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

#include <unistd.h>
#include <fcntl.h>
#include <sched.h>
#include <time.h>

#define ENABLE_TRACING 1
#define BUFSIZE (192 << 12)

static void clear_trace(void)
{
#if ENABLE_TRACING
  int fd = open("/sys/kernel/debug/tracing/trace", O_WRONLY);
  if (fd < 0) perror("open trace");

  int res = write(fd, &fd, sizeof fd);
  if (res != sizeof fd) perror("clear trace");

  close(fd);
#endif
}

static void dump_trace(char *save)
{
#if ENABLE_TRACING
  int fd = open("/sys/kernel/debug/tracing/trace", O_RDONLY);
  if (fd < 0) perror("open trace");

  int fd_out = open(save, O_WRONLY | O_CREAT | O_TRUNC, 0600);
  if (fd_out < 0) perror("open save");
  while ( 1 )
  {
    char buf[8000];
    int res = read(fd, buf, sizeof buf);
    if (res <= 0) break;
    write(fd_out, buf, res);
  }
  close(fd_out); close(fd);
#endif
}

static void toggle_tracing(char val)
{
  (void)val;
#if ENABLE_TRACING
  int fd = open("/sys/kernel/debug/tracing/tracing_on", O_WRONLY);
  if (fd < 0) perror("open toggle");

  int res = write(fd, &val, 1);
  if (res != 1) perror(__func__);
  close(fd);
#endif
}

static int64_t now(void)
{
  struct timespec spec;
  clock_gettime(CLOCK_MONOTONIC, &spec);
  return spec.tv_sec*1000000000LL + spec.tv_nsec;
}

int main(void)
{
  struct sched_param param = { .sched_priority = 50 };
  int err = sched_setscheduler(0, SCHED_FIFO, &param);
  printf("sched_setscheduler=%d\n", err);

  char *buf = malloc(BUFSIZE);
  if (buf == NULL) return 2;
  memset(buf, '+', BUFSIZE);

  int flags = O_WRONLY | O_CREAT | O_TRUNC | O_SYNC;
  int fd = open("foob", flags, 0600);
  if (fd < 0) return 3;

  clear_trace();
  strcpy(buf, "WARM UP");
  write(fd, buf, 4096);
  strcpy(buf, "SHOW TIME");

  int64_t t0 = now();
  toggle_tracing('1');
  write(fd, buf, BUFSIZE);
  toggle_tracing('0');
  int64_t t1 = now();

  close(fd);
  printf("LAT = %9lld\n", t1-t0);
  dump_trace("/tmp/trace.one");
  return 0;
}