RTEMS支持多种调度算法，默认情况下是基于优先级的调度器，为了了解这些调度器算法，本文列出RTEMS支持的调度器种类，方便后续逐一进行调度器的测试研究

一、简单优先级调度

Simple Priority Scheduler是基于优先级调度的一种简化算法，其初始化如下：

    #define CONFIGURE_SCHEDULER_TABLE_ENTRIES \
      RTEMS_SCHEDULER_TABLE_SIMPLE( dflt, CONFIGURE_SCHEDULER_NAME )
 
 
#define SCHEDULER_SIMPLE_ENTRY_POINTS \
  { \
    _Scheduler_simple_Initialize,         /* initialize entry point */ \
    _Scheduler_simple_Schedule,           /* schedule entry point */ \
    _Scheduler_simple_Yield,              /* yield entry point */ \
    _Scheduler_simple_Block,              /* block entry point */ \
    _Scheduler_simple_Unblock,            /* unblock entry point */ \
    _Scheduler_simple_Update_priority,    /* update priority entry point */ \
    _Scheduler_default_Map_priority,      /* map priority entry point */ \
    _Scheduler_default_Unmap_priority,    /* unmap priority entry point */ \
    SCHEDULER_DEFAULT_SMP_OPERATIONS \
    _Scheduler_default_Node_initialize,   /* node initialize entry point */ \
    _Scheduler_default_Node_destroy,      /* node destroy entry point */ \
    _Scheduler_default_Release_job,       /* new period of task */ \
    _Scheduler_default_Cancel_job,        /* cancel period of task */ \
    _Scheduler_default_Start_idle         /* start idle entry point */ \
    SCHEDULER_DEFAULT_SET_AFFINITY_OPERATION \
  }

二、优先级调度

Priority Scheduler是默认的基于优先级的调度器，其初始化如下

#define RTEMS_SCHEDULER_TABLE_PRIORITY( name, obj_name ) \
  { \
    &SCHEDULER_PRIORITY_CONTEXT_NAME( name ).Base.Base, \
    SCHEDULER_PRIORITY_ENTRY_POINTS, \
    RTEMS_ARRAY_SIZE( \
      SCHEDULER_PRIORITY_CONTEXT_NAME( name ).Ready \
    ) - 1, \
    ( obj_name ) \
    SCHEDULER_CONTROL_IS_NON_PREEMPT_MODE_SUPPORTED( true ) \
  }
#define SCHEDULER_PRIORITY_ENTRY_POINTS \
  { \
    _Scheduler_priority_Initialize,       /* initialize entry point */ \
    _Scheduler_priority_Schedule,         /* schedule entry point */ \
    _Scheduler_priority_Yield,            /* yield entry point */ \
    _Scheduler_priority_Block,            /* block entry point */ \
    _Scheduler_priority_Unblock,          /* unblock entry point */ \
    _Scheduler_priority_Update_priority,  /* update priority entry point */ \
    _Scheduler_default_Map_priority,      /* map priority entry point */ \
    _Scheduler_default_Unmap_priority,    /* unmap priority entry point */ \
    SCHEDULER_DEFAULT_SMP_OPERATIONS \
    _Scheduler_priority_Node_initialize,  /* node initialize entry point */ \
    _Scheduler_default_Node_destroy,      /* node destroy entry point */ \
    _Scheduler_default_Release_job,       /* new period of task */ \
    _Scheduler_default_Cancel_job,        /* cancel period of task */ \
    _Scheduler_default_Start_idle         /* start idle entry point */ \
    SCHEDULER_DEFAULT_SET_AFFINITY_OPERATION \
  }

三、最早截止时间优先调度

Earliest Deadline First调度按照任务的截止时间来确定任务优先级，其初始化如下

#define RTEMS_SCHEDULER_TABLE_EDF( name, obj_name ) \
  { \
    &SCHEDULER_EDF_CONTEXT_NAME( name ).Base, \
    SCHEDULER_EDF_ENTRY_POINTS, \
    SCHEDULER_EDF_MAXIMUM_PRIORITY, \
    ( obj_name ) \
    SCHEDULER_CONTROL_IS_NON_PREEMPT_MODE_SUPPORTED( true ) \
  }
#define SCHEDULER_EDF_ENTRY_POINTS \
  { \
    _Scheduler_EDF_Initialize,       /* initialize entry point */ \
    _Scheduler_EDF_Schedule,         /* schedule entry point */ \
    _Scheduler_EDF_Yield,            /* yield entry point */ \
    _Scheduler_EDF_Block,            /* block entry point */ \
    _Scheduler_EDF_Unblock,          /* unblock entry point */ \
    _Scheduler_EDF_Update_priority,  /* update priority entry point */ \
    _Scheduler_EDF_Map_priority,     /* map priority entry point */ \
    _Scheduler_EDF_Unmap_priority,   /* unmap priority entry point */ \
    SCHEDULER_DEFAULT_SMP_OPERATIONS \
    _Scheduler_EDF_Node_initialize,  /* node initialize entry point */ \
    _Scheduler_default_Node_destroy, /* node destroy entry point */ \
    _Scheduler_EDF_Release_job,      /* new period of task */ \
    _Scheduler_EDF_Cancel_job,       /* cancel period of task */ \
    _Scheduler_default_Start_idle    /* start idle entry point */ \
    SCHEDULER_DEFAULT_SET_AFFINITY_OPERATION \
  }

四、恒定带宽调度

Constant Bandwidth Server调度是基于EDF的扩展，默认情况下，给任务分配固定的带宽（预算），然后再通过截止时间进行调度

#define RTEMS_SCHEDULER_TABLE_CBS( name, obj_name ) \
  { \
    &SCHEDULER_CBS_CONTEXT_NAME( name ).Base, \
    SCHEDULER_CBS_ENTRY_POINTS, \
    SCHEDULER_CBS_MAXIMUM_PRIORITY, \
    ( obj_name ) \
    SCHEDULER_CONTROL_IS_NON_PREEMPT_MODE_SUPPORTED( true ) \
  }
#define SCHEDULER_CBS_ENTRY_POINTS \
  { \
    _Scheduler_EDF_Initialize,       /* initialize entry point */ \
    _Scheduler_EDF_Schedule,         /* schedule entry point */ \
    _Scheduler_EDF_Yield,            /* yield entry point */ \
    _Scheduler_EDF_Block,            /* block entry point */ \
    _Scheduler_CBS_Unblock,          /* unblock entry point */ \
    _Scheduler_EDF_Update_priority,  /* update priority entry point */ \
    _Scheduler_EDF_Map_priority,     /* map priority entry point */ \
    _Scheduler_EDF_Unmap_priority,   /* unmap priority entry point */ \
    SCHEDULER_DEFAULT_SMP_OPERATIONS \
    _Scheduler_CBS_Node_initialize,  /* node initialize entry point */ \
    _Scheduler_default_Node_destroy, /* node destroy entry point */ \
    _Scheduler_CBS_Release_job,      /* new period of task */ \
    _Scheduler_CBS_Cancel_job,       /* cancel period of task */ \
    _Scheduler_default_Start_idle    /* start idle entry point */ \
    SCHEDULER_DEFAULT_SET_AFFINITY_OPERATION \
  }

五、基于SMP的调度器扩展

为了支持SMP，简单优先级，优先级，EDF都做了SMP的扩展支持，如下

#define RTEMS_SCHEDULER_TABLE_SIMPLE_SMP( name, obj_name ) \
  { \
    &SCHEDULER_SIMPLE_SMP_CONTEXT_NAME( name ).Base.Base, \
    SCHEDULER_SIMPLE_SMP_ENTRY_POINTS, \
    SCHEDULER_SIMPLE_SMP_MAXIMUM_PRIORITY, \
    ( obj_name ) \
    SCHEDULER_CONTROL_IS_NON_PREEMPT_MODE_SUPPORTED( false ) \
  }
#define RTEMS_SCHEDULER_TABLE_PRIORITY_SMP( name, obj_name ) \
  { \
    &SCHEDULER_PRIORITY_SMP_CONTEXT_NAME( name ).Base.Base.Base, \
    SCHEDULER_PRIORITY_SMP_ENTRY_POINTS, \
    RTEMS_ARRAY_SIZE( \
      SCHEDULER_PRIORITY_SMP_CONTEXT_NAME( name ).Ready \
    ) - 1, \
    ( obj_name ) \
    SCHEDULER_CONTROL_IS_NON_PREEMPT_MODE_SUPPORTED( false ) \
  }
#define RTEMS_SCHEDULER_TABLE_EDF_SMP( name, obj_name ) \
  { \
    &SCHEDULER_EDF_SMP_CONTEXT_NAME( name ).Base.Base.Base, \
    SCHEDULER_EDF_SMP_ENTRY_POINTS, \
    SCHEDULER_EDF_MAXIMUM_PRIORITY, \
    ( obj_name ) \
    SCHEDULER_CONTROL_IS_NON_PREEMPT_MODE_SUPPORTED( false ) \
  }

六、其他调度器

除了上述调度器，还有基于CPU亲和性优先级的SMP调度器，如RTEMS_SCHEDULER_TABLE_PRIORITY_AFFINITY_SMP，他会根据CPU的亲和性来调整基于优先级的调度器，可以确保某个任务只允许运行在某个CPU上

还有基于抢占式的SMP调度器，并支持CPU的亲和性设置

七、系统默认调度器

系统默认调度器是优先级调度，其定义如下：

#if !defined(CONFIGURE_SCHEDULER_CBS) \
  && !defined(CONFIGURE_SCHEDULER_EDF) \
  && !defined(CONFIGURE_SCHEDULER_EDF_SMP) \
  && !defined(CONFIGURE_SCHEDULER_PRIORITY) \
  && !defined(CONFIGURE_SCHEDULER_PRIORITY_AFFINITY_SMP) \
  && !defined(CONFIGURE_SCHEDULER_PRIORITY_SMP) \
  && !defined(CONFIGURE_SCHEDULER_SIMPLE) \
  && !defined(CONFIGURE_SCHEDULER_SIMPLE_SMP) \
  && !defined(CONFIGURE_SCHEDULER_STRONG_APA) \
  && !defined(CONFIGURE_SCHEDULER_USER)
  #if defined(RTEMS_SMP) && _CONFIGURE_MAXIMUM_PROCESSORS > 1
    #define CONFIGURE_SCHEDULER_EDF_SMP
  #else
    #define CONFIGURE_SCHEDULER_PRIORITY
  #endif
#endif

其配置作用如下：

#ifdef CONFIGURE_SCHEDULER
  /*
   * Ignore these warnings:
   *
   * - invalid use of structure with flexible array member
   *
   * - struct has no members
   */
  #pragma GCC diagnostic push
  #pragma GCC diagnostic ignored "-Wpedantic"
  CONFIGURE_SCHEDULER;
  #pragma GCC diagnostic pop
#endif
 
const Scheduler_Control _Scheduler_Table[] = {
  CONFIGURE_SCHEDULER_TABLE_ENTRIES
};

然后我们将宏定义展开，如下：

  static struct { Scheduler_priority_Context Base; Chain_Control Ready[ ( 255 + 1 ) ]; } _Configuration_Scheduler_priority_dflt;
const Scheduler_Control _Scheduler_Table[] = {
  { &_Configuration_Scheduler_priority_dflt.Base.Base, { _Scheduler_priority_Initialize, _Scheduler_priority_Schedule, _Scheduler_priority_Yield, _Scheduler_priority_Block, _Scheduler
_priority_Unblock, _Scheduler_priority_Update_priority, _Scheduler_default_Map_priority, _Scheduler_default_Unmap_priority,

可以看到我们配置的默认调度器就是优先级调度，且有且就1个。还记得之前查看初始化代码的时候，我们在rtems_initialize_data_structures函数，会调用_Scheduler_Handler_initialization，其实现如下

void _Scheduler_Handler_initialization(void)
{
  size_t n;
  size_t i;
 
  n = _Scheduler_Count;
 
  for ( i = 0 ; i < n ; ++i ) {
    const Scheduler_Control *scheduler;
#if defined(RTEMS_SMP)
    Scheduler_Context       *context;
#endif
 
    scheduler = &_Scheduler_Table[ i ];
#if defined(RTEMS_SMP)
    context = _Scheduler_Get_context( scheduler );
#endif
    _ISR_lock_Initialize( &context->Lock, "Scheduler" );
    ( *scheduler->Operations.initialize )( scheduler );
  }
}

这里会直接调用调度器的initialize函数。至此就开展调度器的初始化了