[linux-audio-user] [ANN] rt_watchdog

Florian Schmidt mista.tapas at gmx.net
Fri Nov 4 14:21:51 EST 2005




you find a small program which acts as a watchdog daemon that kills
runaway SCHED_FIFO tasks. It does so by setting up two threads:

- one high priority (99) consumer that runs every 3 seconds
- one low priority (1) producer that runs every second

the producer fills a ringbuffer and the consumer drains it. When the
ringbuffer runs empty a shell script will be run (via the system()
function) that tries to change the scheduling policy of all threads in
the system from SCHED_FIFO to SCHED_OTHER. the offending task may
naturally not run at prio 99, otherwise the watchdog would never get to

Here's a potential problem: The shell script potentially changes its own
scheduling policy to SCHED_OTHER prior to chnaging the offending runaway
task. Ugh! Any hint?

Also all IRQ handler threads also have their policy changed. Dunno
whether that's good or not. 

For reference i pasted the script and the c file here:

unfifo_stuff.sh (the two arguments are the two thread id's of the
rt_watchdog threads, as these are not supposed to have their policy

There's also a small test program in the tarball (compiles to test_rt)
that wastes cycles SCHED_FIFO (locking the system) but exits eventually,
so it's quite safe to test the watchdog with it.

Maybe someone else find's it useful. Recommendations, tips, critique are
all welcome..



for i in $( ps -eL -o pid ); do
	if [ "$i" != "$1" -a "$i" != "$2" ]
		chrt -o -p 0 $i


#include <pthread.h>
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <syslog.h>
#include <signal.h>
#include <sys/mman.h>
#include <sys/stat.h>

#include <sys/types.h>
#include <linux/unistd.h>
#include <errno.h>

#include "ringbuffer.h"

/* how long to sleep between checks in the high prio thread */
#define SLEEPSECS     3

/* how long to sleep between writing "alive" messages to the ringbuffer 
   from the low prio thread */
#define LP_SLEEPSECS  1

/* the priority of the high prio thread */
#define PRIO         99

/* the priority of the low prio thread */
#define LP_PRIO       1

/* the ringbuffer used to transfer "alive" messages from low prio producer
   to high prio consumer */
jack_ringbuffer_t *rb;

pthread_t low_prio_thread;

/* the thread id's for the low prio and high prio threads.
   these get passed to the unfifo_stuff.sh script to make sure
   the watchdog doesn't repolicy itself to SCHED_OTHER */
pid_t lp_tid;
pid_t hp_tid;

void signalled(int signal) 


volatile int thread_finish;

/* this is the low prio thread. it simply writes to
   the ringbuffer to signal that it got to run, meaning it is still
   alive */
void *lp_thread_func(void *arg) {
	char data;
	struct timespec tv;

	lp_tid = gettid();
	/* syslog(LOG_INFO, "lp tid: %i", gettid()); */

	data = 0	;
	while(!thread_finish) {
		/* we simply write stuff to the ringbuffer and go back to sleeping
		   we can ignore the return value, cause, when it's full, it's ok
		   the data doesn;t have any meaning. it just needs to be there 
		   running full shouldn't happen anyways */
		jack_ringbuffer_write(rb, &data, sizeof(data));

		/* then sleep a bit. but less than the watchdog high prio thread */
		tv.tv_sec = LP_SLEEPSECS;
		tv.tv_nsec = 0;
		// sleep(LP_SLEEPSECS);
		nanosleep(&tv, NULL);
	return 0;

int main() 
	pid_t pid, sid;
	int done;
	struct sched_param params;
	char data;
	int err;
	int consumed;
	int count;
	struct timespec tv;
	char unfifo_cmd[1000];

	/* Fork off the parent process */
	pid = fork();
	if (pid < 0) {
	/* If we got a good PID, then
	   we can exit the parent process. */
	if (pid > 0) {

	/* Change the file mode mask */

	/* Open any logs here */
	openlog("rt_watchdog", 0, LOG_DAEMON);
	syslog(LOG_INFO, "started");

	/* Create a new SID for the child process */
	sid = setsid();
	if (sid < 0) {
		/* Log any failure here */
		syslog(LOG_INFO, "setsid failed. exiting");

	/* Change the current working directory */
	if ((chdir("/")) < 0) {
		/* Log any failure here */
		syslog(LOG_INFO, "chdir failed. exiting");

	/* Close out the standard file descriptors */

	// syslog(LOG_INFO, "closed fd's");

	/* syslog(LOG_INFO, "hp tid: %i", gettid()); */
	hp_tid = gettid();

	/* not really nessecary, but wtf */

	thread_finish = 0;
	done = 0;

	/* get ourself SCHED_FIFO with prio PRIO */
	params.sched_priority = PRIO;
	if (pthread_setschedparam(pthread_self(), SCHED_FIFO, &params)) {
		syslog(LOG_INFO, "couldn't set realtime prio for main thread.. exiting..");
	// syslog(LOG_INFO, "prio set");
	/* create a ringbuffer for the low prio thread to signal it's alive */
	rb = jack_ringbuffer_create(1024);
	// syslog(LOG_INFO, "ringbuffer created");

	/* create low prio thread */
	err = pthread_create(&low_prio_thread, 0, lp_thread_func, 0);
	if(err) {
		syslog(LOG_INFO, "couldn't create low prio thread, exiting..");

	/* dirty. we really need to wait for the lp thread
	   to have written the lp_tid. (condition variable?) */

	// syslog(LOG_INFO, "created low prio thread");
	/* make the low prio thread sched_fifo */
	params.sched_priority = LP_PRIO;
	if (pthread_setschedparam(low_prio_thread, SCHED_FIFO, &params)) {
		syslog(LOG_INFO, "couldn't set realtime prio for lower prio thread.. exiting..");
		thread_finish = 1;
		pthread_join(low_prio_thread, NULL);


	/* this is the main loop. we somply check whether the low prio thread
	   got to run at all by looking into the ringbuffer. If it's empty,
	   the low prio thread is kinda dead */
	count = 0;
	while(!done) {
		tv.tv_sec = SLEEPSECS;
		tv.tv_nsec = 0;
		nanosleep(&tv, NULL);
		/* sleep(SLEEPSECS); */
		/* syslog(LOG_INFO, "count %i", count); */
		/* see if our little brother got to run */

		if (jack_ringbuffer_read_space(rb) == 0) {
			/* oh oh, it didn't */
			syslog(LOG_INFO, "low prio thread seems to be starved, taking measures...");

			/* we pass our own TID's to the script, so we are excluded from the 
			   scheduling policy change */
			sprintf(unfifo_cmd, "unfifo_stuff.sh %i %i", lp_tid, hp_tid);
			syslog(LOG_INFO, "unfifo command:");
			syslog(LOG_INFO, unfifo_cmd);

			if (system(unfifo_cmd) == -1) {
				syslog(LOG_INFO, "unfifo command failed");
		} else {
			/* consume stuff from ringbuffer */
			consumed = 0;
			while (jack_ringbuffer_read_space(rb)) {
				jack_ringbuffer_read(rb, &data, 1);
			/* syslog(LOG_INFO, "lp alive: %i", consumed); */

	thread_finish = 1;	
	pthread_join(low_prio_thread, NULL);	

	syslog(LOG_INFO, "exiting");


#include <pthread.h>
#include <stdlib.h>
#include <sys/time.h>
#include <time.h>

int main() {
	struct sched_param params;
	int done = 0;
	struct timeval tv;
	time_t startsec;
	unsigned long int loops;
	int outer_loops;

	params.sched_priority = 80;
	pthread_setschedparam(pthread_self(), SCHED_FIFO, &params);

	startsec = tv.tv_sec;
#if 0
	while(!done) {
		gettimeofday(&tv, 0);
		if(tv.tv_sec - startsec > 10)
			done = 1;		
	for (outer_loops = 0; outer_loops < 4; ++outer_loops) {
		for (loops = 0; loops < 1000000000; loops++) {done += 1;}

Palimm Palimm!

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