31. kernel_levels.h内核打印级别

定义了内核打印级别

31.1. include/linux/kern_levels.h

内核源码树的这个文件定义了内核打印级别 或者参考【github】

#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 */

#define KERN_DEFAULT    ""      /* the default kernel loglevel */

在内核模块代码使用printk打印信息：

printk(KERN_INFO "EBB: Hello %s from the BBB LKM!\n", name);

31.1.1. 查看当前系统的打印级别

cat /proc/sys/kernel/printk

使用sysctl可以达到同样效果

sysctl kernel/printk

4       4       1       7

The first value in our output is the current console_loglevel. This is the information we were looking for: the value, 4 in this case, represents the log level currently used. As said before this means that only messages adopting a severity level higher than it, will be displayed on the console.

第一个参数是4， 是当前终端的打印级别

The second value in the output represents the default_message_loglevel. This value is automatically used for messages without a specific log level: if a message is not associated with a log level, this one will be used for it.

第二个参数是4，是/var/log/messages 的默认保存级别

The third value in the output reports the minimum_console_loglevel status. It indicates the minimum loglevel which can be used for console_loglevel. The level here used it’s 1, the highest.

第三个参数是1，是最小化终端的打印级别

Finally, the last value represents the default_console_loglevel, which is the default loglevel used for console_loglevel at boot time.

第四个参数是7，默认终端的打印级别。也就是系统启动时能看到的信息。

31.1.2. 修改内核的打印级别

echo "7"  > /proc/sys/kernel/printk

使用sysctl可以达到同样效果

sudo sysctl -w kernel.printk=7