kdump: does not generate a dump on 14.04, because the reserved memory is too low

To debug kernel crashes, I have installed and configured the kernel crash dumps on a 14.04 system (with Wily HWE Stack, but the problem exist with older an newer kernels too).

I have used the following documentation:

https://wiki.ubuntu.com/Kernel/CrashdumpRecipe
https://help.ubuntu.com/lts/serverguide/kernel-crash-dump.html

The default kernel option is:

crashkernel=384M-:128M

The output of kdump-config show:

# kdump-config show
USE_KDUMP: 1
KDUMP_SYSCTL: kernel.panic_on_oops=1
KDUMP_COREDIR: /var/crash
crashkernel addr: 0x24000000
current state: ready to kdump

kexec command:
  /sbin/kexec -p --command-line="BOOT_IMAGE=/vmlinuz-4.2.0-30-generic root=/dev/mapper/VolGroup00-rootlv ro quiet splash vt.handoff=7 irqpoll maxcpus=1 nousb" --initrd=/boot/initrd.img-4.2.0-30-generic /boot/vmlinuz-4.2.0-30-generic

When using the "echo c > /proc/sysrq-trigger" to induce a crash, nothing happens after the Oops gets printed on the console. I have to manually turn the system off an on again, and of course, no crash dump is found in /var/crash.

After raising the reserved memory to 256MB (crashkernel=384M-:256M), and triggering the crash using the sysrq, the new kernel gets started shortly after the Oops, and after the following reboot the crash dump is in /var/crash, as expected.

The system is a Lenovo T450s with amd64 installation.