Scheduling parameters of a process and its main thread

Scheduling parameters of a process and its main thread

[Olivier Croquette]

Hi

When a process only has one thread, I would expect that sched_get* and 
pthread_getschedparam return equivalent priorities.

But in the example below, it's not the case.
What I do:
- print priorities
- give RT priority to the process
- print priorities
- give RT priority to the thread
- print priorities

1. Process 3938 : policy=0 prio=0
1. Thread 1075147872 : policy=0  prio=0

2.  Process 3938 : policy=1 prio=20
2.  Thread 1075147872 : policy=0  prio=0

3.  Process 3938 : policy=1 prio=30
3.  Thread 1075147872 : policy=1  prio=30

As you can see, at the step to, the process and the thread don't have 
the same parameters.
Is that normal?



#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <pthread.h>

/* gcc schedparam.c -o schedparam -Wall -pthread */

int print_process_sched(pid_t pid) {
   struct sched_param param;
   int prio;
   int policy;

   policy = sched_getscheduler(pid);

   sched_getparam(pid,&param);
   prio = param.sched_priority;

   printf("Process %d : policy=%d prio=%d\n",(int)pid, policy,prio);

   return 0;
}

int utilities_print_thread_sched(pthread_t id) {
   struct sched_param param;
   int policy;
   int prio;
   unsigned long int lid = id;

   if ( pthread_getschedparam(id, &policy, &param) ) {
     perror("pthread_getschedparam()"); return 1;
   }

   prio = param.sched_priority;

   printf("Thread %ld : policy=%d  prio=%d\n", (long)lid, policy,prio);

   return 0;
}

int main(void ) {

   struct sched_param p;

   printf("1. "); print_process_sched(getpid());
   printf("1. "); utilities_print_thread_sched(pthread_self());

   p.sched_priority = 20;
   if ( sched_setscheduler(getpid(),SCHED_FIFO,&p) ) {
     perror("sched_setscheduler()");
     return 1;
   }

   printf("2.  "); print_process_sched(getpid());
   printf("2.  "); utilities_print_thread_sched(pthread_self());

   p.sched_priority = 30;
   if ( pthread_setschedparam(pthread_self(),SCHED_FIFO,&p) ) {
     perror("sched_setscheduler()");
     return 1;
   }

   printf("3.  "); print_process_sched(getpid());
   printf("3.  "); utilities_print_thread_sched(pthread_self());

   return 0;
}