sched_setscheduler.3 and threads (was: For review: pthread_setschedparam.3)

[Loic Domaigne <tech@xxxxxxxxxxxx>]

Hello Michael,

I've investigated the effects of sched_setscheduler() on threads. Things 
seem worse than I expected.

AFAICS, you can change scheduling attribute of a thread using 
sched_setscheduler(3) without the changes being reflected in the 
thread's scheduling parameters as retrieved with pthread_getschedparam(3).

Output from the schedtest program below:

<copy>

main   > call sched_setscheduler(0,FIFO,1)
main   > pthread_getschedparam() : Policy=FIFO, prio=1
main   > sched_getscheduler()    : Policy=FIFO, prio=1
-----
Thread > pthread_getschedparam() : Policy=*NOT* FIFO, prio=0
Thread > sched_getscheduler()    : Policy=*NOT* FIFO, prio=0
-----
Thread > call sched_setscheduler(0,FIFO,2)
Thread > pthread_getschedparam() : Policy=*NOT* FIFO, prio=0
Thread > sched_getscheduler()    : Policy=FIFO, prio=2
-----
main   > pthread_getschedparam() : Policy=FIFO, prio=1
main   > sched_getscheduler()    : Policy=FIFO, prio=1
-----

</copy>

Let's hope that my program is broken!

A+,
Loïc
--


/***************************************************************************/
/* schedtest.c- test interaction between sched_setscheduler and threads
/***************************************************************************/
/*
 * compile with: cc -pthread schedtest.c -o schedtest
 */
#include <errno.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <unistd.h>

/**********************************************************************/
/* our macro for errors checking                                      */
/**********************************************************************/
#define COND_CHECK(func, cond, errv) \
if ( (cond) ) \
{ \
   fprintf(stderr, "\n[CHECK FAILED at %s:%d]\n| %s(...)=%d (%s)\n\n",\
                 __FILE__,__LINE__,func,errv,strerror(errv)); \
                    exit(EXIT_FAILURE); \
}

#define UnixCheck(func,rc)    COND_CHECK(func, (rc==-1), errno)
#define PthreadCheck(func,rc) COND_CHECK(func,(rc!=0), rc)

void
sched_fifo(const char* thr, int prio)
{
   struct sched_param param;
   int rc;

   printf("%s > call sched_setscheduler(0,FIFO,%d)\n", thr, prio);
   param.sched_priority = prio;
   rc = sched_setscheduler(0, SCHED_FIFO, &param);
   UnixCheck("sched_setscheduler", rc);
}

void
sched_info(const char* thr)
{
   struct sched_param param;
   int    policy;
   int    rc;

   rc = pthread_getschedparam(pthread_self(), &policy, &param);
   PthreadCheck("pthread_getschedparam", rc);
   printf("%s > pthread_getschedparam() : Policy=%s, prio=%d\n",
          thr,
          (policy==SCHED_FIFO) ? "FIFO" : "*NOT* FIFO",
	  param.sched_priority);
   policy = sched_getscheduler(0);
   UnixCheck("sched_getscheduler", policy);
   rc = sched_getparam(0, &param);
   UnixCheck("sched_getparam", rc);
   printf("%s > sched_getscheduler()    : Policy=%s, prio=%d\n",
          thr,
          (policy==SCHED_FIFO) ? "FIFO" : "*NOT* FIFO",
	  param.sched_priority);
   printf("-----\n");
}

void*
thread(void* ignore)
{
   sleep(2);
   sched_info("Thread");
   sched_fifo("Thread", 2);
   sched_info("Thread");
   return NULL;
}

int
main()
{
   pthread_t tid;
   int       rc;

   rc = pthread_create(&tid, NULL, thread, NULL);
   PthreadCheck("pthread_create", rc);
   sched_fifo("main  ", 1);
   sched_info("main  ");
   rc = pthread_join(tid, NULL);
   PthreadCheck("pthread_join", rc);
   sched_info("main  ");
   return EXIT_SUCCESS;
}
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