Kernel Timer Demo

Description

Demonstrates kernel timers for periodic execution. Timers run in softirq context and are useful for polling, timeouts, and scheduled tasks.

Features

• Periodic timer execution (fires 10 times)
• Configurable interval via module parameter
• Jiffies and time conversion
• Safe timer cancellation

Build & Test

make

# Default: 1 second interval
sudo insmod timer_demo.ko

# Custom interval: 500ms
sudo insmod timer_demo.ko timer_interval_ms=500

# Watch timer fire
dmesg -w

# Unload (cancels timer)
sudo rmmod timer_demo

Expected Output

=== Kernel Timer Demo ===
Timer interval: 1000 ms
HZ (timer frequency): 250
Jiffies at init: 4294892314
Timer started - will fire 10 times

Timer fired! Count: 1, jiffies: 4294892564
Timer rescheduled for 1000 ms
Timer fired! Count: 2, jiffies: 4294892814
Timer rescheduled for 1000 ms
...
Timer fired! Count: 10, jiffies: 4294895064
Timer finished after 10 callbacks

(On rmmod):
Timer was already inactive
Timer demo: Fired 10 times total

Module Parameters

• timer_interval_ms: Interval in milliseconds (default: 1000)

# Fast timer (100ms)
sudo insmod timer_demo.ko timer_interval_ms=100

# Slow timer (5 seconds)
sudo insmod timer_demo.ko timer_interval_ms=5000

Time Concepts

Jiffies

• Kernel’s tick counter (increments HZ times per second)
• System-dependent: HZ = 100, 250, or 1000

Time Conversion

msecs_to_jiffies(1000)  // 1 second in jiffies
jiffies_to_msecs(HZ)    // HZ jiffies to milliseconds

Use Cases

• Polling: Check device status periodically
• Timeouts: Implement operation timeouts
• Throttling: Rate-limit operations
• Heartbeat: Keep-alive mechanisms
• Delayed work: Alternative to work queues for simple cases

Learning Points

• Timer callback runs in softirq (atomic) context
• Cannot sleep in timer callback
• Use mod_timer() for rescheduling
• Use del_timer_sync() for safe cancellation
• Timers are not perfectly accurate (depends on HZ)

Advanced: High-Resolution Timers

For sub-jiffy precision, use hrtimers (not covered in this example):

#include <linux/hrtimer.h>
// See kernel documentation for hrtimer API