zephyr-threading

Zephyr Threading

Overview

This skill provides expert knowledge on Zephyr kernel threading: creating and managing threads, understanding the scheduling model, selecting appropriate priorities, and using workqueues for deferred processing.

When thread blocking behavior and idle residency matter for battery life or low-power operation, also read ../zephyr-power-management/SKILL.md.

Workflow

1. Determine Thread Type Needed

First, identify what kind of execution context is required:

Need	Solution
Lengthy/complex processing not suitable for ISR	Create a thread
Deferred work from ISR or high-priority thread	Use workqueue
Background processing with timeout control	Use delayable work item
Single-threaded application	Use main thread directly

2. Select Priority Class

Zephyr has two priority classes. Choose based on preemption requirements:

Cooperative threads (negative priority, e.g., -1 to -CONFIG_NUM_COOP_PRIORITIES):

• Never preempted by scheduler until they voluntarily yield
• Best for: device drivers, performance-critical code, short atomic operations
• Use K_PRIO_COOP(x) macro where x=0 is highest cooperative priority

Preemptive threads (non-negative priority, 0 to CONFIG_NUM_PREEMPT_PRIORITIES-1):

• Can be preempted by higher-priority threads at any reschedule point
• Best for: application logic, time-sensitive processing
• Use K_PRIO_PREEMPT(x) macro where x=0 is highest preemptive priority

Meta-IRQ threads (optional, highest priority cooperative):

• Enable with CONFIG_NUM_METAIRQ_PRIORITIES
• Can preempt even cooperative threads — use only for interrupt bottom-half processing

Rule of thumb: Lower numeric value = higher priority. Priority -2 > -1 > 0 > 1.

3. Implementation

Once requirements are clear, implement using the appropriate reference:

Step 3a: For thread creation and lifecycle management:

• Read references/thread-lifecycle.md — Static vs dynamic creation, stacks, start/suspend/resume/abort/join