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readme: explain how to use dynamic debugging for boot messages

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run: pass the console_msg_format=syslog boot parameter by default,
to allow seeing what is the priority of messages on the terminal.

Document printk better.
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Ciro Santilli
2018-04-18 14:58:47 +01:00
parent 5698a5ed2b
commit b5d64142e9
2 changed files with 62 additions and 11 deletions
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70
README.adoc
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@@ -300,17 +300,55 @@ every time.
To automate that, use the methods described at: <<init>>
=== Message control
=== printk
We use `printk` a lot, and it shows on the QEMU terminal by default. If that annoys you (e.g. you want to see stdout separately), do:
We use `printk` a lot, 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
When in <<graphic-mode>>, you can scroll up a bit on the default TTY with:
Do it with a <<kernel-command-line-parameters>> to affect the boot itself:
....
./run -e '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
* `printk.time=y`: add the `[TIMESTAMP]` part
Scroll up in <<graphic-mode>>:
....
Shift-PgUp
@@ -321,15 +359,15 @@ but I never managed to increase that buffer:
* https://askubuntu.com/questions/709697/how-to-increase-scrollback-lines-in-ubuntu14-04-2-server-edition
* https://unix.stackexchange.com/questions/346018/how-to-increase-the-scrollback-buffer-size-for-tty
The superior alternative is to use text mode or a telnet connection.
The superior alternative is to use text mode and GNU screen or tmux.
==== pr_debug
https://stackoverflow.com/questions/28936199/why-is-pr-debug-of-the-linux-kernel-not-giving-any-output/49835405#49835405
Not printable by default without recompile.
Debug messages are not printable by default without recompiling.
But the awesome `CONFIG_DYNAMIC_DEBUG=y` option allows us to do:
But the awesome `CONFIG_DYNAMIC_DEBUG=y` option which we enable by default allows us to do:
....
echo 8 > /proc/sys/kernel/printk
@@ -345,13 +383,19 @@ and we have a shortcut at:
Syntax: https://www.kernel.org/doc/html/v4.11/admin-guide/dynamic-debug-howto.html
Wildcards are also accepted, e.g. enable all messages from all files:
....
echo 'file * +p' > /sys/kernel/debug/dynamic_debug/control
....
TODO: why is this not working:
....
func sys_init_module +p' > /sys/kernel/debug/dynamic_debug/control
echo 'func sys_init_module +p' > /sys/kernel/debug/dynamic_debug/control
....
For modules, we can use:
Enable messages in specific modules:
....
echo 8 > /proc/sys/kernel/printk
@@ -359,12 +403,20 @@ echo 'module myprintk +p' > /sys/kernel/debug/dynamic_debug/control
insmod /myprintk.ko
....
which should now contain the `pr_debug` message:
which now outputs the `pr_debug` message:
....
printk debug
....
Enable `pr_debug` for boot messages as well, before we can reach userland and write to `/proc`:
....
./run -e 'dyndbg="file * +p" loglevel=8'
....
Get ready for the noisiest boot ever, I think it overflows the `printk` buffer and funny things happen.
=== Module documentation
....