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ckoeber
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PostPosted: Fri May 17, 2013 1:10 am    Post subject: Dealing with Kernel Panics when you can't see the error ... Reply with quote

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Hello,

I have a rather simple problem that I can't seem to find an answer for with Google; I am installing Gentoo (Kernel 3.8.13 on x86_64) and after the reboot I get a kernel panic.

The problem is I get the Call Trace and it gives me a bunch of text which doesn't allow me to see what the actual problem was.

Is there an option I can enable, at least temporarily, to remove the call trace to see the error message?

Or, in general, how can I find the actual problem of why I am getting a panic message?

Thank you for your time.


Last edited by ckoeber on Tue May 21, 2013 6:32 pm; edited 1 time in total
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BradN
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PostPosted: Fri May 17, 2013 5:56 am    Post subject: Reply with quote

Look into using console over network or serial port, is the best suggestion I can think of.

Failing that, is your kernel custom configured? If so, consider using a genkernel config or livecd config as a basis and then modify from there. Probably you will miss the option that is causing it to crash (or else, if you know it's not crashing with the defaults you can narrow it down at least!)

Good luck!
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Hu
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PostPosted: Sat May 18, 2013 12:43 am    Post subject: Reply with quote

If you use the framebuffer console, the increased resolution may give you enough extra lines on screen to see the error message.
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DONAHUE
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PostPosted: Sat May 18, 2013 3:28 am    Post subject: Reply with quote

phone or digcam that does movies, record the boot, playback in slow motion or frame by frame.
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krinn
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PostPosted: Sat May 18, 2013 4:20 pm    Post subject: Reply with quote

it will also get record in dmesg, something that will of course get clean on next boot, so you need to access it without booting it, something easy to do with any livecd.
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[n00b@localhost]
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PostPosted: Sat May 18, 2013 4:24 pm    Post subject: Reply with quote

The output from dmesg gets logged to /var/log/dmesg.log which you can read using a liveCD.
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Jaglover
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PostPosted: Sat May 18, 2013 4:35 pm    Post subject: Reply with quote

Did you try Shift+PgUp.
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Hu
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PostPosted: Sat May 18, 2013 4:42 pm    Post subject: Reply with quote

Assuming the OP is correct to call his event a kernel panic, then nothing will be written to the filesystem, so there is no way to retrieve it from /var/log/dmesg.log. Similarly, once the kernel panics, you cannot scroll back.

Some new users mistakenly call any BUG or OOPS event a kernel panic. However, since the OP says this happens right after reboot and says he is installing Gentoo, the odds are good that he is correct to call it a kernel panic and that the problem is some sort of kernel configuration problem that results in a failure to start init.
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BradN
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PostPosted: Sat May 18, 2013 6:50 pm    Post subject: Reply with quote

Hu is right, in addition sometimes BUG/OOPS is followed immediately by something related that locks up the machine, so even in those cases it may not be possible to scroll the screen or retrieve from log files.
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ckoeber
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PostPosted: Tue May 21, 2013 5:23 pm    Post subject: Reply with quote

Jaglover wrote:
Did you try Shift+PgUp.


Just tried this, doesn't work :(

Thanks!
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ckoeber
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PostPosted: Tue May 21, 2013 5:25 pm    Post subject: Reply with quote

BradN wrote:
Look into using console over network or serial port, is the best suggestion I can think of.

Failing that, is your kernel custom configured? If so, consider using a genkernel config or livecd config as a basis and then modify from there. Probably you will miss the option that is causing it to crash (or else, if you know it's not crashing with the defaults you can narrow it down at least!)

Good luck!


I tried to get this to work but I always get the quick message (after a long pause) that the link was not ready (despite the network card being functional + everything working in the LiveCD).

So, I will try the genkernel for now (I really like the manual config, faster bot times...)
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ckoeber
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PostPosted: Tue May 21, 2013 5:35 pm    Post subject: Reply with quote

Hu wrote:
Assuming the OP is correct to call his event a kernel panic, then nothing will be written to the filesystem, so there is no way to retrieve it from /var/log/dmesg.log. Similarly, once the kernel panics, you cannot scroll back.

Some new users mistakenly call any BUG or OOPS event a kernel panic. However, since the OP says this happens right after reboot and says he is installing Gentoo, the odds are good that he is correct to call it a kernel panic and that the problem is some sort of kernel configuration problem that results in a failure to start init.


I see, what I DID find is that I can add a loglevel option to change the output from the boot system.

A little better output can be obtained:

Code:
loglevel=   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


Also, this is helpful:

Code:
logo.nologo   [FB] Disables display of the built-in Linux logo.
         This may be used to provide more screen space for
         kernel log messages and is useful when debugging
         kernel boot problems.


So I answered my own question, in a way. I'll mark this thread as solved (and start a new thread possibly shortly)
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TomWij
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PostPosted: Tue May 21, 2013 8:13 pm    Post subject: Reply with quote

Code:
boot_delay=   Milliseconds to delay each printk during boot.
         Values larger than  (10000) are changed to
         no delay (0).
         Format: integer
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