watermark_signals.c

#define _GNU_SOURCE

#include <stdio.h>
#include <unistd.h>
#include <sys/mman.h>
#include <linux/perf_event.h>
#include <sys/syscall.h>
#include <poll.h>
#include <stdlib.h>
#include <string.h>
#include <sys/ioctl.h>
#include <fcntl.h>
#include <signal.h>

static void do_child() {
  for (int i = 0; i < 20; ++i) {
    sleep(1);
  }
  exit(0);
}

static void handle_signal(__attribute__((unused)) int sig) {
  write(STDERR_FILENO, "SIGIO\n", 6);
}

/* Undefined this to use poll instead, which works */
#define USE_SIGNALS 1

int main() {
  pid_t child = fork();
  if (!child) {
    do_child();
  }

  signal(SIGIO, handle_signal);

  struct perf_event_attr attr;
  memset(&attr, 0, sizeof(attr));
  attr.size = sizeof(attr);
  attr.type = PERF_TYPE_SOFTWARE;
  attr.config = PERF_COUNT_SW_DUMMY;
  attr.sample_period = 1;
  attr.disabled = 1;
  attr.watermark = 1;
  attr.context_switch = 1;
  attr.wakeup_watermark = 100;
  int fd = syscall(SYS_perf_event_open, &attr, child, -1, -1, 0);
  fcntl(fd, F_SETFL, FASYNC);
  fcntl(fd, F_SETOWN, getpid());
  fcntl(fd, F_SETSIG, SIGIO);
  struct perf_event_mmap_page* p = (struct perf_event_mmap_page*)
    mmap(NULL, 8192, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
  ioctl(fd, PERF_EVENT_IOC_ENABLE, 0);

#ifdef USE_SIGNALS
  sleep(1000);
#else
  char buf[20][8192];
  for (int i = 0; i < 20; ++i) {
    struct pollfd pfd = { fd, POLLIN, 0 };
    poll(&pfd, 1, 1000000000);
    printf("Polled %d\n", i);
    memcpy(buf[i], p, sizeof(buf[i]));
  }
#endif

  return 0;
}