=========================

Write a Linux kernel module, and stand-alone Makefile, that when loaded
prints to the kernel debug log level, "Hello World!"  Be sure to make
the module able to be unloaded as well

=========================

obj-m :=task01.o
KVER := `uname -r`
KDIR = /lib/modules/$(KVER)/build

all:
	make -C $(KDIR) M=`pwd` modules
clean:
	rm -f *.ko *.o *.mod.o *.mod.c *.symvers

#include <linux/module.h>

static int task01_init (void)
{
	printk(KERN_INFO "Hello World!\n");
	return 0;
}

static void task01_exit (void)
{
	printk(KERN_INFO "task01 exit...\n");
}

module_init(task01_init);
module_exit(task01_exit);

MODULE_AUTHOR("");
MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("task01");

这样是不符合要求的, 后来查得, 在内核源码目录include/linux/kernel.h(2.6.32.61), include/linux/kern_levels.h(3.17.0)文件中

#define KERN_EMERG	KERN_SOH "0"	/* system is unusable */
#define KERN_ALERT	KERN_SOH "1"	/* action must be taken immediately */
#define KERN_CRIT	KERN_SOH "2"	/* critical conditions */
#define KERN_ERR	KERN_SOH "3"	/* error conditions */
#define KERN_WARNING	KERN_SOH "4"	/* warning conditions */
#define KERN_NOTICE	KERN_SOH "5"	/* normal but significant condition */
#define KERN_INFO	KERN_SOH "6"	/* informational */
#define KERN_DEBUG	KERN_SOH "7"	/* debug-level messages */

共8个级别, 控制台也有一个日志级别console_loglevel, 当printk的输出级别小于console_loglevel时才会显示出来, 详细见http://blog.sina.com.cn/s/blog_4a4832fe0100cvrk.html

把源文件中KERN_INFO改成KERN_DEBUG即可