when call vPortYield(), what they do is below;

portSAVE_CONTEXT();
vTaskSwitchContext();
portRESTORE_CONTEXT();

but if scheduler is locked or there is no higher or the other same priority task in readylist , we have to return to the current task again. In these cases, portSAVE_CONTEXT() and portRESTORE_CONTEXT() waste much system resources.

so i think we should let vTaskSwitchContext() return a value to solve this problem like this:

signed portBASE_TYPE vTaskSwitchContext()
{
................
................
if we have switched to another task
return pdTRUE;
else
return pdFALSE
}

thus vPortYield() maybe change to
void vPortYield( void )
{
signed portBASE_TYPE xReturn;
.....
portENTER_CRITICAL();
xReturn = vTaskSwitchContext();
portEXIT_CRITICAL();
if (xReturn == pdFALSE) return;
portSAVE_CONTEXT();
portRESTORE_CONTEXT();
.....
}

And this change doesn't afffect vPortYieldFromTick or other ISRs.

sorry vPortYield() should be

void vPortYield( void )
{
__signed portBASE_TYPE xReturn;
.....
__portENTER_CRITICAL();

__xReturn = vTaskSwitchContext();

__if (xReturn == pdFALSE)
__{
____portEXIT_CRITICAL();
____return;
__}

__portSAVE_CONTEXT();
__portRESTORE_CONTEXT();

__portEXIT_CRITICAL();
.....
}

this definately will not work. SwitchContext must come between save_context and restore_context otherwise you save the context of one task into the tcb of another.

as

1\ task a is running.

2\ tick happens

3\ SwitchContext decides another task (b) should run and switches the current tcb.

4\ as your code SwitchContext returns true and save_context is called.

5\ save_context saves task a context into task b stack.

6\ restore_context then immediately removes task a context from task b stack.

now task a is running but the current tcb is pointing to task b.

yes, you are right.

But i think it's better we need a method to avoid portSAVE_CONTEXT() and portRESTORE_CONTEXT() when scheduler is locked or there is no higher or the other same priority task in readylist.

of course , my method will not work , i know now. But it will has another way to do this.

which port do you use. there is very little overhead in hc12 port, but more in say pic port.

sorry , now i have not run it acctually.

I am reading the source and try to understand all ot it and post my problems here