Date: Sat, 30 Dec 2006 18:29:38 +0000 (GMT) From: Robert Watson <rwatson@FreeBSD.org> To: Karol Kwiatkowski <freebsd@orchid.homeunix.org> Cc: freebsd-stable@freebsd.org Subject: Re: debugging kernel options (was: Re: 6.2-PRE: Fatal Trap?) Message-ID: <20061230182714.O18740@fledge.watson.org> In-Reply-To: <4596AF32.2060205@orchid.homeunix.org> References: <20061230035722.L39715@thebighonker.lerctr.org> <20061230151933.P18740@fledge.watson.org> <20061230093730.I47579@thebighonker.lerctr.org> <20061230160353.K18740@fledge.watson.org> <45969487.1020806@orchid.homeunix.org> <20061230170358.K50974@fledge.watson.org> <4596AF32.2060205@orchid.homeunix.org>
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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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