Re: [PATCH 0/2] Kexec jump: The first step to kexec base hibernation

[Rafael J. Wysocki]

On Thursday, 12 July 2007 20:42, david@xxxxxxx wrote:
> On Thu, 12 Jul 2007, Rafael J. Wysocki wrote:
> 
> > On Thursday, 12 July 2007 08:43, david@xxxxxxx wrote:
> >> On Wed, 11 Jul 2007, Jeremy Fitzhardinge wrote:
> >>
> >>> Andrew Morton wrote:
> >>>>  On Wed, 11 Jul 2007 15:30:31 +0000
> >>>>  "Huang, Ying" <ying.huang@xxxxxxxxx> wrote:
> >>>>
> >>>>>  1. Boot a kernel A
> >>>>>  2. Work under kernel A
> >>>>>  3. Kexec another kernel B in kernel A
> >>>>>  4. Work under kernel B
> >>>>>  5. Jump from kernel B to kernel A
> >>>>>  6. Continue work under kernel A
> >>>>>
> >>>>>  This is the first step to implement kexec based hibernation. If the
> >>>>>  memory image of kernel A is written to or read from a permanent media
> >>>>>  in step 4, a preliminary version of kexec based hibernation can be
> >>>>>  implemented.
> >>>>>
> >>>>>  The kernel B is run as a crashdump kernel in reserved memory
> >>>>>  region. This is the biggest constrains of the patch. It is planed to
> >>>>>  be eliminated in the next version. That is, instead of reserving memory
> >>>>>  region previously, the needed memory region is backuped before kexec
> >>>>>  and restored after jumping back.
> >>>>>
> >>>>>  Another constrains of the patch is that the CONFIG_ACPI must be turned
> >>>>>  off to make kexec jump work. Because ACPI will put devices into low
> >>>>>  power state, the kexeced kernel can not be booted properly under
> >>>>>  it. This constrains can be eliminated by separating the suspend method
> >>>>>  and hibernation method of the devices as proposed earlier in the LKML.
> >>>>>
> >>>>>  The kexec jump is implemented in the framework of software suspend. In
> >>>>>  fact, the kexec based hibernation can be seen as just implementing the
> >>>>>  image writing and reading method of software suspend with a kexeced
> >>>>>  Linux kernel.
> >>>>>
> >>>
> >>> I guess I'm (still) confused by the terminology here.  Do you mean that it
> >>> fits into suspend-to-disk as a disk-writing mechanism, or in suspend-to-ram
> >>> as a way of going to sleep?
> >>
> >> Suspend-to-ram involves stopping the system and shutting down devices to
> >> go into low-power mode, then on wakeup restarting devices and resuming
> >> operation
> >>
> >> so the steps would be.
> >>
> >> 1. stop userspace
> >>
> >> 2. walk the system device tree and put devices to sleep
> >>
> >> 3. go into the lowest power mode available and wait for a wakeup signal
> >>
> >> later
> >>
> >> 4. walk the system device tree and wake up devices
> >>
> >> 5. resume userspace scheduling.
> >
> > Note that we are going to phase out steps 1 and 5.
> 
> what I'm referring to in #1 and #5 is not the current freezer, it's just 
> the kernel not allocating any cpu time to userspace while it's shutting 
> things down (this could be as simple as unplugging all non-boot cpu's and 
> then doing the rest of the work without letting the scheduler run.

This is not what we're going to do.  There is a plan to block tasks on I/O if
they request it during a suspend.

> >> note that what devices get put to sleep could be configurable, potentially
> >> to the extreme of things like the OLPC (that have hardware designed for
> >> cheap sleeping) going into a light suspend-to-ram state between keystrokes
> >> if nothing else has a timer event scheduled before that.
> >>
> >> Suspend-do-disk (Hibernate) involves stopping the system, makeing a
> >> snapshot of ram, writing the snapshot to somewhere and powering off the
> >> box. on wakeup (power-on) a helper kernel boots, loads the snapshot into
> >> ram and jumps to the kernel in the snapshot to resume operation.
> >>
> >> as I understand the proposal the thought is to do the following
> >>
> >> 1. system kernel does suspend-to-ram to put the devices into a known safe
> >> state.
> >
> > Not necessarily suspend-to-RAM.  I'd much prefer it if devices were not put
> > into low power states but quiesced (ie. no DMA, no interrupts).
> 
> as I asked in another message, is it really worth having two (or more) 
> modes here?

I think so.  The suspend-to-RAM mode is quite specific and on some platform
(ie. ACPI) it requires platform support.

We've already reached the conclusion that it's better to separate suspend from
hibernation, as far as device drivers are concerned, and let's not repeat the
discussion.

> >> 2. system kernel uses kexec to start hibernate kernel
> >>
> >> 3. hibernate kernel wakes up devices it needs as if it was doing a
> >> resume-from-ram
> >
> > I think that the devices should be initialized from scratch in this step.
> >
> >> 4. hibernate kernel copies ram image somewhere
> >
> > In this step some userland may be involved (started from the "hibernate"
> > kernel).
> 
> yes,, I should have been much clearer that it's the userspace for the 
> hibernate kernel that does this.
> 
> >> 5. hibernate kernel shuts down the box
> >>
> >> later
> >>
> >> 6. hibernate kernel boots
> >>
> >> 7. hibernate kernel copies ram image from somewhere
> >>
> >> 8. hibernate kernel does syspend-to-ram to put the devices into a known
> >> safe state.
> >
> > Again, the devices should be quiesced rather then suspended in this step.
> >
> >> 9. hibernate kernel uses kexec to start system kernel
> >>
> >> 10. system kernel wakes up devices it needs as if it was doing a
> >> resume-from-ram.
> >
> > I think it should reconfigure devices from scratch (ie. reprobe).
> 
> probably a good idea, especially since there will be devices that the 
> hibernate krenel hasn't touched.

Exactly.

Greetings,
Rafael


-- 
"Premature optimization is the root of all evil." - Donald Knuth
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