Message-ID: <Pine.LNX.4.64.0707141400320.14672@asgard.lang.hm>
Date:	Sat, 14 Jul 2007 14:13:16 -0700 (PDT)
From:	david@...g.hm
To:	"Rafael J. Wysocki" <rjw@...k.pl>
cc:	Jeremy Maitin-Shepard <jbms@....edu>,
	"Huang, Ying" <ying.huang@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Pavel Machek <pavel@....cz>, nigel@...el.suspend2.net,
	linux-kernel@...r.kernel.org, linux-pm@...ts.linux-foundation.org
Subject: Re: [PATCH 0/2] Kexec jump: The first step to kexec base hibernation

On Sat, 14 Jul 2007, Rafael J. Wysocki wrote:

> On Saturday, 14 July 2007 22:34, david@...g.hm wrote:
>> in the past, Rafael J. Wysocki wrote:
>>
>>> BTW, please read this message and tell me what you think:
>>>
>>> http://lkml.org/lkml/2007/7/13/265
>>>
>>> Greetings,
>>> Rafael
>>>
>>>
>>>
>>
>> since I've deleted this message here's the relavent portion of it
>>
>>> Okay, I have thought it through and I think that, as an initial step, we
>>> can do something like this:
>>>
>>> - preload the image-saving kernel before hibernation
>>> - in the hibernation code path replace device_suspend() with the shutting
>>> down of all devices without unregistering them (not very nice, but should be
>>> sufficient for a while)
>>> - when we've called device_power_down() and save_processor_state(), jump
>>> to the image-saving kernel and let it run
>>> - make the image-saving kernel set up everything, save the image without
>>>  starting any user space (we may use the existing image-saving code for
>>> this purpose, with some modifications) and power off the system (or make it
>>> enter S4)
>>> - use the existing restoration code to load the image and jump to the
>>>  hibernated kernel
>>> - in the restore code patch replace device_resume() with the reprobing of
>>> all devices.
>>>
>>> Comments?
>>
>> I think this is far more complicated then it needs to be.
>>
>> it sounds like it should be possible to do the following
>>
>> 1. figure out what pages should be backed up (creating a data structure to
>> hold them)
>
> That should be done after step 2, because the memory contents can change
> in this step.

no, this needs to be done by the main kernel, becouse only it knows how to 
find this info. the kernel that you kexec into could be very different 
(including different versions) and the ways to identify this data is not 
part of any existing API

>> 2. kexec into the hibernate kernel (this step handles all device
>> transitions today)
>>
>> 3. have the hibernate userspace find the data structures created in step
>> #1
>>
>> 4. have the hibernate userspace write the pages somewhere in the suspend
>> format.
>
> You don't need to run any hibernate userspace to carry out steps 3 and 4.

you should though.

by doing this write in usespace you can add in all the eye-candy (aka 
progress bars), network I/O, etc that you want since it doesn't affect 
things

trying to do this in the kernel makes the kernel have to know/decide too 
much policy (and many things that people want to do are things that do not 
belong in the kernel in the first place)

>> 5. have the hibernate kernel power down the box.
>>
>> the only things here that sounds like they're not available in stock
>> kernels are steps #1 and #3.
>
> Correct, up to the first remark above.
>
>> now this won't do the fancier suspend-to-ram-and-disk and it won't let you
>> go back from the hibernate kernel to the main kernel, but it should be
>> enough to let you do the suspend safely and reliably.
>>
>> for the restore, as I understand it the process is
>>
>> 1. boot a kernel, any working kernel.
>>
>> 2. read the suspend formatted data from wherever it was saved and feed it
>> to /dev/suspend
>
> Yes, something like this, but you really should pay more attention to devices.
>
> There are things that you shouldn't do to them (like unregistering), because
> some processes may be using them while we're trying to hibernate (for now
> we have the freezer, but I though you'd like to do all that to eliminate it).
>
> Generally, you need to ensure that the devices are handled in consistent ways
> by both the hibernated and image-saving kernels and that's a big piece of the
> jigsaw that's missing now.

the kexec process should handle the device state for the transition from 
one kernel to another (it has to do this now, this isn't a new 
requirement), so this should solve the problem during the hibernate stage.

during the wakeup stage, I thought you said that al that was needed was to 
feed the suspend image to /dev/suspend and the kernel in the suspend image 
would re-probe, or otherwise re-initialize all the devices it needs. am I 
misunderstanding this?

David Lang
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