CODEWITHSUNDEEP
linux-kernelscheduler
user4089193
user4089193

Reputation:

What does the linux scheduler return when there are no tasks left in the queue

The linux scheduler calls for an scheduler algorithm that finds the next task in the task list.

What does the scheduling algorithm return if there are no tasks left?

Below is a piece of code

struct rt_info* sched_something(struct list_head *head, int flags) { /* some logic */ return some_task; // What task's value if there are no tasks left. }

Upvotes: 2

Views: 901

Answers (1)

osgx
osgx

Reputation: 94245

There is special "idle" task with PID=0, sometimes it is called swapper (Why do we need a swapper task in linux?). This task is scheduled to CPU core when there is no other task ready to run. There are several idle tasks - one for each CPU core

Source kernel/sched/core.c http://lxr.free-electrons.com/source/kernel/sched/core.c?v=3.16#L3160

3160 /**
3161  * idle_task - return the idle task for a given cpu.
3162  * @cpu: the processor in question.
3163  *
3164  * Return: The idle task for the cpu @cpu.
3165  */
3166 struct task_struct *idle_task(int cpu)
3167 {
3168         return cpu_rq(cpu)->idle;
3169 }
3170

So, pointer to this task is stored in runqueue (struct rq) kernel/sched/sched.h:

502  * This is the main, per-CPU runqueue data structure. ...  */
508 struct rq {
557         struct task_struct *curr, *idle, *stop;

There is some init code in sched/core.c:

4517 /**
4518  * init_idle - set up an idle thread for a given CPU
4519  * @idle: task in question
4520  * @cpu: cpu the idle task belongs to
4524  */
4525 void init_idle(struct task_struct *idle, int cpu)

I think, idle task will run some kind of loop with special asm commands to inform CPU core that there is no useful job...

This post http://duartes.org/gustavo/blog/post/what-does-an-idle-cpu-do/ says idle task executes cpu_idle_loop kernel/sched/idle.c (there may be custom version of loop for arch and CPU - called with cpu_idle_poll(void) -> cpu_relax();):

 40 #define cpu_relax()     asm volatile("rep; nop")
 45 static inline int cpu_idle_poll(void)
 ..
 50         while (!tif_need_resched())
 51                 cpu_relax();

221                         if (cpu_idle_force_poll || tick_check_broadcast_expired())
222                                 cpu_idle_poll();
223                         else
224                                 cpuidle_idle_call();

Upvotes: 1

Related Questions