Notes about timer in Linux kernel.内容主要转载自Timer Example。

1. Prerequisite

jiffies记录了系统启动以来，经过了多少tick。
一个tick代表多长时间，在内核的CONFIG_HZ中定义。比如CONFIG_HZ=200，则一个jiffies对应5ms时间。所以内核基于jiffies的定时器精度也是5ms。

2. Timer API

A timer is represented in the kernel as an instance of timer_list:

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#include <linux/timer.h>

struct timer_list {
	/*
	 * All fields that change during normal runtime grouped to the
	 * same cacheline
	 */
	struct hlist_node	entry;
	unsigned long		expires;
	void			(*function)(struct timer_list *);
	u32			flags;

#ifdef CONFIG_LOCKDEP
	struct lockdep_map	lockdep_map;
#endif
};

expires is an absolute value in jiffies. entry is a doubley linked list, and a callback function.

3. Timer setup initialization

The following are steps to initialize timers:

  • Setting up the timer: Set up the timer, feeding the user-defined callback function.

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    /**
     * timer_setup - prepare a timer for first use
     * @timer: the timer in question
     * @callback: the function to call when timer expires
     * @flags: any TIMER_* flags
     *
     * Regular timer initialization should use either DEFINE_TIMER() above,
     * or timer_setup(). For timers on the stack, timer_setup_on_stack() must
     * be used and must be balanced with a call to destroy_timer_on_stack().
     */
    #define timer_setup(timer, callback, flags)			\
    	__init_timer((timer), (callback), (flags))

  • Setting the expiration time: When the timer is initialized, we need to set its expiration before the callback gets fired:

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    int mod_timer(struct timer_list *timer, unsigned long expires);

  • Releasing the timer: When you are done with the timer, it needs to be released:

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    int del_timer(struct timer_list * timer);
    int del_timer_sync(struct timer_list *timer);

del_timer() return 0 on an inactive timer, and return 1 on an active timer, del_timer_sync waits for the handler to finish its execution, even those that may happen on another CPU. You should not hold a lock preventing the handler’s completion, otherwise it will result in a dead lock. You should release the timer in the module cleanup routine. You can independently check whether the timer is running or not:

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int timer_pending(const struct timer_list * timer);

4. Example

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#include <linux/init.h>
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/timer.h>

static struct timer_list my_timer;

static void my_timer_callback(struct timer_list *timer)
{
	printk("%s called (%ld)\n", __func__, jiffies);
}

static int __init my_init(void)
{
	int ret;
	
	pr_info("%s: Timer module loaded\n", __func__);
	
	timer_setup(&my_timer, my_timer_callback, 0);
	pr_info("%s: Setup timer to fire in 2s (%ld)\n", __func__, jiffies);

	ret = mod_timer(&my_timer, jiffies + msecs_to_jiffies(2000));
	if (ret)
		pr_err("%s: Timer firing failed\n", __func__);
	
	return 0;
}

static void __exit my_exit(void)
{
	int ret;
	
	ret = del_timer(&my_timer);
	if (ret)
		pr_err("%s: The timer is still is use ...\n", __func__);
	
	pr_info("%s: Timer module unloaded\n", __func__);
}

module_init(my_init);
module_exit(my_exit);
MODULE_AUTHOR("Yannik Li <yannik520@gmail.com>");
MODULE_DESCRIPTION("Timer example");
MODULE_LICENSE("GPL");
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$ dmesg
[1208385.932488] my_init: Timer module loaded
[1208385.932581] my_init: Setup timer to fire in 2s (5503072803)
[1208387.945145] my_timer_callback called (5503074816)
[1208425.037823] my_exit: Timer module unloaded

参考资料:

  1. Timer Example
  2. jiffies背景介绍