STM32F103 porting FreeRTOS series ten: suspension and recovery of task scheduler

 If there is a lot of code in the critical section, a long execution time will cause a delay interrupt. If the interrupt is not responded to in this way, it does not conform to the real-time operating system.

use case

	void vTaskDelay( const TickType_t xTicksToDelay )
	{
	BaseType_t xAlreadyYielded = pdFALSE;

		/* A delay time of zero just forces a reschedule. */
		if( xTicksToDelay > ( TickType_t ) 0U )
		{
			configASSERT( uxSchedulerSuspended == 0 );
			vTaskSuspendAll();
			{
				traceTASK_DELAY();

				/* A task that is removed from the event list while the
				scheduler is suspended will not get placed in the ready
				list or removed from the blocked list until the scheduler
				is resumed.

				This task cannot be in an event list as it is the currently
				executing task. */
				prvAddCurrentTaskToDelayedList( xTicksToDelay, pdFALSE );
			}
			xAlreadyYielded = xTaskResumeAll();
		}
		else
		{
			mtCOVERAGE_TEST_MARKER();
		}

		/* Force a reschedule if xTaskResumeAll has not already done so, we may
		have put ourselves to sleep. */
		if( xAlreadyYielded == pdFALSE )
		{
			portYIELD_WITHIN_API();
		}
		else
		{
			mtCOVERAGE_TEST_MARKER();
		}
	}

Let's explain the specific underlying implementation of these two API functions

 We know that the system performs task switching through the pendsv interrupt, so how is this variable connected to this interrupt?

in delay.c

In the M3 Definitive Guide

portNVIC_INT_CTRL_REGis defined as a pointer to a register address. By assigning portNVIC_PENDSVSET_BITthe value of (an integer) to this pointer, the value of the register is actually set to portNVIC_PENDSVSET_BITthe bit pattern represented by .

back to delay.c

 go to go in

Then   

 At this time, it is equivalent to the task scheduler being suspended, which is also a nested function

There is also a function to suspend the task scheduler. There are too many screenshots to take. You should be able to watch the video after reading the above.

 Equal to 0 here means that the variable uxSchedulerSupended is equal to 0