RapidReboot

Revision 5 as of 2006-09-12 15:28:37

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NOTE: This page is part of the Ubuntu Specification process. Please check the status and details in Launchpad before editing. If the spec is Approved then you should contact the Assignee, or another knowledgeable person, before making changes.

Summary

This spec defines the use of kexec where we know the user does not want to see the boot loader.

Rationale

There are a few cases where we can safely assume the user does not want to see the boot loader; in these cases, we should use kexec to avoid long reboots, BIOS POSTs, and boot loader time. This can take 10 seconds in optimal situations; but with SCSI or RAID BIOS and network boot roms, the time can climb to 20, 30, or even over 60 seconds, even in cases where the time between loading the kernel and seeing the log-in screen is 30-60 seconds.

Use cases

  • Bob has just upgraded his kernel; update-notifier informs him he must reboot for the changes to take effect, and the 'Restart' button uses kexec to make this faster.

  • Alice just upgraded dbus; update-notifier tells her to restart, and uses kexec.

  • Seveas has performed several updates in the past month, and now is under three times as much memory pressure due to different programs using different copies of shared libraries. He decides to reboot to clear this up, and uses a "Quick Reboot" to make this faster.

  • Keybuk just installed Ubuntu Edgy and wants to boot into it. The LiveCD loads the target kernel and initrd with kexec; umounts all disk-based file systems; sync; and then uses kexec to start the new system rather than rebooting.

Scope

We are concerned with kexec in three cases:

  • Reboots from update-manager.

  • Reboots from "Quick Reboot" in the Shutdown menu.
  • Reboots from the LiveCD into a newly installed Ubuntu system.

Design

The design will utilize kexec to facilitate fast rebooting. kexec can load the initrd and the kernel image for Linux kernels and append a command line to the kernel.

We will also have to decide on a method for producing a "Quick Reboot" in the Shutdown menu. Possibilities are:

  • Add a fourth Shutdown button labeled "Quick Reboot."

  • Switch the buttons on modifier, such that holding SHIFT or CTRL will change the "Restart" button to say "Quick Restart" and function as such.

The kernel will only be loaded by kexec when the user has made the decision to take a kexec action; this is because whenever we do a reboot, the reboot script init calls attempts to kexec any loaded kernel.

Implementation

We will need to modify /etc/init.d/reboot to call kexec before calling reboot.

A script must be produced to determine the latest Ubuntu kernel and initrd from the currently running system. We can derive this from update-grub.

A script must be produced to extract the kernel command line as it would be in /boot/grub/menu.lst for the selected kernel. We can also derive this from update-grub, or possibly read it from menu.lst.

A script must be produced to derive the kernel and its command line using the above two scripts. This script should then call the following: 

kexec -l --initrd=${INITRD} --append="${COMMAND_LINE}" ${KERNEL}

The above script should be able to run whenever all of the following conditions are met. These requirements will allow the LiveCD to chroot /target to prepare for kexec into the new system.

  • The /boot file system is readable

  • The /usr file system is readable

  • The / file system is readable

  • /proc is mounted and readable

Code

The below version of do_stop() should be used in /etc/init.d/reboot: 

do_stop () {
        # End message in newline for kFreeBSD bug #323749
        log_action_msg "Will now restart"
        # Reboot normally if kexec isn't installed correctly
        if [ -x /sbin/kexec ]; then
            kexec -e
        else
            reboot -d -f -i
        fi
}

--JeffSchroeder: Improved do_stop logic

We need the other three scripts.

Data preservation and migration

None.

Unresolved issues

I've got absolutely no clue how to get scripts up to detect the kernel; and only a vague idea on a cheap and inaccurate hack to get a command line out of grub.conf. Anyone want to take up the tab? --JohnMoser

For the default commandline minus the initrd or root volume: 

grep '^#.defoptions' /boot/grub/menu.lst | sed 's/^#.defoptions=//''

For the currently running kernel, /proc/cmdline contains the full commandline. If a new default kernel is installed, try this: 

egrep '/vmlinuz.root' menu.lst | egrep -v '^#|single' | sed 's/^kernel.*z //'

I like this spec enought that I'm going to work on implementing it. Now that upstart is in edgy replacing sysv init script, contacting Keybuk would be wise. --JeffSchroeder

BoF agenda and discussion

CategorySpec CategorySpec