Hello there !
Assumably, I've discovered a bug in Linux kernel (version 2.6.16), at:
kernel\sched.c function set_user_nice()
Problem description:
After you execute nice() system call, the dynamic priority is set to the new
value of the static priority, instead of being adjusted by a difference
between new and old nice values.
In other words:
What you have before you execute nice(): p->prio == static_prio - bonus
After you execute nice(): p->prio == "a new" static_prio (no bonus)
BUG Fix:
Here I paste the whole of function set_user_nice() (thanks god, it's short)
with the BUG highlighted and fixed:
void set_user_nice(task_t *p, long nice)
{
unsigned long flags;
prio_array_t *array;
runqueue_t *rq;
int old_prio, new_prio, delta;
if (TASK_NICE(p) == nice || nice < -20 || nice > 19)
return;
/*
* We have to be careful, if called from sys_setpriority(),
* the task might be in the middle of scheduling on another CPU.
*/
rq = task_rq_lock(p, &flags);
/*
* 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:
*/
if (rt_task(p)) {
p->static_prio = NICE_TO_PRIO(nice);
goto out_unlock;
}
array = p->array;
if (array)
dequeue_task(p, array);
//-------------------------------------------------
/*
//BUGGED FORMULA : 5 lines
old_prio = p->prio;
new_prio = NICE_TO_PRIO(nice);
delta = new_prio - old_prio;
p->static_prio = NICE_TO_PRIO(nice);
p->prio += delta;
*/
//BUG FIX : 5 lines
old_prio = p->static_prio;
new_prio = NICE_TO_PRIO(nice);
delta = new_prio - old_prio;
p->static_prio = new_prio;
p->prio += delta;
//-------------------------------------------------
if (array) {
enqueue_task(p, array);
/*
* If the task increased its priority or is running and
* lowered its priority, then reschedule its CPU:
*/
if (delta < 0 || (delta > 0 && task_running(rq, p)))
resched_task(rq->curr);
}
out_unlock:
task_rq_unlock(rq, &flags);
}
Thank you,
Andrey Gelman,
Haifa, ISRAEL
9-Jun-2006
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