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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.
This commit is contained in:
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>> 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 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 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 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://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 * 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 ==== pr_debug
https://stackoverflow.com/questions/28936199/why-is-pr-debug-of-the-linux-kernel-not-giving-any-output/49835405#49835405 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 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 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: 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 echo 8 > /proc/sys/kernel/printk
@@ -359,12 +403,20 @@ echo 'module myprintk +p' > /sys/kernel/debug/dynamic_debug/control
insmod /myprintk.ko insmod /myprintk.ko
.... ....
which should now contain the `pr_debug` message: which now outputs the `pr_debug` message:
.... ....
printk debug 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 === Module documentation
.... ....

3
run
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@@ -10,12 +10,11 @@ debug=false
kgdb=false kgdb=false
kvm=false kvm=false
# norandmaps: Don't use address space randomization. Equivalent to echo 0 > /proc/sys/kernel/randomize_va_space. # norandmaps: Don't use address space randomization. Equivalent to echo 0 > /proc/sys/kernel/randomize_va_space.
# printk.time=y: log in format: "[time ] msg" for all printk messages.
# nokaslr: # nokaslr:
# - https://unix.stackexchange.com/questions/397939/turning-off-kaslr-to-debug-linux-kernel-using-qemu-and-gdb # - https://unix.stackexchange.com/questions/397939/turning-off-kaslr-to-debug-linux-kernel-using-qemu-and-gdb
# - https://stackoverflow.com/questions/44612822/unable-to-debug-kernel-with-qemu-gdb/49840927#49840927 # - https://stackoverflow.com/questions/44612822/unable-to-debug-kernel-with-qemu-gdb/49840927#49840927
# Turned on by default since v4.12 # Turned on by default since v4.12
extra_append='nokaslr norandmaps printk.devkmsg=on printk.time=y' extra_append='console_msg_format=syslog nokaslr norandmaps printk.devkmsg=on printk.time=y'
extra_append_after_dash= extra_append_after_dash=
extra_flags= extra_flags=
extra_flags_qemu= extra_flags_qemu=