resend: KERNEL BUG: nice level should not affect SCHED_RR timeslice

Chris Friesen
2007-03-07T23:20:01Z

I still haven't seen any replies, so I'm resending with a few more
people directly in the TO list.

The timeslice of a SCHED_RR process currently varies with nice level the
same way that it does for SCHED_OTHER. I've included a small app below
that demonstrates the issue. So while niceness doesn't affect the
priority of a SCHED_RR task, it does impact how much cpu it gets
relative to other SCHED_RR tasks.

SUSv3 indicates, "Any processes or threads using SCHED_FIFO or SCHED_RR
shall be unaffected by a call to setpriority()."

In addition, the code in set_user_nice() has a comment that leads me to
believe the current behaviour is accidental (although I think the "not"
in the last line of the comment isn't meant to be there):

/*
* The RT priorities are set via sched_setscheduler(), but we still
* allow the 'normal' nice value to be set - but as expected
* it wont have any effect on scheduling until the task is
* not SCHED_NORMAL/SCHED_BATCH:
*/

It appears that the desired behaviour is to allow setting the nice level
of a realtime task, but to not have it affect anything until (and
unless) it drops that realtime status. This seems reasonable, but
doesn't match current behaviour.

Chris

#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
#include <sched.h>
#include <errno.h>
#include <string.h>
#include <sys/syscall.h>
#include <sys/time.h>
#include <sys/resource.h>

#define THRESHOLD_USEC 2000

unsigned long long stamp()
{
struct timeval tv;
gettimeofday(&tv, 0);
return (unsigned long long) tv.tv_usec + ((unsigned long long)
tv.tv_sec)*1000000;
}

void chewcpu(int cpu)
{
unsigned long long thresh_ticks = THRESHOLD_USEC;
unsigned long long cur,last;

last = stamp();
while(1) {
cur = stamp();
unsigned long long delta = cur-last;
if (delta > thresh_ticks) {
printf("pid %d, out for %llu ms\n", getpid(), delta/1000);
cur = stamp();
}
last = cur;
}

}

int main()
{
int cpu;
cpu_set_t cpumask;
CPU_ZERO(&cpumask);
CPU_SET(0, &cpumask);

int kidpid = fork();

struct sched_param p;
p.sched_priority = 1;
sched_setscheduler(0, SCHED_RR, &p);

struct timespec ts;

if (kidpid) {
setpriority(PRIO_PROCESS, 0, 19);
printf("pid %d, prio of %d\n", getpid(), getpriority(PRIO_PROCESS, 0));
sched_rr_get_interval(0, &ts);
printf("pid %d, interval of %d nsec\n", getpid(), ts.tv_nsec);
} else {
setpriority(PRIO_PROCESS, 0, -19);
printf("pid %d, prio of %d\n", getpid(), getpriority(PRIO_PROCESS, 0));
sched_rr_get_interval(0, &ts);
printf("pid %d, interval of %d nsec\n", getpid(), ts.tv_nsec);
}

int rc = syscall(__NR_sched_setaffinity, 0, sizeof(cpumask), &cpumask);
if (rc < 0)
printf("unable to set affinity: %m\n");

sleep(1);

chewcpu(cpu);
return 0;
}