Re: [Xenomai-help] Xenomai 2.0.1 - Posix Skin - realtime priorities - what I'm doing wrong?

[Paolo Gai <pj@domain.hid>]

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Gilles Chanteperdrix wrote:

>Paolo Gai wrote:
> > Gilles Chanteperdrix wrote:
> > >Working around this issue means using calls to unlocked versions of libc
> > >functions protected with Xenomai POSIX mutexes, such as, for example,
> > >myputs and myputchar (sufficient for Paolo example) defined as :
> > >[...]
> > >  
> > >
> > Ok! I tried it, and I also tried another slightly modified version of 
> > the demo, that simply replaces putchars with an unprotected array of 
> > chars (let's suppose there are no race conditions) that is printed out 
> > to stdout at the end of the game.
>
>There is no race condition in the SCHED_FIFO case or ahem, there should
>be none.
>
Yes, that's true...

> Please also note that the "volatile" qualifier seem misplaced
>in your declaration of buf_current, it may matter since the compiler
>will probably inline calls to myputchar in the "medium" function, and
>buf_current will end up in a register. But this would have no effect
>only in the SCHED_FIFO case.
>  
>
Ouch! I just added it in the last tries and I forgot to remove it - sorry;
However, I checked the source code produced by the compiler on i386 with 
the default compiler of FC3 and the code produced is the same with and 
without volatile;

> > the results are the following, where the first part is RR and the second 
> > is FIFO (it's quite strange for me that FIFO has more contect changes 
> > than RR (?))
>
>Please also note that any thread created with the SCHED_RR attribute
>will be a plain Linux thread, and due to a bug in glibc, the thread will
>use the default policy. The results are strange indeed for SCHED_FIFO,
>it may be a bug, this needs a closer look... Maybe threads are not
>Xenomai threads using SCHED_FIFO policy at all ?
>  
>
Ok... I've done some more experiments. here are some results, with 
screenshots :-)

First thing, I found this snippet
http://sources.redhat.com/ml/glibc-bugs/2004-05/msg00003.html
that shows how on Linux and NPTL setting realtime priorities simply 
fails... I tried it on my FC3, and the behavior is the same, confirming 
the bug on my distro.

Then, I slightly modified the example of the previous posts, setting the 
thread priorities with pthread_setschedparam.

----------------------------
Case 1/Linux: Printing on the console using stdio, Linux real-time threads

Screenshot:
[root@domain.hid xenomai-demos]# ./ex_rr3
thread id = 0xb7f0dbb0, policy = SCHED_OTHER, priority = 0
thread id = 0xb7f0dbb0, policy = SCHED_RR, priority = 3
...........###########............############...........###########...........###########............############...........###########...........###########............############...........###########............###########...........############...........###########............############...........###########...........###########............############...........###########...........###########............############...........###########............###########...........############...........###########............############...........###########...........###########.....#####LOW 
priority thread!!!
............................................................................................................................................................................................................................................................................................................############################################################################################################################################################################################################################################################################################################LOW 
priority thread!!!
[root@domain.hid xenomai-demos]#

everything is fine, SCHED_RR and SCHED_FIFO works as expected, while the 
demo runs the XServer is freezed.

----------------------------

Case 1/Xenomai: Printing on the console using stdio, Xenomai real-time 
threads

Screenshot:
[root@domain.hid xenomai-demos]# ./rt_ex_rr3
thread id = 0xb7fc7bb0, policy = SCHED_OTHER, priority = 0
pthread_setschedparam failed: Success
thread id = 0xb7fc7bb0, policy = SCHED_OTHER, priority = 0
pthread_setschedparam failed: Success
pthread_setschedparam failed: Success
.....########LOW priority thread!!!
##################################################################################################....................................................................................................##################################################################################################................................................................................................################################################################################################...............................................................................................................................................................................................................................................................................................................................................................................................................############################################################################################################################################################################################################################################################################################################LOW 
priority thread!!!
[root@domain.hid xenomai-demos]#

1) why do i get "pthread_setschedparam failed: Success" ?? Why the 
policy is not changed (am I still calling the Linux functions?)
2) The SCHED_FIFO seems to work as expected, whereas SCHED_RR seems to 
have a strange behavior when it starts (the low priority thread is 
started before the others...
3) the XServer freezes but for a SHORTER time - I guess they are the 
Xenomai threads, one thing I do not understand is why with Linux the 
XServer stops for around 10 seconds, and in Xenomai less than one second!!!

----------------------------

Case 2/Linux: Printing on the console using a shared buffer, Linux 
real-time threads

Screenshot:
[root@domain.hid xenomai-demos]# ./ex_rr4
thread id = 0xb7fcabb0, policy = SCHED_OTHER, priority = 0
thread id = 0xb7fcabb0, policy = SCHED_RR, priority = 3
.............############...........###########...........###########...........############............###########...........###########...........############............###########...........###########...........###########............############...........###########...........###########...........############............###########...........###########...........############............###########...........###########............############...........###########...........############............###########...........###########...........############............###########....#####L   
............................................................................................................................................................................................................................................................................................................############################################################################################################################################################################################################################################################################################################L
[root@domain.hid xenomai-demos]#

works as expected, the same behavior as Case1/Linux.

----------------------------

Case 2/xenomai: Printing on the console using a shared buffer, Xenomai 
real-time threads

Screenshot:
[root@domain.hid xenomai-demos]# ./rt_ex_rr4
thread id = 0xb7f25bb0, policy = SCHED_OTHER, priority = 0
pthread_setschedparam failed: Success
thread id = 0xb7f25bb0, policy = SCHED_OTHER, priority = 0
pthread_setschedparam failed: Success
pthread_setschedparam failed: Success
...............#############L................................................................................................................................................................##################################################################################################################################################################.............................................................................................................................#############################################################################################################################   
............................................................................................................................................................................................................................................................................................................############################################################################################################################################################################################################################################################################################################L
[root@domain.hid xenomai-demos]#

Again, the same behavior as before in Case 1/Xenomai

which make me think that
- the behavior on Xenomai does not depend a lot on the fact i called 
stdio functions
- the SCHED_RR maybe still has some problems (???)



ok, that's all for now, sorry again for the long mail.

Paolo

[-- Attachment #2: ex_rr3.c --]
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/*
 * this version uses pthread_setschedparam as a workaround of the
 * glibc bug, prints using stdio
 */


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

/* for sched_param.c */
#include <sched.h> 

/* --------------------------------------------- */
/* source from http://sources.redhat.com/ml/glibc-bugs/2004-05/msg00003.html */

#define LPRIO  1
#define MPRIO  2
#define HPRIO 3

void show_sched(void)
{
	struct sched_param sp;
	int policy;
	const char *pstr;
	
	if (pthread_getschedparam(pthread_self(), &policy, &sp) != 0) {
		perror("pthread_getschedparam failed");
		return;
	}

	switch(policy) {
		case SCHED_OTHER:
			pstr = "SCHED_OTHER";
			break;
		case SCHED_FIFO:
			pstr = "SCHED_FIFO";
			break;
		case SCHED_RR:
			pstr = "SCHED_RR";
			break;
		default:
			pstr = "unknown";
	}

	printf("thread id = 0x%lx, policy = %s, priority = %d\n",
			pthread_self(), pstr, sp.sched_priority);

	return;
}

void set_sched(int policy, int priority)
{
	struct sched_param schedp;

	memset(&schedp, 0, sizeof(schedp));
	schedp.sched_priority = priority;
	if (pthread_setschedparam(pthread_self(), policy, &schedp) != 0)
		perror("pthread_setschedparam failed");

}

/* --------------------------------------------- */



void *low(void *arg)
{
  set_sched(SCHED_FIFO, LPRIO);
  printf("LOW priority thread!!!\n");
  return NULL;
}

struct medium_par_t {
  int policy;
  char *c;
} mp1, mp2;

void *medium(void *arg)
{
  int i,j;

  struct medium_par_t *p = (struct medium_par_t *)arg;

  set_sched(p->policy, MPRIO);

  for (i=0; i<300; i++) {
    for (j=0; j<1000000; j++) ;
    printf(p->c);
  }

  return NULL;
}

void my_create(int policy)
{
  pthread_t th1, th2, th3;
  pthread_attr_t medium_attr, low_attr;
  struct sched_param medium_policy, low_policy;

  pthread_attr_init(&medium_attr);
  pthread_attr_setschedpolicy(&medium_attr, policy);
  medium_policy.sched_priority = 2;
  pthread_attr_setschedparam(&medium_attr, &medium_policy);

  pthread_attr_init(&low_attr);
  pthread_attr_setschedpolicy(&low_attr, SCHED_FIFO);
  low_policy.sched_priority = 1;
  pthread_attr_setschedparam(&low_attr, &low_policy);

  mp1.policy = mp2.policy = policy;
  mp1.c = ".";
  mp2.c = "#";

  pthread_create(&th1, &medium_attr, medium, (void *)&mp1);
  pthread_create(&th2, &medium_attr, medium, (void *)&mp2);
  pthread_create(&th3, &low_attr, low, NULL);
  
  pthread_attr_destroy(&medium_attr);
  pthread_attr_destroy(&low_attr);

  pthread_join(th1, NULL);
  pthread_join(th2, NULL);
  pthread_join(th3, NULL);
}


void *high(void *arg)
{
  show_sched();
  set_sched(SCHED_RR, HPRIO);
  show_sched();

  /* first experiment:
     - two medium priority thread scheduled with RR
     - one low priority thread scheduled with FIFO
  */
  my_create(SCHED_RR);

  /* second experiment:
     - two medium priority thread scheduled with FIFO
     - one low priority thread scheduled with FIFO
  */
  my_create(SCHED_FIFO);

  return NULL;

}

int main()
{
  pthread_t mythread;
  pthread_attr_t myattr;
  struct sched_param myparam;

  int err;
  int parameter;
  void *returnvalue;

  /* initializes the thread attribute */
  pthread_attr_init(&myattr);
  pthread_attr_setschedpolicy(&myattr, SCHED_FIFO);
  myparam.sched_priority = 3;
  pthread_attr_setschedparam(&myattr, &myparam);

  err = pthread_create(&mythread, &myattr, high, (void *)&parameter);

  if (err) {
    perror("ERROR");
    exit(1);
  }

  pthread_attr_destroy(&myattr);

  /* wait the end of the thread we just created */
  pthread_join(mythread, &returnvalue);

  return 0;
}


[-- Attachment #3: ex_rr4.c --]
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/*
 * this version uses pthread_setschedparam as a workaround of the
 * glibc bug, prints using a shared buffer
 */

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

/* for sched_param.c */
#include <sched.h> 


/* --------------------------------------------- */
/* source from http://sources.redhat.com/ml/glibc-bugs/2004-05/msg00003.html */

#define LPRIO  1
#define MPRIO  2
#define HPRIO 3

void show_sched(void)
{
	struct sched_param sp;
	int policy;
	const char *pstr;
	
	if (pthread_getschedparam(pthread_self(), &policy, &sp) != 0) {
		perror("pthread_getschedparam failed");
		return;
	}

	switch(policy) {
		case SCHED_OTHER:
			pstr = "SCHED_OTHER";
			break;
		case SCHED_FIFO:
			pstr = "SCHED_FIFO";
			break;
		case SCHED_RR:
			pstr = "SCHED_RR";
			break;
		default:
			pstr = "unknown";
	}

	printf("thread id = 0x%lx, policy = %s, priority = %d\n",
			pthread_self(), pstr, sp.sched_priority);

	return;
}

void set_sched(int policy, int priority)
{
	struct sched_param schedp;

	memset(&schedp, 0, sizeof(schedp));
	schedp.sched_priority = priority;
	if (pthread_setschedparam(pthread_self(), policy, &schedp) != 0)
		perror("pthread_setschedparam failed");

}

/* --------------------------------------------- */



/* ------------------------------------------------------------ */
/* unprotected buffer to avoid migrations */

char buf[10000];
char *buf_current=buf;

void myputchar(char c)
{
  *buf_current++ = c;
}

void myprintbuf(void)
{
  *buf_current=0;
  puts(buf);
}
/* ------------------------------------------------------------ */


void *low(void *arg)
{
  set_sched(SCHED_FIFO, LPRIO);
  myputchar('L');
  return NULL;
}

struct medium_par_t {
  int policy;
  char *c;
} mp1, mp2;

void *medium(void *arg)
{
  int i,j;

  struct medium_par_t *p = (struct medium_par_t *)arg;

  set_sched(p->policy, MPRIO);

  for (i=0; i<300; i++) {
    for (j=0; j<1000000; j++) ;
    myputchar(p->c[0]);
  }

  return NULL;
}

void my_create(int policy)
{
  pthread_t th1, th2, th3;
  pthread_attr_t medium_attr, low_attr;
  struct sched_param medium_policy, low_policy;

  pthread_attr_init(&medium_attr);
  pthread_attr_setschedpolicy(&medium_attr, policy);
  medium_policy.sched_priority = 2;
  pthread_attr_setschedparam(&medium_attr, &medium_policy);

  pthread_attr_init(&low_attr);
  pthread_attr_setschedpolicy(&low_attr, SCHED_FIFO);
  low_policy.sched_priority = 1;
  pthread_attr_setschedparam(&low_attr, &low_policy);

  mp1.policy = mp2.policy = policy;
  mp1.c = ".";
  mp2.c = "#";

  pthread_create(&th1, &medium_attr, medium, (void *)&mp1);
  pthread_create(&th2, &medium_attr, medium, (void *)&mp2);
  pthread_create(&th3, &low_attr, low, NULL);
  
  pthread_attr_destroy(&medium_attr);
  pthread_attr_destroy(&low_attr);

  pthread_join(th1, NULL);
  pthread_join(th2, NULL);
  pthread_join(th3, NULL);
  
  myputchar(' ');
  myputchar(' ');
  myputchar(' ');
}


void *high(void *arg)
{
  show_sched();
  set_sched(SCHED_RR, HPRIO);
  show_sched();

  /* first experiment:
     - two medium priority thread scheduled with RR
     - one low priority thread scheduled with FIFO
  */
  my_create(SCHED_RR);

  /* second experiment:
     - two medium priority thread scheduled with FIFO
     - one low priority thread scheduled with FIFO
  */
  my_create(SCHED_FIFO);

  return NULL;

}

int main()
{
  pthread_t mythread;
  pthread_attr_t myattr;
  struct sched_param myparam;

  int err;
  void *returnvalue;


  /* initializes the thread attribute */
  pthread_attr_init(&myattr);
  pthread_attr_setschedpolicy(&myattr, SCHED_FIFO);
  myparam.sched_priority = 3;
  pthread_attr_setschedparam(&myattr, &myparam);

  err = pthread_create(&mythread, &myattr, high, NULL);

  if (err) {
    perror("ERROR");
    exit(1);
  }

  pthread_attr_destroy(&myattr);

  /* wait the end of the thread we just created */
  pthread_join(mythread, &returnvalue);

  myprintbuf();

  return 0;
}