17.4. printk
printk is the most simple and widely used way of getting information from the kernel, so you should familiarize yourself with its basic configuration.
We use printk a lot in our kernel modules, and it shows on the terminal by default, along with stdout and what you type.
Hide all printk messages:
dmesg -n 1
or equivalently:
echo 1 > /proc/sys/kernel/printk
See also: https://superuser.com/questions/351387/how-to-stop-kernel-messages-from-flooding-my-console
Do it with a Kernel command line parameters to affect the boot itself:
./run --kernel-cli 'loglevel=5'
and now only boot warning messages or worse show, which is useful to identify problems.
Our default printk format is:
<LEVEL>[TIMESTAMP] MESSAGE
e.g.:
<6>[ 2.979121] Freeing unused kernel memory: 2024K
where:
-
LEVEL: higher means less serious -
TIMESTAMP: seconds since boot
This format is selected by the following boot options:
-
console_msg_format=syslog: add the<LEVEL>part. Added in v4.16. -
printk.time=y: add the[TIMESTAMP]part
The debug highest level is a bit more magic, see: Section 17.4.3, “pr_debug” for more info.