which kernel debugger is "best"?

What are people using? neither kdb or kgdb appear to support 2.5.7 (kdb does 2.5.5)... or do real men debug with prink() ?

kgdb. Tried kdb and (sorry, Keith), it's not in the same league. Not by miles.

General answer to this is to go for a foray in
http://www.zip.com.au/~akpm/linux/patches/

Which turns up

	http://www.zip.com.au/~akpm/linux/patches/2.5/2.5.7/kgdb.patch

I have done it both ways, extensively, for long periods. The printk method is comically inefficient. The amount of transparency whch kgdb gives to kernel internals is extraordinary.

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kdb and kgdb are aimed at different debugging environments. kgdb requires a second machine containing the kernel compiled with -g, kdb lets you debug directly on the machine that failed, with or without compiling with -g. Almost all the differences flow from that design decision.

Another important niggle to me is that kgdb requires the kernel to be compiled with frame pointers, because that is all that gdb understands. On ix86 the extra register pressure from dedicating ebp to frame pointers can cause Heisenbugs. kdb works with and without frame pointers.

Can kgdb handle the special hard wired calls that do not add frame pointers, such as __down_failed? I doubt that gdb knows how to handle those.

I am not knocking kgdb, it has its place. I see a spectrum of debugging tools from UML through kgdb to kdb, each tool is aimed at a different debugging environment. Pick the right tool.

I guess the distinction here is that I use kgdb for "development", not for "debugging".

Displaying data structures, values of variables. Seeing what state all tasks in the system are in, where they're sleeping, where they're spending CPU, etc.

When adding ad-hoc inxtrumentation to the kernel, you don't need to bother printing it out - just increment the counters and go in take a look when desired.

And yes, kgdb mucks up call chains across down() because of the lack of a frame pointer - backtraces don't display who called down() - it loses the innermost frame. That's irritating, but not enough to have motivated me to soil my hands with x86 assembly yet.

I haven't had any problems with -fno-omit-frame-pointer at any time.

I *have* had problems with -fno-inline. I'd very much like to be able to turn that on, but the presence of extern inline' functions causes a link failure with -fno-inline'. I'd suggest that this is a gcc shortcoming. I actually had a poke yesterday at teaching gcc to convert extern inline to static inline if flag_no_inline, but it didn't work out.

kgdb is damned inconvenient. You have to set up a cross-build machine, serial cable and generally get organised to use it. In reality, this would take an hour or so but it is some friction.

I would like to see kdb shipped in the mainline kernel, so that we can get better diagnostic reports from users/testers.

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From: Andrew Morton <[email protected]>
Date: Fri, 29 Mar 2002 20:24:05 -0800

I *have* had problems with -fno-inline. I'd very much like to be able to turn that on, but the presence of extern inline' functions causes a link failure with -fno-inline'.

Feel free to submit the patch that converts the remaining extern inline into static inline. That is the correct solution.

GCC has every right not to inline and expect the function name to be referencable externally if you say extern inline, so this is another reason to fix the remaining extern inline instances.

Whoops, I think the same. And also something like a crash dump utility would be nice in the mainline kernels.

Without them it's hard to get qualified bug reports from production machines...

Hi,

It's time we have a webpage comparing the two debuggers. I put-up one at http://kgdb.sourceforge.net/whichdebugger.html

Please let me know if I have missed anything.

Amit Kale
Veritas Software India ( http://www.veritasindia.com/ )

[other stuff cut]

Here, Here! That and crash dumps are important to providing high quality support to enterprise customers.

Phil

Philip R. Auld, Ph.D.                  Technical Staff 
Egenera, Inc.                        [email protected]
165 Forest St., Marlboro, MA 01752       (508)786-9444

I just uploaded kdb patches for 2.5.7 to ftp://oss.sgi.com/projects/kdb/download/v2.1. They compile but have not been booted, I don't have much time to work on 2.5 kernels. I have no idea if it will work with a preemptible kernel or not.