ecos kernel 是个典型的抢占式多任务的rtos,我这里想从代码上,把它的实现搭个框架出来。
分时的多任务系统是靠定时时间中断实现的,所以我从这里做切入点
有kernel 的ecos重写了interrupt 处理代码,原来的在drv_api.c里实现的,现在的在kernel/intr/intr.cxx里,时间中断的注册在kernel/common/clock.cxx里
Cyg_RealTimeClock Cyg_RealTimeClock::rtc CYG_INIT_PRIORITY( CLOCK );
// -------------------------------------------------------------------------
Cyg_RealTimeClock::Cyg_RealTimeClock()
: Cyg_Clock(rtc_resolution),
interrupt(CYGNUM_HAL_INTERRUPT_RTC,
CYGNUM_KERNEL_COUNTERS_CLOCK_ISR_PRIORITY,
(CYG_ADDRWORD)this, isr, dsr)
{
CYG_REPORT_FUNCTION();
HAL_CLOCK_INITIALIZE( CYGNUM_KERNEL_COUNTERS_RTC_PERIOD );
interrupt.attach();
interrupt.unmask_interrupt(CYGNUM_HAL_INTERRUPT_RTC);
Cyg_Clock::real_time_clock = this;
}
中断的注册很好理解,但这里有个有趣的是这个函数是怎样被调用到的,直接搜索ecos所有的代码是找不到的。一般我们有个概念c++的类在声明后就会被自动调用里面和自己名字一样的那个函数,(很久没有接触c++,忘记叫什么名字了)
这里也是这样,这里第一句就是声明这个实例,然后编译器会把这个函数放到一个特殊的段__CTOR_LIST__里面(target.ld),
然后cyg_hal_invoke_constructors()会遍历__CTOR_LIST__并执行所有的函数,cyg_hal_invoke_constructors() 是在vector.S里面被调用到的。这个“自动调用”就是这样实现的。
再看时间中断服务程序,ecos 把中断服务分为两块ISR和DSR,ISR里只做些最简单的事情,发生中断后会被直接调到,以保证kernel快速响应的效果。把其他的事情都放到DSR里面,DSR会被稍后调用,先看DSR里面代码
// -------------------------------------------------------------------------
void Cyg_RealTimeClock::dsr(cyg_vector vector, cyg_ucount32 count, CYG_ADDRWORD data)
{
// CYG_REPORT_FUNCTION();
Cyg_RealTimeClock *rtc = (Cyg_RealTimeClock *)data;
CYG_INSTRUMENT_CLOCK( TICK_START,
rtc->current_value_lo(),
rtc->current_value_hi());
>>这里是提供系统时钟
rtc->tick( count );
#ifdef CYGSEM_KERNEL_SCHED_TIMESLICE
#if 0 == CYGINT_KERNEL_SCHEDULER_UNIQUE_PRIORITIES
// If timeslicing is enabled, call the scheduler to
// handle it. But not if we have unique priorities.
>>分时多任务的处理,它的实现在算法里,我以mlqueue为例
Cyg_Scheduler::scheduler.timeslice();
#endif
#endif
CYG_INSTRUMENT_CLOCK( TICK_END,
rtc->current_value_lo(),
rtc->current_value_hi());
}
timeslice()调用timeslice_cpu(),timeslice_cpu里要找出同一priority任务队列中下一个任务,如果有,则设置reschedule的标志:需要做任务切换。
到这里这条路就断了。但是前面我没有讲到DSR是怎样被调到的,这里要看interrupt_end()
在vector.S里被调到,interrupt_end代码在kernel/intr/intr.cxx里
//-------------------------------------
externC void
interrupt_end(
cyg_uint32 isr_ret,
Cyg_Interrupt *intr,
HAL_SavedRegisters *regs
)
{
// CYG_REPORT_FUNCTION();
#ifdef CYGPKG_KERNEL_SMP_SUPPORT
Cyg_Scheduler::lock();
#endif
// Sometimes we have a NULL intr object pointer.
cyg_vector vector = (intr!=NULL)?intr->vector:0;
CYG_INSTRUMENT_INTR(END, vector, isr_ret);
CYG_UNUSED_PARAM( cyg_vector, vector ); // prevent compiler warning
#ifndef CYGIMP_KERNEL_INTERRUPTS_CHAIN
// Only do this if we are in a non-chained configuration.
// If we are chained, then chain_isr below will do the DSR
// posting.
>>这里把当前的DSR post出去,其实就是加入一个DSR 任务链表里去,之后再拿出来处理
if( isr_ret & Cyg_Interrupt::CALL_DSR && intr != NULL ) intr->post_dsr();
#endif
// Now unlock the scheduler, which may also call DSRs
// and cause a thread switch to happen.
>>这里就是多任务处理的入口了,下面再去看里面的实现
Cyg_Scheduler::unlock();
CYG_INSTRUMENT_INTR(RESTORE, vector, 0);
}
unlock()会调用unlock_inner,unlock_inner是kernel最重要的一个函数了,它是多任务切换的
执行者,来看它的实现,代码很长,只挑其中一段
//-------------------------------------
void Cyg_Scheduler::unlock_inner( cyg_ucount32 new_lock )
{
do {
#ifdef CYGIMP_KERNEL_INTERRUPTS_DSRS
// Call any pending DSRs. Do this here to ensure that any
// threads that get awakened are properly scheduled.
>>调用前面post的所有的DSR,注意里面会有reschedule flag的设置
>>下面就要用到
if( new_lock == 0 && Cyg_Interrupt::DSRs_pending() )
Cyg_Interrupt::call_pending_DSRs();
#endif
Cyg_Thread *current = get_current_thread();
// If the current thread is going to sleep, or someone
// wants a reschedule, choose another thread to run
>>这里有几种情况需要处理,一个是当前的任务主动要求休息(调用yield()),当然就要切换给别的任务;
>>另外一个就是在DSR的timeslice中找到优先级相同的下个任务需要运行;
>>还有一种是当有新的高优先级的任务加入。(所有都会调用set_need_reschedule()) if( current->state != Cyg_Thread::RUNNING || get_need_reschedule() ) {
CYG_INSTRUMENT_SCHED(RESCHEDULE,0,0);
// Get the next thread to run from scheduler
Cyg_Thread *next = scheduler.schedule();
if( current != next )
{
CYG_INSTRUMENT_THREAD(SWITCH,current,next);
// Count this thread switch
thread_switches[CYG_KERNEL_CPU_THIS()]++;
>>上下文切换,在contexts.S里
// Switch contexts
HAL_THREAD_SWITCH_CONTEXT( ¤t->stack_ptr,
&next->stack_ptr );
// Worry here about possible compiler
// optimizations across the above call that may try to
// propogate common subexpresions. We would end up
// with the expression from one thread in its
// successor. This is only a worry if we do not save
// and restore the complete register set. We need a
// way of marking functions that return into a
// different context. A temporary fix would be to
// disable CSE (-fdisable-cse) in the compiler.
// We return here only when the current thread is
// rescheduled. There is a bit of housekeeping to do
// here before we are allowed to go on our way.
>>一般就不会跑到这里了,cpu pc指针已经切换到别的任务上去了,只有等这个任务再次
>>被reschedule时,才会从这里开始执行
current_thread[CYG_KERNEL_CPU_THIS()] = current; // restore current thread pointer
}
#ifdef CYGSEM_KERNEL_SCHED_TIMESLICE
// Reset the timeslice counter so that this thread gets a full
// quantum.
reset_timeslice_count();
#endif
clear_need_reschedule(); // finished rescheduling
}
return;
} while( 1 );
}
至此,整个框架已经出来了,对于schedule,thread,semphone,mutex,flag,mailbox等等其他概念,在ecos 发布的文档上
有比较详细的介绍(ecos reference manual),我就不再赘述了。
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