Re: For review: pthread_setschedparam.3

Michael Kerrisk <mtk.manpages@xxxxxxxxxxxxxx> · Tue, 18 Nov 2008 08:50:23 -0500

Hi Loic,

Okay, here I'll revisit this piece.

The returned priority value is that set by the most recent
.BR pthread_setschedparam (),
.BR pthread_setschedprio (3),
or
.BR pthread_create (3)
call that affected
.IR thread .

Hmm, that's perfectly right from a POSIX point of view. Knowing how 
Linux implements threads, I have been interested about the effect of 
sched_setscheduler() on a MT-process (since NPTL uses 1:1 model, this 
should be a NOP).

Why should it be a NOP?

Thanks,

Michael

I tested the following program against the stable glibc-2.7... 
Apparently, it seems that sched_setscheduler() might affect the main 
thread priority as well.

--
#include <stdio.h>
#include <pthread.h>
#include <sched.h>
#include <unistd.h>

void
print_schedinfo(const char* thread)
{
   struct sched_param param;
   int policy;
   int rc;

   rc = pthread_getschedparam(pthread_self(), &policy, &param);
   if (rc!=0) printf("##%d\n", rc);
   printf("%s > Policy=%s, prio=%d\n",
          thread,
          (policy==SCHED_FIFO) ? "FIFO" : "*NOT* FIFO",
      param.sched_priority);
}

// dummy thread...
//
void*
thread(void* ignore)
{
   sleep(3);
   print_schedinfo("dummy thread");
   pthread_exit(NULL);
}

int
main()
{
   struct sched_param param;
   int    policy;
   int    rc;
   pthread_t tid;

   // create dummy thread
   //
   pthread_create(&tid, NULL, thread, NULL);
   param.sched_priority=1;

   // now we shall change the process policy/prio using
   // sched_setscheduler().
   // Normally: this should be a NOP. But due to the way Linux
   // implements threads, I am suspecting that this shall affect
   // the main thread
   //
   rc=sched_setscheduler(0, SCHED_FIFO, &param);
   if (rc==-1) printf("sched_setscheduler FAILED\n");

   // print my scheduling info
   //
   print_schedinfo("main");

   // join dummy thread and terminate
   //
   pthread_join(tid, NULL);
   return 0;
}

--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
git://git.kernel.org/pub/scm/docs/man-pages/man-pages.git
man-pages online: http://www.kernel.org/doc/man-pages/online_pages.html
Found a bug? http://www.kernel.org/doc/man-pages/reporting_bugs.html

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