Describe the bug

The test_coop_resched_threads test in kernel.multiprocessing.smp fails on 12-core systems for two reasons:

Priority Range Violation
The test uses K_PRIO_COOP(10) as the starting priority, which evaluates to -6. With the default CONFIG_NUM_COOP_PRIORITIES=16, the allowed range: 14 to -16.

The spawn_threads function decrements the priority for each thread created, resulting in priorities -7, -8, -9, ..., -17 for 12 threads. The thread receives priority -17, which exceeds the valid range and triggers an invalid priority assertion.

Insufficient Timing
The test uses a fixed DELAY_US busy wait period to allow threads to execute before checking results. On 12-core systems, this delay is insufficient for all threads to complete execution, causing false failures such as "Comp cooperative thread 4 didn't run."

Regression

• This is a regression.

Steps to reproduce

west build -p -b nsim/nsim_hs5x/smp/12cores tests/kernel/smp -T kernel.multiprocessing.smp -t run

Relevant log output

===================================================================
START - test_coop_resched_threads
ASSERTION FAIL [((((prio)) == 15 && z_is_idle_thread_entry((entry))) || (((15 - 1) >= ((-16))) && ((prio)) >= ((-16)) && ((prio)) <= (15 - 1)))] @ WEST_TOPDIR/zephyr/kernel/thread.c:581
	invalid priority (-17); allowed range: 14 to -16
E:  r0: 0x4  r1: 0x245  r2: 0x1  r3: 0x8000a5a4
E:  r4: 0x0  r5: 0x0  r6: 0x0  r7: 0x1
E:  r8: 0xffffffff  r9: 0x0 r10: 0x1f r11: 0x0
E: r12: 0xa r13: 0x80  pc: 0x8000144e
E:  blink: 0x80006aca status32: 0x80082802
E: >>> ZEPHYR FATAL ERROR 4: Kernel panic on CPU 0
E: Current thread: 0x8000bd18 (test_coop_resched_threads)
wrong error reason
===================================================================

Impact

Annoyance – Minor irritation; no significant impact on usability or functionality.

Environment

OS: Linux
Toolchain (e.g Zephyr SDK, ...): arcmwdt
Commit SHA or Version used: Main 1c15986

Additional Context

No response