Kexec quick reboot

[John Richard Moser]

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I'm looking at kexec and thinking it could be used for fast rebooting in
some situations; I've had a machine that makes it to Grub in 18 seconds,
and I know someone who's got his going 25 seconds, both Athlon 64s.
Someone else I know has a couple RAID/SCSI cards that give him a nice
long wait.

I've tested this on dapper (using an edgy kernel, which has kexec on);
the results are pleasing.  The line I used to load went like this:

 sudo kexec -l --initrd=/boot/initrd.img-2.6.15-26-686 \
 --append="`cat /proc/cmdline`" /boot/vmlinuz-2.6.15-26-686

NOTE:  Rather than do `cat /proc/cmdline` you can do:
   $(cat /boot/grub/menu.lst | grep $(uname -r) | grep "^kernel" | \
     head -n 1 | xargs | cut -f 3 -d ' ')

This loaded the latest Dapper kernel (I was on 2.6.17-6-686 from Edgy)
for kexec, and I went merrily on my way.

Later (20 seconds later, after toying with a few more programs) I tried
to reboot; this failed.  The solution was to modify /etc/init.d/reboot
to have the following block:

do_stop () {
	log_action_msg "Will now reboot"
	# if we have kexec, try to execute a loaded kernel
	which kexec > /dev/null && kexec -e
	# failing that, reboot normally
	reboot -d -f -i
}

I loaded the Dapper kernel again; hit reboot; and, amazingly, it worked!
 uname showed I'd downgraded from 2.6.17 to 2.6.15, and everything works.

I'm thinking there's three scenarios this can be used in:

1.  "System Restart Required."  The dialog has a "Reboot" button in it.
    If we're expecting the user to be restarting Linux, then it makes
    sense to kexec -l the latest Linux kernel before rebooting.

2.  Shutdown->Restart.  We COULD assume the user is restarting Linux,
    and automatically load the latest kernel.  This has issues of
    course, because the user has to power down to get back to the boot
    loader.

2a. Shutdown->Restart.  Again, we could get the normal actions to carry
    out our bidding; but this time, disable the functionality if we
    detect a non-Ubuntu OS.  Unfortunately we can't detect multiboot
    with 100% certainty.

2b. Shutdown->Restart.  Just make it configurable and disable it by
    default.  We may turn it on by default later, but that decision is
    easy to delay.

3.  Shutdown->Quick Restart.  We COULD add a new Quit/Shut Down option
    for a "Quick Restart" that loads the latest kernel.  This is
    harmless; except that it will clutter that dialog with an extra
    button.


The steps that need to be taken for this to be enabled are as follows:

 - Patch /etc/init.d/reboot to call kexec before reboot; a failure with
   kexec will fall through to normal reboot.  (Hint:  Read up)

 - Write a script (A) to figure out the latest kernel.

 - Write a script (B) to divine the command line for a specific kernel.

 - Write a script (C) to call script (A) and script (B) and load kexec
   as appropriate.

 - Set the "System Restart Required" dialog up to call script (C) before
   rebooting.

 - Address any configuration issues with (2a) or (2b) if you want to
   implement (2).

 - Add the button for (3) and have it call script (C) before rebooting,
   if you want to implement (3).

- --
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