Calculating the Stack usage in RTOS application

Question

I am currently working on a project to develop an application in STM32 microcontroller using RTOS (micrium).
Are there any tools to calculate the stack usage of a particular thread in RTOS application?

Answer 1

No tools I know of. However, two simple methods to estimate stack usage have always worked for me.

1. Fill all RAM with a value like 0x55 or 0xAA. Let the program run long enough while using all of the device's options to have the most code execution coverage. Stop (under some debugger), and examine RAM for the above values being overwritten. That should give you a good approximation. This works with or without an OS.

2. Modify the OS just a bit so that on task switches you record to some global variable (array) and for each task the lowest stack pointer found by comparing to the previous value for the same task. After running the app long enough as in [1], examine the counters. Although there is no guarantee the moment a task switch happens you will have the maximum stack used for that task, statistically, after long enough time and assuming preemptive switching, you will have managed to record an accurate enough value.

Answer 2

If you are using GCC or clang -fstack-usage compiler switch generates a stack frame size for each function. You need to combine that information with call-graph information generated by the linker to find the deepest stack usage starting from a specific function. Starting at main(), a task entry-point and and ISR will then give you the worst-case usage for that thread.

Helpfully the work to create such a tool has been done for you as discussed here, using a Perl script from here.

ARM's armcc compiler v5 and earlier (v6 is clang/llvm but uses ARMLINK so has the same functionality) has this functionality built-in and can include detailed stack analysis in the link map, including the worst-case call path and warnings of non-deterministic stack usage (due to recursion or call-backs through function pointers for example). You may be using armcc if you are using Keil ARM MDK for example. Again for multi-threaded systems (tasks/ISRs) you need to look at the stack usage for the thread entry point.

Note also that on ARM Cortex-M, the "system stack" is shared by the main() thread and all ISRs, and if you use the ISR preemption priorities multiple interrupts may be active simultaneously. So in theory worst case stack usage is the sum of the stack usage for each of main() and all ISRs that may occur concurrently. Whilst it is good practice to keep ISRs short and simple, beware of third-party code. ST's USB library for example runs the entire USB device stack in the ISR context for example!