I've been getting this horribly annoying kernel panic in X recently, so I decided to use kdump/kexec to try and get at my vmcore.
I've set up kexec as per the instructions in /Documentation/kdump/kdump.txt
I've enabled sysrq in both the kernel, AND, in the /etc/sysctl.conf
The system responds to sysrq-C when I'm in the console and before startx, which properly loads the dump kernel and I can get access to my /proc/vmcore
However, when I'm in X (after startx), sysrq-c does NOT load up the crash kernel, and INSTEAD responds by hanging my system (NO blinking capslock though).
I'm on a Intel Core 2 Duo, 64-bit, 2GB of memory, NVIDIA QUADRO FX 570M, and note that my CONFIG_PHYSICAL_START is 0x200000, NOT 0x100000, (I wasn't sure if this should be changed as the default was 0x200000). But this shouldn't matter because the kernel is RELOCATABLE(=y)
I've attached my:
.config : http://dpaste.com/105844/
lspci : http://dpaste.com/105846/
lsmod : http://dpaste.com/105848/
dmesg : http://dpaste.com/105849/
grub.conf : http://dpaste.com/105850/
my kexec command is "sudo kexec -p /usr/src/linux-2.6.28-gentoo/arch/x86/boot/bzImage --append=root=/dev/sda3 1 irqpoll maxcpus=1 reset_devices"