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.
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?
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.
David Lang
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