Re: [patch 0/3] [Announcement] Performance Counters for Linux

Ingo Molnar
2008-12-05T08:12:18Z

* David Miller <davem@davemloft.net> wrote:

> From: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Date: Fri, 05 Dec 2008 09:03:36 +0100
>
> > On Fri, 2008-12-05 at 18:57 +1100, Paul Mackerras wrote:
> > > Peter Zijlstra writes:
> > >
> > > > So, while most people would not consider two consecutive read() ops to
> > > > be close or near the same time, due to preemption and such, that is
> > > > taken away by the fact that the counters are task local time based - so
> > > > preemption doesn't affect thing. Right?
> > >
> > > I'm sorry, I don't follow the argument here. What do you mean by
> > > "task local time based"?
> >
> > time only flows when the task is running.
>
> These things aren't measuring time, or even just cycles, they are
> measuring things like L2 cache misses, cpu cycles, and other similar
> kinds of events.
>
> So these counters are going to measure all of the damn crap assosciated
> with doing the read() call as well as the real work the task does.

that's wrong, look at the example we posted - see it pasted below.

When monitoring another task it does _not_ count the read() done in the
monitoring task, it does _not_ include it in the event count. It is a
fundamental property of our code to be as unintrusive as possible. It
only measures the work done by that task.

( You _can_ measure your own overhead of course too, if you want to. It's
a natural special-case of our performance counter abstraction. )

Ingo

---

/*
* Performance counters monitoring test case
*/
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/time.h>
#include <unistd.h>
#include <stdint.h>
#include <stdlib.h>
#include <string.h>
#include <getopt.h>
#include <fcntl.h>
#include <stdio.h>
#include <errno.h>

#define __user

#include "sys.h"

static int count = 10000;
static int eventid;
static int tid;
static char *debuginfo;

static void display_help(void)
{
printf("monitor\n");
printf("Usage:\n"
"monitor options threadid\n\n"
"-e EID --eventid=EID eventid\n"
"-c CNT --count=CNT event count on which IP is sampled\n"
"-d FILE --debug=FILE path to binary file with debug info\n");
exit(0);
}

static void process_options (int argc, char *argv[])
{
int error = 0;

for (;;) {
int option_index = 0;
/** Options for getopt */
static struct option long_options[] = {
{"count", required_argument, NULL, 'c'},
{"debug", required_argument, NULL, 'd'},
{"eventid", required_argument, NULL, 'e'},
{"help", no_argument, NULL, 'h'},
{NULL, 0, NULL, 0}
};
int c = getopt_long(argc, argv, "c:d:e:",
long_options, &option_index);
if (c == -1)
break;
switch (c) {
case 'c': count = atoi(optarg); break;
case 'd': debuginfo = strdup(optarg); break;
case 'e': eventid = atoi(optarg); break;
default: error = 1; break;
}
}
if (error || optind == argc)
display_help ();

tid = atoi(argv[optind]);
}

int main(int argc, char *argv[])
{
char str[256];
uint64_t ip;
ssize_t res;
int fd;

process_options(argc, argv);

fd = perf_counter_open(eventid, count, 1, tid, -1);
if (fd < 0) {
perror("Create counter");
exit(-1);
}

while (1) {
res = read(fd, (char *) &ip, sizeof(ip));
if (res != sizeof(ip)) {
perror("Read counter");
break;
}

if (!debuginfo) {
printf("IP: 0x%016llx\n", (unsigned long long)ip);
} else {
sprintf(str, "addr2line -e %s 0x%llx\n", debuginfo,
(unsigned long long)ip);
system(str);
}
}

close(fd);
exit(0);
}