Re: debugging kernel options (was: Re: 6.2-PRE: Fatal Trap?)


On Sat, 30 Dec 2006, Karol Kwiatkowski wrote:

Robert Watson wrote:
P.S. out of curiosity - now that I have configured kernel with DDB and
KDB options, is there any performance penalty of running such kernel?

No, it shouldn't really have any effect on performance. The one thing to watch out for is that your system will no longer reboot automatically on a panic, as it will drop to the debugger, by default. You can change this by setting debug.debugger_on_panic to 0, in which case you will likely want to set debug.trace_on_panic to 1 so it prints a stack trace before rebooting (which is often sufficient, combined with the trap frame and panic message to debug the problem).

Right now these are sysctls, not tunables, but you can change the default using options KDB_UNATTENDED (which flips the default to not entering the debugger and rebooting) and options KDB_TRACE (which causes a trace to be printed on panic by default). Probably they should also be tunables so that loader.conf entries will work.

Great explanation, thank you. I turned on debugging on my desktop computer which, apart from normal every day use, is 'testing' STABLE by running it :) I'm perfectly fine with the defaults, at least for now.

BTW, if you're running X on your desktop, be aware that it's X that does all the video mode management. If your box enters the debugger while in X, the debugger doesn't know how to switch back to text mode (and X isn't running, obviously), so while you'll be talking to the debugger, the chances you'll see anything comprehensible are actually quite low. For this reason, I normally also use a serial console when debugging desktop boxes: I can always plug my notebook in with a serial cable to see why it's entered the debugger.

Robert N M Watson
Computer Laboratory
University of Cambridge
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