Porting-to-FreeRTOS-SMP-Kernel.md

The FreeRTOS API remains substantially the same between single core and SMP versions except for these additions. Therefore, an application written for the FreeRTOS single core version should compile with the SMP version with minimal to no effort. However, there may be some functional issues, as some assumptions which were true for single core applications may no longer be true for those on multi-cores.

One such common assumption is that a lower priority task cannot run while a higher priority task is running. While this was true on a single core, it is no longer true for multi-cores, as multiple tasks can be running simultaneously. If the application relies on relative task priorities to provide mutual exclusion, it may observe unexpected results in a multi-core environment. The application writer has couple of options to address this:

1. The best option is to update the application so that it does not rely on task priorities and uses synchronization primitives instead.
2. Another option is to pin all the tasks which must not be running simultaneously to one core using the vTaskCoreAffinitySet API.
3. Another option is to define configRUN_MULTIPLE_PRIORITIES to 0 which ensures that multiple tasks will run simultaneously only if they have the same priority. Note that this may result in under utilization and put some cores to idle when they could be used to run other low priority tasks.

One other common assumption is that ISRs cannot run simultaneously with each other or with other tasks. This is no longer true in a multi-core environment and the application writer needs to ensure proper mutual exclusion while accessing shared data between tasks and ISRs. The macros taskENTER_CRITICAL_FROM_ISR() and taskEXIT_CRITICAL_FROM_ISR() can be used in ISRs and the macros taskENTER_CRITICAL() and taskEXIT_CRITICAL() can be used in tasks to provide such mutual exclusion.