Doing kexec reboot right in systemd

Doing kexec reboot right in systemd

[Marti Raudsepp]

Hi list,

I was recently pondering how systemd could use kexec "properly", in a
reasonably general way, to make reboots faster. I exchanged an email
with Poettering on the systemd list and he suggested me to ask here.

My current proposal:
1. Include a template 'kexec-load.service' with kexec-tools, which
distros/users could customize to load the right kernel and initrd via
/sbin/kexec

2. If the user does 'systemctl enable kexec-load.service', then this
service would trigger before every reboot. Loading the kernel any
earlier would waste memory (5+14 MB on Ubuntu Server). Also a common
reason for rebooting is to boot into a newly-installed kernel. Loading
it any earlier would boot you into an older kernel.

3. During a reboot, after systemd has finished shutting down, it
detects via /sys/kernel/kexec_loaded whether the kexec kernel is
loaded. If 1, it performs a 'kexec -e'. If not, it does a normal
reboot.

----

Lennart commented on #2 that kexec might have problems finding
contiguous memory to load the kernel into, this late in the lifecycle.
Do you think this is a deal-breaker for such a feature? If it's rare
enough, then it might not be a problem -- we can always fall back to a
normal reboot.

I'm also assuming that "kexec --load" will fail atomically -- that it
won't try to boot a half-loaded kernel, or boot a kernel whose initrd
failed to load.

Also, #3 doesn't yet work that way in systemd (currently it detects
before shutdown) and Poettering hasn't agreed yet.

Does this proposal seem sane to kexec folks? Did I miss anything?

----

Here are my and Poettering's initial thoughts on this matter:
http://lists.freedesktop.org/archives/systemd-devel/2012-March/004760.html
http://lists.freedesktop.org/archives/systemd-devel/2012-March/004764.html

PS: I hope I haven't broken any rules by cross-posting this. This
question seems relevant to both projects.

Regards,
Marti

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Re: Doing kexec reboot right in systemd

[Simon Horman]

On Thu, Mar 22, 2012 at 09:14:56PM +0200, Marti Raudsepp wrote:
> Hi list,

Hi Marti,

> I was recently pondering how systemd could use kexec "properly", in a
> reasonably general way, to make reboots faster. I exchanged an email
> with Poettering on the systemd list and he suggested me to ask here.
> 
> My current proposal:
> 1. Include a template 'kexec-load.service' with kexec-tools, which
> distros/users could customize to load the right kernel and initrd via
> /sbin/kexec
> 
> 2. If the user does 'systemctl enable kexec-load.service', then this
> service would trigger before every reboot. Loading the kernel any
> earlier would waste memory (5+14 MB on Ubuntu Server). Also a common
> reason for rebooting is to boot into a newly-installed kernel. Loading
> it any earlier would boot you into an older kernel.
> 
> 3. During a reboot, after systemd has finished shutting down, it
> detects via /sys/kernel/kexec_loaded whether the kexec kernel is
> loaded. If 1, it performs a 'kexec -e'. If not, it does a normal
> reboot.
> 
> ----
> 
> Lennart commented on #2 that kexec might have problems finding
> contiguous memory to load the kernel into, this late in the lifecycle.
> Do you think this is a deal-breaker for such a feature? If it's rare
> enough, then it might not be a problem -- we can always fall back to a
> normal reboot.

In general kexec doesn't store the kernel in contiguous memory at load-time.
Rather, when kexec -e is called the kernel is copped from (most likely)
discontiguous pages) into its destination location.

> I'm also assuming that "kexec --load" will fail atomically -- that it
> won't try to boot a half-loaded kernel, or boot a kernel whose initrd
> failed to load.

Yes, that is correct.

However, I speculate that there is a greater chance of the boot of a
kexeced kernel failing than the same kernel booting by more conventional
means on the same hardware. Primarily because there are most likely
shutdown/boot paths, especially in drivers, that have not been well
exercised.

So while it may work well on some hardware, perhaps even most hardware, I
suspect there are cases where it will fail.  I guess that is where
systemctl enable/disable kexec-load.service comes into play.

> Also, #3 doesn't yet work that way in systemd (currently it detects
> before shutdown) and Poettering hasn't agreed yet.
> 
> Does this proposal seem sane to kexec folks? Did I miss anything?

If it is desirable to use kexec as a reboot mechanism then
from a kexec point of view I think your proposal makes sense.

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Re: Doing kexec reboot right in systemd

[Bouchard Louis]

Hello,

Le 22/03/2012 20:14, Marti Raudsepp a écrit :
> Hi list,
> 
> I was recently pondering how systemd could use kexec "properly", in a
> reasonably general way, to make reboots faster. I exchanged an email
> with Poettering on the systemd list and he suggested me to ask here.
> 

While not done by systemd, isn't is what is done on Debian when
kexec-tools is installed ?

It provides a 'coldreboot' script that does the following :


 #!/bin/sh
 NOKEXECFILE=/tmp/no-kexec-reboot

 /bin/rm -f $NOKEXECFILE
 touch $NOKEXECFILE
 /sbin/reboot $*

FWIR, using reboot will quickly kexec into a new kernel.

Kind regards,

...Louis
-- 
Louis Bouchard
Backline Support Analyst
Canonical Ltd
Ubuntu support: http://landscape.canonical.com

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Re: Doing kexec reboot right in systemd

[Marti Raudsepp]

On Fri, Mar 23, 2012 at 11:35, Bouchard Louis
<louis.bouchard@canonical.com> wrote:
> While not done by systemd, isn't is what is done on Debian when
> kexec-tools is installed ?

Thanks for pointing that out. The approach is indeed pretty similar to
what I described above.

I guess that means we're all in agreement.

Regards,
Marti

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Re: Doing kexec reboot right in systemd

[Lennart Poettering]

On Fri, 23.03.12 07:36, Simon Horman (horms@verge.net.au) wrote:

> 
> On Thu, Mar 22, 2012 at 09:14:56PM +0200, Marti Raudsepp wrote:
> > Hi list,
> 
> Hi Marti,
> 
> > I was recently pondering how systemd could use kexec "properly", in a
> > reasonably general way, to make reboots faster. I exchanged an email
> > with Poettering on the systemd list and he suggested me to ask here.
> > 
> > My current proposal:
> > 1. Include a template 'kexec-load.service' with kexec-tools, which
> > distros/users could customize to load the right kernel and initrd via
> > /sbin/kexec

Hmm, so since this would then belong in the shutdown path of systemd,
and only be added for the kexec shutdown path I actually think it would
make more sense to add this into systemd itself instead of kexec-tools,
to ensure the right ordering.

Marti, sorry for changing my mind on this: would be great if you could
prep a patch for this for systemd itself.

> > 3. During a reboot, after systemd has finished shutting down, it
> > detects via /sys/kernel/kexec_loaded whether the kexec kernel is
> > loaded. If 1, it performs a 'kexec -e'. If not, it does a normal
> > reboot.

I'd like to keep kexec.target different from reboot.target by
default, so that people have both options available.

So, how I'd envision this:

a) we introduce kexec-load.service which is always in the kexec.target
   shutdown path, never in reboot.target. Doesn't need to be enabled.

b) kexec-load.service becomes a NOP if a kernel is already loaded

c) "systemctl reboot" continues to check whether a kernel is already
   loaded, and if so results in kexec.target being started rather than
   reboot.target

d) If the user wants that the machine is always rebooted via kexec,
   never with traditional reboot, he can manually create a symlink
   /etc/systemd/system/reboot.target to
   /etc/systemd/system/kexec.target.

That way, that in general both paths always exist, unless the admin
manually loaded a kernel or manually redirected all reboots to kexec.

Does that make sense?

Lennart

-- 
Lennart Poettering - Red Hat, Inc.

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Re: Doing kexec reboot right in systemd

[Marti Raudsepp]

On Mon, Mar 26, 2012 at 21:07, Lennart Poettering
<lennart@poettering.net> wrote:
> Marti, sorry for changing my mind on this: would be great if you could
> prep a patch for this for systemd itself.

D'oh, I already prepared a patch for kexec, although I haven't sent it out yet.

> Hmm, so since this would then belong in the shutdown path of systemd,
> and only be added for the kexec shutdown path I actually think it would
> make more sense to add this into systemd itself instead of kexec-tools,
> to ensure the right ordering.

Um, why is including in systemd necessary for guaranteeing order?
Seems like this should do it...

[Unit]
DefaultDependencies=no
Before=shutdown.target umount.target final.target
[Install]
WantedBy=kexec.target

> I'd like to keep kexec.target different from reboot.target by
> default, so that people have both options available.

Fair enough.

> a) we introduce kexec-load.service which is always in the kexec.target
>   shutdown path, never in reboot.target. Doesn't need to be enabled.
>
> b) kexec-load.service becomes a NOP if a kernel is already loaded
>
> c) "systemctl reboot" continues to check whether a kernel is already
>   loaded, and if so results in kexec.target being started rather than
>   reboot.target
>
> d) If the user wants that the machine is always rebooted via kexec,
>   never with traditional reboot, he can manually create a symlink
>   /etc/systemd/system/reboot.target to
>   /etc/systemd/system/kexec.target.

Yeah, agreed.

Regards,
Marti

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Re: Doing kexec reboot right in systemd

[Lennart Poettering]

On Mon, 26.03.12 21:25, Marti Raudsepp (marti@juffo.org) wrote:

> 
> On Mon, Mar 26, 2012 at 21:07, Lennart Poettering
> <lennart@poettering.net> wrote:
> > Marti, sorry for changing my mind on this: would be great if you could
> > prep a patch for this for systemd itself.
> 
> D'oh, I already prepared a patch for kexec, although I haven't sent it
> out yet.

OK, I am fine with either.

> > Hmm, so since this would then belong in the shutdown path of systemd,
> > and only be added for the kexec shutdown path I actually think it would
> > make more sense to add this into systemd itself instead of kexec-tools,
> > to ensure the right ordering.
> 
> Um, why is including in systemd necessary for guaranteeing order?

It isn't, but it's easier to keep early boot and late shutdown stuff in
systemd.

Lennart

-- 
Lennart Poettering - Red Hat, Inc.

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Re: Doing kexec reboot right in systemd

[Simon Horman]

On Tue, Mar 27, 2012 at 12:01:36PM +0200, Lennart Poettering wrote:
> On Mon, 26.03.12 21:25, Marti Raudsepp (marti@juffo.org) wrote:
> 
> > 
> > On Mon, Mar 26, 2012 at 21:07, Lennart Poettering
> > <lennart@poettering.net> wrote:
> > > Marti, sorry for changing my mind on this: would be great if you could
> > > prep a patch for this for systemd itself.
> > 
> > D'oh, I already prepared a patch for kexec, although I haven't sent it
> > out yet.
> 
> OK, I am fine with either.

FWIW, I think systemd is a better place for this change.

> > > Hmm, so since this would then belong in the shutdown path of systemd,
> > > and only be added for the kexec shutdown path I actually think it would
> > > make more sense to add this into systemd itself instead of kexec-tools,
> > > to ensure the right ordering.
> > 
> > Um, why is including in systemd necessary for guaranteeing order?
> 
> It isn't, but it's easier to keep early boot and late shutdown stuff in
> systemd.
> 
> Lennart
> 
> -- 
> Lennart Poettering - Red Hat, Inc.
> 

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