Periodic thread fails real-time in Xenomai

Question

I'm creating a periodic thread which outputs a square signal on an analogic output. I'm using Posix Skin and Analogy from the Xenomai API.

I tested the real-time performance of my code using an oscilloscope and looking at the latency on the square signal (whose frequency is 1kHz). I am supposed to achieve <100us latency. However, the signal is strongly (>250us latency) perturbated by common interruption signals, like moving the mouse, starting a new program, etc.

The flags in my makefile are set up as such:

gcc  -I/usr/xenomai/include -D_GNU_SOURCE -D_REENTRANT -D__XENO__ -I/usr/xenomai/include/posix    
main_posix.c -Xlinker -rpath -Xlinker /usr/xenomai/lib -Wl,@/usr/xenomai/lib/posix.wrappers 
-L/usr/xenomai/lib -lpthread_rt -lxenomai -lpthread -lrt  -lanalogy -lrtdm -o main_posix

and this is the code:

#define PERIOD 1e6 
#define FILENAME "analogy0"
#define ANALOG_SUBD 1
#define CHANNEL 0
#define SCAN_SIZE 2
#define DELAI 5

static char *filename = FILENAME;
static int idx_subd = ANALOG_SUBD;
static int idx_chan = CHANNEL;
static int valueUp = 450000;
static int valueDown = 98500;

void *TaskCode(void *arg)
{
   unsigned char sgnl = 0;
   unsigned long overruns_r = 0;

   a4l_desc_t dsc = { .sbdata = NULL };
   a4l_chinfo_t *chinfo;
   int err = 0;
   unsigned int scan_size = SCAN_SIZE;

   err = a4l_open(&dsc, filename);
   if (err < 0) {
        fprintf(stderr,
            "insn_write: a4l_open %s failed (err=%d)\n",
            filename, err);
        return NULL;
   }    

   while(1) {
        pthread_wait_np( &overruns_r );
    if(sgnl)
        err = a4l_sync_write(&dsc,
                 idx_subd, CHAN(idx_chan), 0, &valueUp, scan_size);
    else
        err = a4l_sync_write(&dsc,
                 idx_subd, CHAN(idx_chan), 0, &valueDown, scan_size);
    if (err < 0) {
        fprintf(stderr,
            "insn_write: a4l_sync_write failed (err=%d)\n", err);
        goto out_insn_write;
    }

    sgnl = (sgnl + 1) % 2;
   }

   out_insn_write:

    if (dsc.sbdata != NULL)
        free(dsc.sbdata);

    a4l_close(&dsc);

    return NULL;

}

int main(void)
{
   mlockall( MCL_CURRENT | MCL_FUTURE );

   pthread_t thread;
   int rc, i;
   int prio = 99;
   struct timespec rqtp, rmtp;
   rqtp.tv_sec = 0;
   rqtp.tv_nsec = PERIOD;

   struct sched_param sparam;
   sparam.sched_priority = 99;

   rc = pthread_create(&thread, NULL, TaskCode, NULL);
   assert(0 == rc);

   rc = pthread_setschedparam(&thread, SCHED_FIFO, &sparam);
   assert(0 == rc);

   rc = clock_gettime( CLOCK_REALTIME, &rmtp );
   assert(0 == rc);
   rmtp.tv_sec = rmtp.tv_sec + DELAI;   

   rc = pthread_make_periodic_np(thread, &rmtp, &rqtp);
   if(rc == ETIMEDOUT) printf("Début dépassé \n");
   else if(rc == ESRCH) printf("Thread invalide \n");
   assert(0 == rc);

   rc = pthread_join(thread, NULL);

   exit(EXIT_SUCCESS);
}

I am strongly suspecting (by looking at the Xenomai scheduler) that my program somehow enters secondary mode. I tried to remove the "assert" statements as well as the relevant printf's, but this was not successful. Any idea how to fix it?

Answer 1

As always, the devil is in the details.

I enabled the -Wall option in gcc, which shows all the warnings. It turned out the headers for pthread_* were not properly loaded, which prevented me from seeing that the first argument of pthread_setschedparam was wrong, and was supposed to be thread and not &thread.