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

the linux scheduler calls scheduler algorithm finds next task in task list.

what scheduling algorithm return if there no tasks left?

below piece of code

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

there special "idle" task pid=0, called swapper (why need swapper task in linux?). task scheduled cpu core when there no other task ready run. there several idle tasks - 1 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 idle task given cpu. 3162  * @cpu: processor in question. 3163  * 3164  * return: idle task cpu @cpu. 3165  */ 3166 struct task_struct *idle_task(int cpu) 3167 { 3168         return cpu_rq(cpu)->idle; 3169 } 3170  

so, pointer task stored in runqueue (struct rq) kernel/sched/sched.h:

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

there init code in sched/core.c:

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

i think, idle task run kind of loop special asm commands inform cpu core there 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 custom version of loop arch , cpu - called 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();