2.7 时间片轮转机制

概述

TencentOS tiny操作系统内核是一个抢占式内核，抢占式内核的特点是，如果最高优先级的任务不放弃CPU（调用tos_task_delay、tos_task_yeild等主动放权，或者任务间同步通信机制的pend接口等），那么CPU将会一直被此任务独占。

假设这样一种场景：系统中包含多个同等优先级的任务，且这几个任务体中都没有放弃CPU的行为，则会出现的情况是，这几个任务始终只有第一个被得到调度的那个在运行，因为第一个得到调度的任务体中不会主动放弃CPU，而其他任务优先级上与其相等无法抢占。此种场景下，其他任务会因得不到CPU而陷入饥饿状态。

时间片轮转机制提供了按时间片占用调度的策略，可以解决上述场景下的任务饥饿问题。

API讲解

编程实例

1、在tos_config.h中，配置时间片轮转组件开关TOS_CFG_ROUND_ROBIN_EN：

#define TOS_CFG_ROUND_ROBIN_EN 1u

2、编写main.c示例代码：

  1. #include "tos.h"
  2. #include "mcu_init.h"
  3.  
  4. #define STK_SIZE_TASK_DEMO      512
  5. #define STK_SIZE_TASK_SAMPLE    512
  6.  
  7. /*
  8. 此代码中创建了两个同等优先级（PRIO_TASK_DEMO）的任务task_demo1、task_demo2，两个任务体中做的操作是不断对各自的计数器（demo1_counter、demo2_counter）做自增操作（没有放弃CPU的操作），时间片分别为timeslice_demo1、timeslice_demo2。
  9. 同时创建了一个优先级比task_demo1、task_demo2较高的采样任务task_sample，此任务不间歇的对两个计数器进行采样。在开启时间片轮转的情况下，task_demo1、task_demo2得到运行的时间比例应该是timeslice_demo1与timeslice_demo2的比例，那么demo1_counter和demo2_counter值的比例应该也差不多是timeslice_demo1与timeslice_demo2的比例。
  10. */
  11.  
  12. // task_demo1和task_demo2的优先级
  13. #define PRIO_TASK_DEMO          4
  14. // 采样任务的优先级
  15. #define PRIO_TASK_SAMPLE        (PRIO_TASK_DEMO - 1)
  16.  
  17. // task_demo1的时间片，在tos_task_create时传入
  18. const k_timeslice_t timeslice_demo1 = 10;
  19. // task_demo2的时间片，在tos_task_create时传入
  20. const k_timeslice_t timeslice_demo2 = 20;
  21.  
  22. k_stack_t stack_task_demo1[STK_SIZE_TASK_DEMO];
  23. k_stack_t stack_task_demo2[STK_SIZE_TASK_DEMO];
  24. k_stack_t stack_task_sample[STK_SIZE_TASK_SAMPLE];
  25.  
  26. k_task_t task_demo1;
  27. k_task_t task_demo2;
  28. k_task_t task_sample;
  29.  
  30. extern void entry_task_demo1(void *arg);
  31. extern void entry_task_demo2(void *arg);
  32. extern void entry_task_sample(void *arg);
  33.  
  34. uint64_t demo1_counter = 0;
  35. uint64_t demo2_counter = 0;
  36.  
  37. void entry_task_demo1(void *arg)
  38. {
  39.     while (K_TRUE) {
  40.         ++demo1_counter;
  41.     }
  42. }
  43.  
  44. void entry_task_demo2(void *arg)
  45. {
  46.     while (K_TRUE) {
  47.         ++demo2_counter;
  48.     }
  49. }
  50.  
  51. void entry_task_sample(void *arg)
  52. {
  53.     while (K_TRUE) {
  54.         ++demo2_counter;
  55.         printf("demo1_counter: %lld\n", demo1_counter);
  56.         printf("demo2_counter: %lld\n", demo2_counter);
  57.         printf("demo2_counter / demo1_counter = %f\n",
  58.             (double)demo2_counter / demo1_counter);
  59.         printf("should almost equals to:\n");
  60.         printf("timeslice_demo2 / timeslice_demo1 = %f\n\n", (double)timeslice_demo2 / timeslice_demo1);
  61.         tos_task_delay(1000);
  62.     }
  63. }
  64.  
  65. int main(void)
  66. {
  67.     board_init();
  68.     tos_knl_init();
  69.     // 配置robin机制参数
  70.     tos_robin_config(TOS_ROBIN_STATE_ENABLED, (k_timeslice_t)500u);
  71.     (void)tos_task_create(&task_demo1, "demo1", entry_task_demo1, NULL,
  72.                             PRIO_TASK_DEMO, stack_task_demo1, STK_SIZE_TASK_DEMO,
  73.                             timeslice_demo1);
  74.     (void)tos_task_create(&task_demo2, "demo2", entry_task_demo2, NULL,
  75.                             PRIO_TASK_DEMO, stack_task_demo2, STK_SIZE_TASK_DEMO,
  76.                             timeslice_demo2);
  77.     (void)tos_task_create(&task_sample, "sample", entry_task_sample, NULL,
  78.                             PRIO_TASK_SAMPLE, stack_task_sample, STK_SIZE_TASK_SAMPLE,
  79.                             0);
  80.     tos_knl_start();
  81. }

运行效果

demo1_counter: 0demo2_counter: 1demo2_counter / demo1_counter = 0.000000should almost equals to:timeslice_demo2 / timeslice_demo1 = 2.000000

demo1_counter: 1915369demo2_counter: 3720158demo2_counter / demo1_counter = 1.942267should almost equals to:timeslice_demo2 / timeslice_demo1 = 2.000000

demo1_counter: 3774985demo2_counter: 7493508demo2_counter / demo1_counter = 1.985043should almost equals to:timeslice_demo2 / timeslice_demo1 = 2.000000

demo1_counter: 5634601demo2_counter: 11266858demo2_counter / demo1_counter = 1.999584should almost equals to:timeslice_demo2 / timeslice_demo1 = 2.000000

demo1_counter: 7546896demo2_counter: 14987015demo2_counter / demo1_counter = 1.985852should almost equals to:timeslice_demo2 / timeslice_demo1 = 2.000000

demo1_counter: 9406512demo2_counter: 18759340demo2_counter / demo1_counter = 1.994293should almost equals to:timeslice_demo2 / timeslice_demo1 = 2.000000

demo1_counter: 11266128demo2_counter: 22531664demo2_counter / demo1_counter = 1.999947should almost equals to:timeslice_demo2 / timeslice_demo1 = 2.000000

demo1_counter: 13177398demo2_counter: 26251821demo2_counter / demo1_counter = 1.992185should almost equals to:timeslice_demo2 / timeslice_demo1 = 2.000000

demo1_counter: 15037014demo2_counter: 30023632demo2_counter / demo1_counter = 1.996649should almost equals to:timeslice_demo2 / timeslice_demo1 = 2.000000

demo1_counter: 16896630demo2_counter: 33795443demo2_counter / demo1_counter = 2.000129should almost equals to:timeslice_demo2 / timeslice_demo1 = 2.000000

demo1_counter: 18807900demo2_counter: 37515600demo2_counter / demo1_counter = 1.994672should almost equals to:timeslice_demo2 / timeslice_demo1 = 2.000000

实例代码