perf_event_open - how to monitoring multiple events

Question

does anyone know how to set perf_event_attr struct that can trigger PMU to monitoring multiple (type) event via perf_event_open()?

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Answer 1

That's a bit tricky.

We create first counter as usual. Additionally, we pass PERF_FORMAT_GROUP and PERF_FORMAT_ID to be able to work with multiple counters simultaneously. This counter will be our group leader.

struct perf_event_attr pea;
int fd1, fd2;
uint64_t id1, id2; 

memset(&pea, 0, sizeof(struct perf_event_attr));
pea.type = PERF_TYPE_HARDWARE;
pea.size = sizeof(struct perf_event_attr);
pea.config = PERF_COUNT_HW_CPU_CYCLES;
pea.disabled = 1;
pea.exclude_kernel = 1;
pea.exclude_hv = 1;
pea.read_format = PERF_FORMAT_GROUP | PERF_FORMAT_ID;
fd1 = syscall(__NR_perf_event_open, &pea, 0, -1, -1, 0);

Next, we retrieve identifier for the first counter:

ioctl(fd1, PERF_EVENT_IOC_ID, &id1);

Second (and all further counters) are created in the same fashion with only one exception: we pass fd1 value as group leader argument:

memset(&pea, 0, sizeof(struct perf_event_attr));
pea.type = PERF_TYPE_SOFTWARE;
pea.size = sizeof(struct perf_event_attr);
pea.config = PERF_COUNT_SW_PAGE_FAULTS;
pea.disabled = 1;
pea.exclude_kernel = 1;
pea.exclude_hv = 1;
pea.read_format = PERF_FORMAT_GROUP | PERF_FORMAT_ID;
fd2 = syscall(__NR_perf_event_open, &pea, 0, -1, fd1, 0); // <-- here
ioctl(fd2, PERF_EVENT_IOC_ID, &id2);

Next we need to declare a data structure to read multiple counters at once. You have to declare different set of fields depending on what flags you pass to perf_event_open. Manual page mentions all possible fields. In our case, we passed PERF_FORMAT_ID flag which adds id field. This will allow us to distinguish between different counters.

struct read_format {
    uint64_t nr;
    struct {
        uint64_t value;
        uint64_t id;
    } values[/*2*/];
};

Now we call standard profiling ioctls:

ioctl(fd1, PERF_EVENT_IOC_RESET, PERF_IOC_FLAG_GROUP);
ioctl(fd1, PERF_EVENT_IOC_ENABLE, PERF_IOC_FLAG_GROUP);
do_something();
ioctl(fd1, PERF_EVENT_IOC_DISABLE, PERF_IOC_FLAG_GROUP);

Finally, we read the counters from group leader file descriptor. Both counters are returned in single read_format structure that we declared:

char buf[4096];
struct read_format* rf = (struct read_format*) buf;
uint64_t val1, val2;

read(fd1, buf, sizeof(buf));
for (i = 0; i < rf->nr; i++) {
  if (rf->values[i].id == id1) {
    val1 = rf->values[i].value;
  } else if (rf->values[i].id == id2) {
    val2 = rf->values[i].value;
  }
}
printf("cpu cycles: %"PRIu64"\n", val1);
printf("page faults: %"PRIu64"\n", val2);

---

Below is the full program listing:

#define _GNU_SOURCE
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/syscall.h>
#include <string.h>
#include <sys/ioctl.h>
#include <linux/perf_event.h>
#include <linux/hw_breakpoint.h>
#include <asm/unistd.h>
#include <errno.h>
#include <stdint.h>
#include <inttypes.h>

struct read_format {
  uint64_t nr;
  struct {
    uint64_t value;
    uint64_t id;
  } values[];
};

void do_something() {
  int i;
  char* ptr;

  ptr = malloc(100*1024*1024);
  for (i = 0; i < 100*1024*1024; i++) {
    ptr[i] = (char) (i & 0xff); // pagefault
  }
  free(ptr);
}

int main(int argc, char* argv[]) {
  struct perf_event_attr pea;
  int fd1, fd2;
  uint64_t id1, id2;
  uint64_t val1, val2;
  char buf[4096];
  struct read_format* rf = (struct read_format*) buf;
  int i;

  memset(&pea, 0, sizeof(struct perf_event_attr));
  pea.type = PERF_TYPE_HARDWARE;
  pea.size = sizeof(struct perf_event_attr);
  pea.config = PERF_COUNT_HW_CPU_CYCLES;
  pea.disabled = 1;
  pea.exclude_kernel = 1;
  pea.exclude_hv = 1;
  pea.read_format = PERF_FORMAT_GROUP | PERF_FORMAT_ID;
  fd1 = syscall(__NR_perf_event_open, &pea, 0, -1, -1, 0);
  ioctl(fd1, PERF_EVENT_IOC_ID, &id1);

  memset(&pea, 0, sizeof(struct perf_event_attr));
  pea.type = PERF_TYPE_SOFTWARE;
  pea.size = sizeof(struct perf_event_attr);
  pea.config = PERF_COUNT_SW_PAGE_FAULTS;
  pea.disabled = 1;
  pea.exclude_kernel = 1;
  pea.exclude_hv = 1;
  pea.read_format = PERF_FORMAT_GROUP | PERF_FORMAT_ID;
  fd2 = syscall(__NR_perf_event_open, &pea, 0, -1, fd1 /*!!!*/, 0);
  ioctl(fd2, PERF_EVENT_IOC_ID, &id2);


  ioctl(fd1, PERF_EVENT_IOC_RESET, PERF_IOC_FLAG_GROUP);
  ioctl(fd1, PERF_EVENT_IOC_ENABLE, PERF_IOC_FLAG_GROUP);
  do_something();
  ioctl(fd1, PERF_EVENT_IOC_DISABLE, PERF_IOC_FLAG_GROUP);


  read(fd1, buf, sizeof(buf));
  for (i = 0; i < rf->nr; i++) {
    if (rf->values[i].id == id1) {
      val1 = rf->values[i].value;
    } else if (rf->values[i].id == id2) {
      val2 = rf->values[i].value;
    }
  }

  printf("cpu cycles: %"PRIu64"\n", val1);
  printf("page faults: %"PRIu64"\n", val2);

  return 0;
}