Pros/cons/advantages/disavantages of log levels?

Lounge Linux lounge
1

I am just an ordinary, (if there is such a thing,) user. I have never considered log levels in my many years of using Linux…so what s up with log levels?

2

The Kernel Docs have a good roundup:

loglevel=       [KNL,EARLY]
                All Kernel Messages with a loglevel smaller than the
                console loglevel will be printed to the console. It can
                also be changed with klogd or other programs. The
                loglevels are defined as follows:

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

You can query the default console log levels using

$ cat /proc/sys/kernel/printk

or

$ sysctl kernel.printk

Output will look like:

kernel.printk = 4	4	1	4

where

  • 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.

  • 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.

  • 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.

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

3

[g@g-xps8940 ~]$ cat /proc/sys/kernel/printk
4 4 1 4

Your example is my situation. Interesting, but in my case, I am guessing log level is probably nothing I need to be concerned with. I can think of no reason why log level should matter to me. At-any-rate, thanks for your explanation.:saluting_face:

4

So does setting kernel parameter loglevel=5 or loglevel=6 in GRUB or SystemD-boot actually changes any of these values? If yes then which of the values changes?

5

The first, so you see what’s current and can still check the defaults:

Example, setting kernel loglevel=5:

$ cat /proc/sys/kernel/printk
5	4	1	4
6

Thanks and any other way to change the other values, i.e. default_message_loglevel, minimum_console_loglevel and default_console_loglevel?

7

I never found the need to change the others, don’t know, tbh. Maybe these are even kernel constants, who knows…

EDIT: man 2 syslog has a bit more information.