從裸機到rtt任務切換有感

閑來無事，作為第一次使用實時操作系統的小白想看看rtthread的調度方式。

總結：

主要調度使用scheduler.c和context——gcc.s。

void rt_system_scheduler_start(void)
{
register struct rt_thread *to_thread;
rt_ubase_t highest_ready_priority;
to_thread = _get_highest_priority_thread(&highest_ready_priority);
rt_current_thread = to_thread;
rt_schedule_remove_thread(to_thread);
to_thread->stat = RT_THREAD_RUNNING;
rt_hw_context_switch_to((rt_ubase_t)&to_thread->sp);//跳轉至.s文件執行
}

第一次會調用此函數用來執行一個.s文件

c
rt_hw_context_switch_to:
LDR r1, =rt_interrupt_to_thread
STR r0, [r1] / 將rt_interrupt_to_thread寫入一個sp指針待會會用到 /
/* set interrupt flag to 1 /
LDR r1, =rt_thread_switch_interrupt_flag
MOV r0, #1
STR r0, [r1]
...
LDR r0, =NVIC_INT_CTRL / trigger the PendSV exception (causes context switch) /
LDR r1, =NVIC_PENDSVSET
STR r1, [r0] / NVIC_INT_CTRL為ICSR寄存器將28位置1會導致一個sv異常，任務切 換也在sv中進行 /
/ restore MSP */
LDR r0, =SCB_VTOR / VTOR保存著內存地址偏移，取出偏移值賦值給msp寄存器 /
LDR r0, [r0]
LDR r0, [r0]
NOP
MSR msp, r0
...
第一次觸發的sv異常

PendSV_Handler:
/* disable interrupt to protect context switch /
MRS r2, PRIMASK
CPSID I
/ get rt_thread_switch_interrupt_flag /
LDR r0, =rt_thread_switch_interrupt_flag
LDR r1, [r0]
CBZ r1, pendsv_exit / pendsv already handled /
/ clear rt_thread_switch_interrupt_flag to 0 /
MOV r1, #0x00
STR r1, [r0]
LDR r0, =rt_interrupt_from_thread / 第一次執行所以此函數我們并沒有賦值所以會執行跳轉 /
LDR r1, [r0]
CBZ r1, switch_to_thread / skip register save at the first time /
MRS r1, psp / get from thread stack pointer /
STMFD r1!, {r4 - r11} / push r4 - r11 register /
LDR r0, [r0]
STR r1, [r0] / update from thread stack pointer /
switch_to_thread:
LDR r1, =rt_interrupt_to_thread / 獲取需要切換的任務的sp指針 /
LDR r1, [r1]
LDR r1, [r1] / load thread stack pointer /
LDMFD r1!, {r4 - r11} / pop r4 - r11 register /
MSR psp, r1 / update stack pointer /
/ 此時psp寄存器已指向我們目標任務的sp /
pendsv_exit:
/ restore interrupt */
MSR PRIMASK, r2
ORR lr, lr, #0x04
BX lr

在后續的輪轉中我們會調用

void rt_schedule(void)
{
rt_base_t level;
struct rt_thread to_thread;
struct rt_thread from_thread;
/ disable interrupt /
level = rt_hw_interrupt_disable();
/ check the scheduler is enabled or not /
if (rt_scheduler_lock_nest == 0)
{
rt_ubase_t highest_ready_priority;
if (rt_thread_ready_priority_group != 0)
{
/ need_insert_from_thread: need to insert from_thread to ready queue /
int need_insert_from_thread = 0;
/ 獲取任務列表中優先級最高的我們假設有比當前任務更高的優先級執行39行，此時目標任務為高優先級任務 /
to_thread = _get_highest_priority_thread(&highest_ready_priority);
...
if (to_thread != rt_current_thread)
{
/ if the destination thread is not the same as current thread /
/ 開始進行轉換，首先將源任務設置為當前任務，再將當前任務設置為目標任務 /
rt_current_priority = (rt_uint8_t)highest_ready_priority;
from_thread = rt_current_thread;
rt_current_thread = to_thread;
...
#ifdef RT_USING_OVERFLOW_CHECK
_rt_scheduler_stack_check(to_thread);
#endif
if (rt_interrupt_nest == 0)
{
extern void rt_thread_handle_sig(rt_bool_t clean_state);
/ 這句進行任務切換 /
rt_hw_context_switch((rt_ubase_t)&from_thread->sp,
(rt_ubase_t)&to_thread->sp);
/ enable interrupt /
rt_hw_interrupt_enable(level);
goto __exit;
}
else
{
RT_DEBUG_LOG(RT_DEBUG_SCHEDULER, ("switch in interruptn"));
rt_hw_context_switch_interrupt((rt_ubase_t)&from_thread->sp,
(rt_ubase_t)&to_thread->sp);
}
}
else
{
rt_schedule_remove_thread(rt_current_thread);
rt_current_thread->stat = RT_THREAD_RUNNING | (rt_current_thread->stat & ~RT_THREAD_STAT_MASK);
}
}
}
/ enable interrupt /
rt_hw_interrupt_enable(level);
__exit:
return;
}
rt_hw_context_switch:
/ set rt_thread_switch_interrupt_flag to 1 /
LDR r2, =rt_thread_switch_interrupt_flag
LDR r3, [r2]
CMP r3, #1
/ 沒有設置interrupt_flag跳過執行 /
BEQ _reswitch
/ 再次置1用于sv異常 /
MOV r3, #1
STR r3, [r2]
/ 將源任務sp賦值給rt_interrupt_from_thread，此時from是有值的 /
LDR r2, =rt_interrupt_from_thread /* set rt_interrupt_from_thread /
STR r0, [r2]
_reswitch:
/ 將目標任務sp賦值給rt_interrupt_to_thread*/
LDR r2, =rt_interrupt_to_thread /* set rt_interrupt_to_thread /
STR r1, [r2]
/ 再次進入sv異常處理 /
LDR r0, =NVIC_INT_CTRL / trigger the PendSV exception (causes context switch) */
LDR r1, =NVIC_PENDSVSET
STR r1, [r0]
BX LR

此時sv處理與上次相比多了一個源任務的r4-r11的壓棧處理其他相同。