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

[Ilpo Järvinen]

On Fri, 24 May 2024, Reinette Chatre wrote:
> On 5/24/24 12:57 AM, Ilpo Järvinen wrote:
> > On Thu, 23 May 2024, Reinette Chatre wrote:
> > > 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.
> > 
> > (b) cannot be the cause for returning the same value again. It would
> > not be able to reread the number at all if file position is not moved.
> 
> I know. This was not intended to explain the duplicate answer but instead
> describe another change required to use current code in a loop. I
> specifically said in (a) that "(this seems to be the part that may cause
> the same value to be returned)".
> 
> > I certainly tried with fseek() and it is when I got same value on the
> > second read which is when I just went to two files solution.
> > 
> > > 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;
> > > }
> > 
> > Okay, so perhaps it's your explanation (a) but can libc be trusted to not
> > do buffering/caching for FILE *? So to be on the safe side, it would
> 
> Coding with expectation that libc cannot be trusted sounds strange to me.
> 
> > need to use syscalls directly to guarantee it's read the file twice.
> > 
> > If I convert it into fds, fscanf() cannot be used which would complicate
> > the string processing by adding extra steps.
> > 
> 
> It is not clear to me why you think that fscanf() cannot be used.

This was related to fscanf() not being able to read from an fd which is 
different interface than what libc's FILE * is.

> Could you please elaborate what the buffering issues are?

I'm pretty sure that by default libc does some buffering (even std* 
streams are line buffered and others streams even more). I'm not entirely 
sure about the extent of that buffering but here we need to always read 
the up to date value from the file itself, not from some buffer.

Maybe there never is any problem that the earlier read values are returned 
from some libc buffer when lseek/rewind is used, I just don't know that 
for sure. You seem to be more certain but I've not seen on what basis 
(other than the anecdotial test you provided).

> It is not necessary to open and close the file every time a value needs
> to be read from it.

-- 
 i.