Boot debugging

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Note: Content moved from GRUB#Advanced_Debugging.

The kernel provides for a convenient way to configure all sorts of advanced settings to enable you to quickly and conveniently boot into your existing system with varying levels of debugging output extended kernel parameters. It is very easy and useful to create several levels of debugging just by adding additional entries to your bootloader configuration. And if you ever have issues or problems down the road due to a power-failure or hardware failure, it can save you hours of trouble, and of course nothing can beat debugging output when it comes to learning about your system.

Useful Entries

If you are interested in debugging, then you deserve some grub entries for powerusers, here are a few examples that you can add to your bootloader configuration (grub-legacy config /boot/grub/menu.lst used as an example).

title Shutdown the Computer

title Reboot the Computer

title Command Line

title Install GRUB to hd0 MBR
root (hd0,0)
setup (hd0)

title Matrix
color green/black light-green/green

title Scan for /boot/grub/menu.lst
find --set-root --ignore-floppies /boot/grub/menu.lst
configfile /boot/grub/menu.lst

title Scan for /boot/menu.lst
find --set-root --ignore-floppies /menu.lst
configfile /boot/menu.lst

title    Run x86test (CPU Info)
kernel /boot/x86test_zImage.bin

title    Run memtest86+ (Memory Testing)
kernel /boot/memtest86+-4.20.bin

Light Debug

A quick way to see more verbose messages on your console is to boot up your bootloader entry after appending verbose to the kernel line. This simple word added to your kernel line turns on more logging thanks to the /etc/rc.sysinit file, which at the top of the file runs:

if /bin/grep -q " verbose" /proc/cmdline; then /bin/dmesg -n 8; fi

Very simple way to get a bit more messages and debug output in your logs.

title  Arch Linux DEBUG Light
kernel /vmlinuz-linux root=/dev/disk/by-label/ROOT ro rootwait verbose
initrd /initramfs-linux.img

Medium Debug

This example menu.lst entry turns on real logging that is set by the kernel and not in an init script. Adding the debug kernel parameter to your kernel line is recognized by a lot of linux internals and enables quite a bit of debugging compared to the default.

title Arch Linux DEBUG Medium
kernel /vmlinuz-linux root=/dev/disk/by-label/ROOT ro rootdelay=5 panic=10 debug
initrd /initramfs-linux.img

Heavy Debug

An even more impressive kernel parameter is the ignore_loglevel, which causes the system to ignore any loglevel and keeps the internal loglevel at the maximum debugging level, basically rendering dmesg unable to lower the debug level.

title Arch Linux DEBUG Heavy
kernel /vmlinuz-linux root=/dev/disk/by-label/ROOT ro rootdelay=5 panic=10 debug ignore_loglevel
initrd /initramfs-linux.img

Extreme Debug

If the "Heavy Debug" seemed like a lot of output, that's about 1/2 of the logging that occurs with this example. This does a couple things, it uses the earlyprintk parameter to setup your kernel for "early" "printing" of messages to your "vga" screen. The keep just lets it stay on the screen longer. This will let you see logs that normally are hidden due to the boot-up process.

This also changes the log buffer length to 10MB, and also instructs that any fatal signals be printed with print_fatal_signals. The last one, sched_debug, you can look up in the very excellent kernel documentation on kernel parameters.

title Arch Linux DEBUG Extreme
kernel /vmlinuz-linux root=/dev/disk/by-label/ROOT ro debug ignore_loglevel log_buf_len=10M print_fatal_signals=1 LOGLEVEL=8 earlyprintk=vga,keep sched_debug
initrd /initramfs-linux.img

Insane Debug

The first few debugging examples showed some really nice kernel parameters to turn on really verbose debugging. This kind of debugging is absolutely critical if you want to max out your system or just learn more about what is going on behind the scenes. But there is a final trick that is my favorite, it is the ability to set both environment variables, and more importantly, module parameters at boot.

As an example, here is the boot line that I am using at the moment on an older Dell Desktop, just to illustrate module parameters and environment vars.

title  Arch Linux X-256
kernel /vmlinuz-linux root=/dev/disk/by-label/ROOT ro rootwait pause_on_oops=5 panic=60 i915.modeset=1 no_console_suspend ipv6.disable=1 TERM=xterm-256color quiet 5
initrd /initramfs-linux.img

Since it is low on both memory and CPU, I disable ipv6. I also turn on kernel modesetting for the i915 video card, set my terminal to be xterm-256color, and boot straight into X. This lets me use a very optimized arch-linux configuration, amazing how fast thanks to using slim as the login manager, ratpoison as my window manager, and terminal with tmux as my login shell, all from boot, as the pstree shows (plus Synergy!).

  |   |-X,3098 -nolisten tcp vt07 -auth /var/run/slim.auth
  |   `-ratpoison,3107,askapache
  |       |-terminal,5341 -x sh -c exec /usr/bin/tmux -2 -l -u -q attach -d -t tmux-askapache
  |       |   |-bash,11165
  |       |   |-tmux,5345 -2 -l -u -q attach -d -t tmux-askapache
  |       |   `-{terminal},5346
  |       `-xscreensaver,3113 -no-splash
  |-synergyc,6121,galileo -f --name galileo-fire --restart
  `-tmux,5348,askapache -2 -l -u -q attach -d -t tmux-askapache
      |   `-ssh,9969
         `-vim,11149 -p sda1/grub/menu.lst /boot/grub/menu.lst

That kind of optimized system is only possible if you first can figure out your system, by debugging both the kernel as previously illustrated, debugging the init process, and most importantly, by debugging the modules enabled for your system's hardware/firmware/software. Debugging modules is challenging but worth the effort, and then you are able to do some truly insane debugging from grub like the following example, note that the actual grub entry is all on one line, but I split it into 4 lines so you could see it all. This basically turns on every module on this little Dell desktop to be at the absolute max debug level. There is so much logging when I boot this that the system grinds to a halt and is slower than a TI-89 calculator (See Improve Boot Performance).

title  Arch Linux DEBUG INSANE
kernel /vmlinuz-linux root=/dev/disk/by-label/ROOT ro rootwait ignore_loglevel debug debug_locks_verbose=1 sched_debug initcall_debug mminit_loglevel=4 udev.log_priority=8 
       loglevel=8 earlyprintk=vga,keep log_buf_len=10M print_fatal_signals=1 apm.debug=Y i8042.debug=Y drm.debug=1 scsi_logging_level=1 usbserial.debug=Y 
       option.debug=Y pl2303.debug=Y firewire_ohci.debug=1 hid.debug=1 pci_hotplug.debug=Y pci_hotplug.debug_acpi=Y shpchp.shpchp_debug=Y apic=debug 
       show_lapic=all hpet=verbose lmb=debug pause_on_oops=5 panic=10 sysrq_always_enabled
initrd /initramfs-linux.img

A couple key items from that grub entry are sysrq_always_enabled which forces on the sysrq magic, which really is a lifesaver when debugging at this level as your machine will freeze/stop-responding sometimes and it is nice to use sysrq to kill all tasks, change the loglevel, unmount all filesystems, or do a hard reboot. Another key parameter is the initcall_debug, which debugs the init process in excruciating detail. Very useful at times. The last parametery I find very useful is the udev.log_priority=8 to turn on udev logging.

Module Parameters

Here is a nice bash function to be run as root that will show a list of all the loaded modules and all of their parameters, including the current value of the parameter. It uses a mix of sysfs, procfs, and modinfo.

function aa_mod_parameters () 
    C=`tput op` O=$(echo -en "\n`tput setaf 2`>>> `tput op`");
    for mod in $(cat /proc/modules|cut -d" " -f1);
        [[ ! -d $md ]] && continue;
        d=`modinfo -d $m 2>$N | tr "\n" "\t"`;
        echo -en "$O$m$C";
        [[ ${#d} -gt 0 ]] && echo -n " - $d";
        for mc in $(cd $md; echo *);
            de=`modinfo -p $mod 2>$N | grep ^$mc 2>$N|sed "s/^$mc=//" 2>$N`;
            echo -en "\t$mc=`cat $md/$mc 2>$N`";
            [[ ${#de} -gt 1 ]] && echo -en " - $de";

Here is some sample output.

# aa_mod_parameters
>>> soundcore - Core sound module
        preclaim_oss=0 - preclaim_oss: (int)

>>> processor - ACPI Processor Driver
        ignore_ppc=-1 - ignore_ppc:If the frequency of your machine gets wronglylimited by BIOS, this should help (int)
        ignore_tpc=0 - ignore_tpc:Disable broken BIOS _TPC throttling support (int)
        latency_factor=2 - latency_factor: (uint)

>>> usb_storage - USB Mass Storage driver for Linux
        delay_use=1 - delay_use:seconds to delay before using a new device (uint)
        option_zero_cd=1 - option_zero_cd:ZeroCD mode (1=Force Modem (default), 2=Allow CD-Rom (uint)
        quirks= - quirks:supplemental list of device IDs and their quirks (string)
        swi_tru_install=1 - swi_tru_install:TRU-Install mode (1=Full Logic (def), 2=Force CD-Rom, 3=Force Modem) (uint)

>>> sr_mod - SCSI cdrom (sr) driver
        xa_test=0 - xa_test: (int)

>>> uhci_hcd - USB Universal Host Controller Interface driver
        ignore_oc=N - ignore_oc:ignore hardware overcurrent indications (bool)

Break Into Init

For instance, If you add break=y to your kernel cmdline, init will pause early in the boot process (after loading modules) and launch an interactive sh shell which can be used for troubleshooting purposes. (Normal boot continues after logout.) This is very similar to the shell that shows up if your computer gets turned off before it is able to shutdown properly. But using this parameter lets you enter into this mode differently at will.

title  Arch Linux Init Break
kernel /vmlinuz-linux root=/dev/disk/by-label/ROOT ro rootwait break=y
initrd /initramfs-linux.img

Debugging init

This awesome parameter udev.log_priority=8 does the same thing as editing the file /etc/udev/udev.conf except it executes earlier, turning on debugging output for udev. If you want to know your hardware, that is the key parameter right there. Another trick is if you change the /etc/udev/udev.conf to be verbose, then you can make your initrd image include that file to turn on verbose udeb debugging by adding it to your /etc/mkinitcpio.conf like:

FILES="/etc/modprobe.d/modprobe.conf /etc/udev/udev.conf"

, which on arch is as easy as

# mkinitcpio -p linux

Debugging udev is key because the initrd performs a root change at the end of its run to usually launch a program like /sbin/init as part of a chroot, and unless the new file system has a valid /dev directory, udev must be initialized before invoking chroot in order to provide /dev/console.

exec chroot . /sbin/init <dev/console >dev/console 2>&1

So basically, you are not able to view the logs that are generated before /dev/console is initialized by udev or by a special initrd you compiled yourself. One method the kernel developers use to be able to still get the log messages generated before /dev/console is available is to provide an alternative console that you can enable or disable from grub.

Net Console

If you read through the kernel documentation regarding debugging, you will hear about Netconsole, which can be loaded from the kernel line in GRUB, compiled into your kernel, or loaded at runtime as a module. Having a netconsole entry in your menu.lst is most excellent for debugging slower computers like old laptops or thin-clients. It is easy to use. Just setup a 2nd computer (running arch) to accept syslog requests on a remote port, very fast and quick to do on arch-linux, 1 line to syslog.conf. Then you could use a log-color-parser like ccze to view all syslog logs, or just tail your everything.log. Then on your laptop, boot up and select the netconsole entry from the grub menu, and you will start seeing as much logging as you want on your syslog system. This logging lets you view even earlier log output than is available with the earlyprintk=vga kernel parameter, as netconsole is used by kernel hackers and developers, so it is very powerful.

title  Arch Linux DEBUG Netconsole
kernel /vmlinuz-linux root=/dev/disk/by-label/ROOT ro netconsole=514@ debug ignore_loglevel
initrd /initramfs-linux.img

Hijacking cmdline

If you do not have access to GRUB or the kernel boottime cmdline, like on a server or virtual machine, as long as you have root permissions you can still enable this kind of simplistic verbose logging using a neat hack. While you cannot modify the /proc/cmdline even as root, you can place your own cmdline file on top of /proc/cmdline, so that accessing /proc/cmdline actually accesses your file.

For example if I cat /proc/cmdline, I have the following:

root=/dev/disk/by-label/ROOT ro console=tty1 logo.nologo quiet

So I use a simple sed command to replace quiet with verbose like:

sed 's/ quiet/ verbose/' /proc/cmdline > /root/cmdline

Then I bind mount /root/cmdline so that it becomes /proc/cmdline, using the -n option to mount so that this mount will not be recorded in the systems mtab.

mount -n --bind -o ro /root/cmdline /proc/cmdline

Now if I cat /proc/cmdline, I have the following:

root=/dev/disk/by-label/ROOT ro console=tty1 logo.nologo verbose


Repairing with Arch live-cd

In case grub is unable to boot your kernel, or if your initramfs is broken, you can boot into a safe system using an Arch live-cd. Once finished with repairs, unmount the broken system and reboot.

Mounting and Chrooting broken system

Once booted and at a console prompt, use the following to mount and repair your broken system (where /dev/sda3 is / and /dev/sda1 is /boot):

First create the mount-point and mount your root / filesystem to it, then cd into it.

# mkdir /mnt/arch
# mount /dev/sda3 /mnt/arch
# cd /mnt/arch

Now create the proc, sysfs, and dev filesystems

# mount -t proc proc proc/
# mount -t sysfs sys sys/
# mount -o bind /dev dev/

Next mount the boot partition if you use one.

# mount /dev/sda1 boot/

Finally chroot into /mnt/arch which will become /.

# chroot .

Turn on networking with

# /etc/rc.d/networking restart
# /etc/rc.d/net-profiles restart

Reinstalling with Pacman

The author uses pacman in a chrooted broken system to reinstall the kernel, grub, initramfs, udev, and any other packages that may be broken and/or needed to get the system up and running.

This will reinstall the kernel and initramfs so check that /etc/mkinitcpio.conf is correct or remove the file entirely and re-install mkinitcpio.

# pacman -Syy mkinitcpio linux udev

You may need to add the --force option.

Afterwards, unmount and reboot.

See Also

External Links