[ECOS] Thread scheduling
Wed Mar 3 20:50:00 GMT 2004
Giovanni Perbellini wrote:
> Hi all,
> I don't understand the behavoir of the scheduler.
I hope this isn't your homework ;-).
> I have a simple application running two threads with euqal priority
> (priority = 12).
> When the execution starts the ISR function is executed and then the DSR
> function is called.The DSR function executes the tick method to calculate the tick SW and
> finally the timeslice function is executed to assign a time slot to a
> thread (let's suppose to have a single cpu with the threads timeslice
> disabled...unique timeslice associates to 1 cpu).
> The timeslice calls timeslice_cpu function that checks if the current
> thread is running, then tries to rotate the run queue and finally the
> need_reschule flag is set.
> if( thread->get_state() == Cyg_Thread::RUNNING )
> Cyg_Scheduler *sched = &Cyg_Scheduler::scheduler;
> CYG_INSTRUMENT_MLQ( TIMESLICE, thread, 0);
> CYG_ASSERTCLASS( thread, "Bad current thread");
> CYG_ASSERTCLASS( sched, "Bad scheduler");
> cyg_priority pri = thread->priority;
> Cyg_RunQueue *queue = &sched->run_queue[pri];
> #ifdef CYGPKG_KERNEL_SMP_SUPPORT
> // In SMP systems we set the head of the queue to point to
> // the thread immediately after the current
> // thread. schedule() will then pick that thread, or one
> // after it to run next.
> queue->to_head( thread->get_next() );
> if( queue->get_head() != thread )
> timeslice_count[cpu_this] = CYGNUM_KERNEL_SCHED_TIMESLICE_TICKS;
> What happens when this function is finished?
Execution returns from the DSR.
> Is the schedule method called?
> I guess that the scheduler executes the same prevoius thread even if it
> should the context switch. Why?
No. After the DSR returns, the HAL's interrupt processing in vectors.S
will then call interrupt_end in src/intr/intr.cxx. This calls
Cyg_Scheduler::unlock, which, if the scheduler count is 0 will call
unlock_inner in src/sched/sched.cxx. That function actually does the
context switch - have a look.
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