Variable sized dynamic thread stacks from pool

[PhilippFinke1988]

Hey everybody,

I'm currently working on a project where I need dynamically created threads having dynamically allocated stacks (via k_thread_stack_alloc()). It seems that I'm not able to use heap-allocated stacks (via CONFIG_DYNAMIC_THREAD_ALLOC) as my CPU, an ARM Cortex-A9 with MMU, doesn't support dynamic kernel objects so I'm bound to use the the stack pool via CONFIG_DYNAMIC_THREAD_POOL_SIZE.

The thread stack sizes in my system range from 2kB to 16kB. This means, with the current implementation of the thread stack pool, that each item in the pool needs to be at least 16kB in size which in turn wastes a lot of memory.

In order to get around this problem, I could imagine one of the following solutions:

1. Allocate specific number of consecutive items from single stack pool: As far as I understand, the stack pool created with K_THREAD_STACK_ARRAY_DEFINE() is a multidimensional array whose addresses are aligned to CPU word boundaries. This means it should be possible to allocate more than one item of the pool for a thread as long as the items are contiguous. Then, I could configure the pool item size to the smallest stack size needed (2kB in my case). The smallest stack of 2kB would then occupy one pool item and the largest with 16kB eight pool items.
2. Introduce additional thread stack pools that allow for different stack sizes: This would imply having additional Kconfig symbols in order to configure the size of each pool individually. When allocating a stack via k_thread_stack_alloc() it could then be decided which pool to be used based on the desired stack size.

What are your thoughts about it? Please let me know if these are feasible solutions or if you have other ideas.

Best regards,

Philipp