
>  			membw_ioctl_perf_event_ioc_reset_enable(imc, j);

Here also. I find these loops unnecessary. I do not think it optimizes anything
and it makes the code harder to understand.

>  	}
> @@ -326,6 +333,24 @@ static int get_mem_bw_imc(int cpu_no, char *bw_report, float *bw_imc)
>  		for (j = 0; j < 2; j++)
>  			membw_ioctl_perf_event_ioc_disable(imc, j);

Same here.

>  	}
> +}
> +
> +/*
> + * get_mem_bw_imc - Memory band width as reported by iMC counters
> + * @bw_report:		Bandwidth report type (reads, writes)
> + *
> + * Memory B/W utilized by a process on a socket can be calculated using
> + * iMC counters. Perf events are used to read these counters.

In the above there are three variations of the same: "band width", "Bandwidth",
and "B/W". Please just use one and use it consistently.

> + *
> + * Return: = 0 on success. < 0 on failure.
> + */
> +static int get_mem_bw_imc(char *bw_report, float *bw_imc)
> +{
> +	float reads, writes, of_mul_read, of_mul_write;
> +	int imc, j;
> +
> +	/* Start all iMC counters to log values (both read and write) */
> +	reads = 0, writes = 0, of_mul_read = 1, of_mul_write = 1;
>  
>  	/*
>  	 * Get results which are stored in struct type imc_counter_config
> @@ -593,10 +618,9 @@ static void initialize_llc_occu_resctrl(const char *ctrlgrp, const char *mongrp,
>  }
>  
>  static int measure_vals(const struct user_params *uparams,
> -			struct resctrl_val_param *param,
> -			unsigned long *bw_resc_start)
> +			struct resctrl_val_param *param)
>  {
> -	unsigned long bw_resc, bw_resc_end;
> +	unsigned long bw_resc, bw_resc_start, bw_resc_end;
>  	float bw_imc;
>  	int ret;
>  
> @@ -607,22 +631,27 @@ static int measure_vals(const struct user_params *uparams,
>  	 * Compare the two values to validate resctrl value.
>  	 * It takes 1sec to measure the data.
>  	 */
> -	ret = get_mem_bw_imc(uparams->cpu, param->bw_report, &bw_imc);
> +	ret = perf_open_imc_mem_bw(uparams->cpu);
>  	if (ret < 0)
>  		return ret;
>  
> +	ret = get_mem_bw_resctrl(&bw_resc_start);
> +	if (ret < 0)
> +		return ret;
> +
> +	do_imc_mem_bw_test();
> +
>  	ret = get_mem_bw_resctrl(&bw_resc_end);
>  	if (ret < 0)
>  		return ret;
>  
> -	bw_resc = (bw_resc_end - *bw_resc_start) / MB;
> -	ret = print_results_bw(param->filename, bm_pid, bw_imc, bw_resc);
> -	if (ret)
> +	ret = get_mem_bw_imc(param->bw_report, &bw_imc);
> +	if (ret < 0)
>  		return ret;
>  
> -	*bw_resc_start = bw_resc_end;
> +	bw_resc = (bw_resc_end - bw_resc_start) / MB;
>  
> -	return 0;
> +	return print_results_bw(param->filename, bm_pid, bw_imc, bw_resc);
>  }
>  
>  /*
> @@ -696,7 +725,6 @@ int resctrl_val(const struct resctrl_test *test,
>  		struct resctrl_val_param *param)
>  {
>  	char *resctrl_val = param->resctrl_val;
> -	unsigned long bw_resc_start = 0;

In the current implementation the first iteration's starting measurement
is, as seen above, 0 ... which makes the first measurement unreliable
and dropped for both the MBA and MBM tests. In this enhancement, the
first measurement is no longer skewed so much so I wonder if this enhancement
can be expanded to the analysis phase where first measurement no longer
needs to be dropped?

>  	struct sigaction sigact;
>  	int ret = 0, pipefd[2];
>  	char pipe_message = 0;
> @@ -838,7 +866,7 @@ int resctrl_val(const struct resctrl_test *test,
>  
>  		if (!strncmp(resctrl_val, MBM_STR, sizeof(MBM_STR)) ||
>  		    !strncmp(resctrl_val, MBA_STR, sizeof(MBA_STR))) {
> -			ret = measure_vals(uparams, param, &bw_resc_start);
> +			ret = measure_vals(uparams, param);
>  			if (ret)
>  				break;
>  		} else if (!strncmp(resctrl_val, CMT_STR, sizeof(CMT_STR))) {

Reinette