Disable CPU idle states using PM QoS framework

low_latency.c

// Disable CPU idle states using PM QoS framework
//
// Directly related:
// https://github.com/torvalds/linux/blob/master/Documentation/power/pm_qos_interface.txt
// https://lwn.net/Articles/465195/
// https://access.redhat.com/articles/65410
//
// Network:
// https://elinux.org/images/f/f9/Elc2008_pm_qos_slides.pdf
//
// Per-CPU idle control:
// https://github.com/torvalds/linux/blob/master/Documentation/admin-guide/pm/cpuidle.rst

#include <stdio.h>  // fprintf
#include <fcntl.h>  // open()
#include <string.h> // strerror()
#include <errno.h>  // errno
#include <stdlib.h> // exit()
#include <unistd.h> // write()

static int pm_qos_fd = -1;
static char * dev_path = "/dev/cpu_dma_latency";

void start_low_latency(void)
{
  int32_t target = 0;

  if (pm_qos_fd >= 0) {
    fprintf(stderr, "Low latency already started; doing nothing\n");
    return;
  }
  fprintf(stderr, "Starting low latency\n");
  fprintf(stderr, "   target size:  %ld\n", sizeof(target));
  pm_qos_fd = open(dev_path, O_RDWR);
  fprintf(stderr, "Opened device %s on fd %d\n", dev_path, pm_qos_fd);

  if (pm_qos_fd < 0) {
    fprintf(stderr, "Failed to open PM QOS file: %s\n",
            strerror(errno));
    exit(errno);
  }
  write(pm_qos_fd, &target, sizeof(target));
  fprintf(stderr, "Wrote to device %s\n", dev_path);
}

void sleep_forever(void)
{
  fprintf(stderr, "Sleeping foverer\n");
  while (1) {
    sleep(100);
  }
  // Shouldn't get here
  fprintf(stderr, "Done sleeping\n");
}

void stop_low_latency(void)
{
  if (pm_qos_fd >= 0) {
    fprintf(stderr, "Stopping disabled CPU idle states\n");
    close(pm_qos_fd);
  } else {
    fprintf(stderr, "Disabled CPU idle states already stopped; doing nothing\n");
  }
}

int main( int argc, const char* argv[] )
{
  start_low_latency();
  sleep_forever();
  stop_low_latency();  // Never gets here
}