linux-kernel - Re: [PATCH v4 02/16] selftests/resctrl: Calculate resctrl FS derived mem bw over sleep(1) only

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Date: Thu, 23 May 2024 17:10:41 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
	<linux-kselftest@...r.kernel.org>, Shuah Khan <shuah@...nel.org>, Babu Moger
	<babu.moger@....com>, Maciej Wieczór-Retman
	<maciej.wieczor-retman@...el.com>
CC: <linux-kernel@...r.kernel.org>, Fenghua Yu <fenghua.yu@...el.com>, "Shuah
 Khan" <skhan@...uxfoundation.org>
Subject: Re: [PATCH v4 02/16] selftests/resctrl: Calculate resctrl FS derived
 mem bw over sleep(1) only

Hi Ilpo,

On 5/20/24 5:30 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. Open two the resctrl mem bw files twice to avoid opening
> after the test during measurement period (reading the same file twice
> returns the same value so two files are needed).

I think this is only because of how the current reading is done, resctrl
surely supports keeping a file open and reading from it multiple times.

There seems to be two things that prevent current code from doing this
correctly:
(a) the fscanf() code does not take into account that resctrl also
     prints a "\n" ... (this seems to be the part that may cause the same
     value to be returned).
     So:
	if (fscanf(fp, "%lu", mbm_total) <= 0) {
     should be:
	if (fscanf(fp, "%lu\n", mbm_total) <= 0) {
(b) the current reading does not reset the file position so a second
     read will attempt to read past the beginning. A "rewind(fp)"
     should help here.

A small program like below worked for me by showing different values
on every read:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

const char *mbm_total_path = "/sys/fs/resctrl/mon_data/mon_L3_00/mbm_total_bytes";

int main(void)
{
	unsigned long mbm_total;
	FILE *fp;
	int count;

	fp = fopen(mbm_total_path, "r");
	if (!fp) {
		perror("Opening data file\n");
		exit(1);
	}
	for (count = 0; count < 100; count++) {
		if (fscanf(fp, "%lu\n", &mbm_total) <= 0) {
			perror("Unable to read from data file\n");
			exit(1);
		}
		printf("Read %d: %lu\n",count ,mbm_total );
		sleep(1);
		rewind(fp);
	}
	fclose(fp);
	return 0;
}

Reinette