Computing priority:
[The kernel uses a simpler scale ranging from 0 to 139 inclusive to represent priorities internally. Again, lower values mean higher priorities. The range from 0 to 99 is reserved for real-time processes. The nice values [−20, +19] are mapped to the range from 100 to 139]
Three priorities must be taken into account:
dynamic priority (task_struct->prio),
normal priority (task_struct->normal_prio),
and static priority (task_struct->static_prio).
static priority is the starting point of calculations. Assume that it has been set and that the kernel want to compute the other priority. This is done by one line:
p->prio = effective_prio(p);
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/*
* Calculate the current priority, i.e. the priority
* taken into account by the scheduler. This value might
* be boosted by RT tasks, or might be boosted by
* interactivity modifiers. Will be RT if the task got
* RT-boosted. If not then it returns p->normal_prio.
*/
static int effective_prio(struct task_struct *p)
{
p->normal_prio = normal_prio(p);
/*
* If we are RT tasks or we were boosted to RT priority,
* keep the priority unchanged. Otherwise, update priority
* to the normal priority:
*/
if (!rt_prio(p->prio))
return p->normal_prio;
return p->prio;
}
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/*
* Calculate the expected normal priority: i.e. priority
* without taking RT-inheritance into account. Might be
* boosted by interactivity modifiers. Changes upon fork,
* setprio syscalls, and whenever the interactivity
* estimator recalculates.
*/static inline int normal_prio(struct task_struct *p)
{
int prio;
if (task_has_rt_policy(p))
prio = MAX_RT_PRIO-1 - p->rt_priority;
else
prio = __normal_prio(p);
return prio;
}
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/*
* __normal_prio - return the priority that is based on the static prio
*/
static inline int __normal_prio(struct task_struct *p)
{
return p->static_prio;
}
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p->prio is set with the method shown above when a newly created task is woken up with wake_up_new_task(), and when the static priority was changed with system call nice().Notice that when a process forks off a child process, the current static priority will be inherited from parent. The dynamic priority is set to normal priority of parent, in case priority booted by RT-Mutexes pass to child.
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