[Linux] [kernel module] Create kthread in kernel module

Notes on how to create a kthread in a kernel module

Please refer to the following for how to create and build a loadable kernel module. [Linux] [kernel module] Build and load a simple loadable kernel module-Qiita

Source

testmod.c

#include <linux/module.h>
#include <linux/sched.h>
#include <linux/kthread.h>

MODULE_LICENSE("MIT");

struct task_struct *k;

static void kthread_main(void)
{
	schedule();
}

static int kthread_func(void* arg)
{
	printk(KERN_INFO "[%s] start kthread\n", k->comm);

	while (!kthread_should_stop()) {
		kthread_main();
	}

	printk(KERN_INFO "[%s] stop kthread\n", k->comm);

	return 0;
}

static int __init testmod_init(void)
{
	printk(KERN_INFO "driver loaded\n");

	k = kthread_run(kthread_func, NULL, "testmod kthread");

	return 0;
}

static void __exit testmod_exit(void)
{
	kthread_stop(k);
	printk(KERN_INFO "driver unloaded\n");
}

module_init(testmod_init);
module_exit(testmod_exit);

Basic flow

Main

• Create a thread with the kthread_run (3, ...) macro with the init function
• Exit kthread with exit function

The kthread_run (3, ...) macro looks like this:

kthread_run

#define kthread_run(threadfn, data, namefmt, ...)                          \
({                                                                         \
       struct task_struct *__k                                            \
               = kthread_create(threadfn, data, namefmt, ## __VA_ARGS__); \
       if (!IS_ERR(__k))                                                  \
               wake_up_process(__k);                                      \
       __k;                                                               \
})

After executing kthread_create (3, ...), wake_up_process (1) is executed. For namefmt, you can use format specifiers like printf and set thread names using variables.

kthread side

• New thread calls kthread_func
• Create a loop that is called repeatedly until the end (the end of the thread is detected by kthread_should_stop ())
• Sleep for a certain period with schedule ()

TODO: Investigate the behavior of schedule ()

Execution example

Execution example

$ sudo insmod testmod.ko
$ ps aux | head -1 && ps auwx | grep "\[testmod kthread\]"
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root     28716 84.8  0.0      0     0 ?        R    23:14  16:07 [testmod kthread]
$ sudo rmmod testmod.ko
$ dmesg | tail
[187744.330108] driver loaded
[187744.330326] [testmod kthread] start kthread
[187812.178718] [testmod kthread] stop kthread
[187812.178743] driver unloaded

It was confirmed that the thread was started and terminated in the kernel module. I was also able to check the thread created with the ps command,

Relation

[Linux] [kernel module] Build and load a simple loadable kernel module-Qiita [Linux] [kernel module] How to pass parameters as arguments when loading a loadable kernel module --Qiita [Linux] [kernel module] Specify / limit the execution CPU of kthread --Qiita

reference

Implementing kernel thread looping and stopping in a kernel module-the future of humans and the web kthread_run macro --Linux memorandum ... (to table of contents) [PDF] kthreads