-1

in a non preemptive system, after an ISR finishes execution, will the interrupted task continue execution even if a higher priority task was activated?

Muhammad Ali
  • 11
  • 2
  • Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. – Community Apr 07 '22 at 01:47
  • You have made a statement rather than asking a question (adding a ? at the end does not really make it a question; just grammatically confused ). What makes you think that might be true? – Clifford Apr 07 '22 at 06:49
  • @Clifford I am sorry for that I copied the statement of the question which its answer was either the statement is true or false. I edited the question now – Muhammad Ali Apr 07 '22 at 14:20
  • The answer may be RTOS specific. Is there a specific RTOS in your mind? FreeRTOS maybe? – Tagli Apr 07 '22 at 15:04
  • @Tagli but FreeRTOS (and RTOS in general) is pre-emptive, so presumably not that. – Clifford Apr 07 '22 at 18:58
  • Presumably this refers to cooperative multitasking, where a task must explicitly yield for a scheduling decision to be made. Draw your on conclusion. – Clifford Apr 07 '22 at 18:59
  • @Clifford , FreeRTOS is pre-emptive by default, but it can also be configured to be **non** pre-emptive. – Tagli Apr 07 '22 at 20:30

1 Answers1

0

This answer is specific to FreeRTOS, and may not be relevant to other RTOS'es.

FreeRTOS is preemptive by default. However, it can also be configured to be non-preemptive by the config option in FreeRTOSConfig.h

#define configUSE_PREEMPTION 0

Normally, a return from ISR does not trigger a context switch. But in preemptive systems it's often desirable, so in most FreeRTOS examples, you see portYIELD_FROM_ISR(xHigherPriorityTaskWoken); at the end of the ISR, which triggers a context switch if xHigherPriorityTaskWoken is pdTRUE.

xHigherPriorityTaskWoken is initialized to pdFALSE at the start of the ISR (manually by the user), and operations which can cause a context switch, such as vTaskNotifyGiveFromISR() , xQueueSendToBackFromISR() etc. , take it as an argument and set it to pdTRUE if a context switch is required after the system call.

In a non-preemptive configuration, you simply pass NULL to such system calls instead of xHigherPriorityTaskWoken, and do not call portYIELD_FROM_ISR() at the end of the ISR. In this case, even if a higher priority task is awakened by the ISR, execution returns to the currently running task and remains there until this task yields or makes a blocking system call.

You may mix ISR yield mechanism with preemption method. For example, you can force a context switch (preemption) from ISR even when configUSE_PREEMPTION is 0, but this may cause problems if the interrupted/preempted task doesn't expect it to happen, so I don't recommend it.

Tagli
  • 2,412
  • 2
  • 11
  • 14