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

david
2007-07-12T18:48:28Z

On Thu, 12 Jul 2007, Rafael J. Wysocki wrote:

> On Thursday, 12 July 2007 08:43, david@lang.hm 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@intel.com> 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