Message-ID: <Pine.LNX.4.64.0707130227310.25614@asgard.lang.hm>
Date:	Fri, 13 Jul 2007 02:38:06 -0700 (PDT)
From:	david@...g.hm
To:	"Rafael J. Wysocki" <rjw@...k.pl>
cc:	"Huang, Ying" <ying.huang@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Pavel Machek <pavel@....cz>, nigel@...el.suspend2.net,
	Jeremy Maitin-Shepard <jbms@....edu>,
	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 Fri, 13 Jul 2007, Rafael J. Wysocki wrote:

> On Friday, 13 July 2007 05:06, david@...g.hm wrote:
>> On Thu, 12 Jul 2007, Rafael J. Wysocki wrote:
>>
>>> On Thursday, 12 July 2007 20:57, david@...g.hm wrote:
>>>> On Thu, 12 Jul 2007, Rafael J. Wysocki wrote:
>>>>
>>>>>> 2. Do not reserve memory for kexec kernel. That is, backup needed memory
>>>>>> before kexec and restore them after kexec.
>>>>>
>>>>> I don't think this is very important initially.
>>>>
>>>> I agree, a stipped down hibernate kernel can be very small, not allocating
>>>> this memory until it's needed is a step for the final polishing.
>>>
>>> I'm not sure if I agree with that.  In any case, having to use two different
>>> kernels for hibernation would be a big drawback.
>>
>> I see it as a big advantage to not have to use the main kernel for the
>> suspend. please keep it as an option at least.
>
> That depends on what we would like to support.  For example, we may need to use
> the same kernel for the suspend-to-disk-and-RAM feature.

I missed this discussion. is this idea to suspend, write to disk, but 
leave things in ram so that if you wakeup soon enough you have everything 
for ram, but if you don't and the battery dies you can restore from disk?

if so I think it's a mistake to mix the two. it would be better to just 
suspend to ram, and wake up once in a while to check the battery state and 
when the battery gets low enough do the suspend to disk.

otherwise you end up mixing the requirements of the two types of suspend, 
which is how things got so ugly in the first place.

>>>>>> 5. A smooth resume process. Maybe it is not needed to kexec a new kernel
>>>>>> for resume. For example, in the first stage of kernel boot, just first
>>>>>> 16M (or a little more) RAM is used, if the resume image is found, the
>>>>>> saved kernel image is resumed; if the resume image is not found, turn on
>>>>>> the remaining RAM. This will depends on 3.
>>>>>
>>>>> I think that this is the most difficult part of the whole thing.
>>>>
>>>> don't try to get too fancy right now. stick with a simple 'boot hibernate
>>>> kernel, it's userspace looks for an image to resume, and if it doesn't
>>>> find one reboot to the normal system'
>>>>
>>>> I don't know how to do this with grub, but it would be a trivial shell
>>>> script with lilo
>>>
>>> I think it's most portable to use initrd for that, which already makes things
>>> complicated.  Then, we'll have to load the image and jump to the hibernated
>>> kernel in such a way that it would be able to continue from where it stopped
>>> before.  I don't think that is trivial.
>>
>> I was talking about the scripts that would be used inside the initrd (or
>> boot partition of whatever type)
>>
>> to start with don't worry about how the kexec kernel gets it's /
>> filesystem (for testing just use a real partition on your disk. after you
>> get everything working let the people who really understand the initrd and
>> consider it trivial switch it to an initrd image)
>
> Well, my experience shows that you need to do everything yourself up to a
> certain point.

true, but if you get something that can work reliably, even if ugly, you 
get a lot more people willing to polish it then is you are asking them to 
help you implement the core features.

remember release early, release often (with something that functions)

>> fo rthe current stage where we are trying to make things work don't worry
>> about packaging everything tight with initrd and re-useing partitions or
>> kernel images. once everything is working reliably then it's time to look
>> at useing the same kernel for multiple functions, writing to a partition
>> that's i use for other things, etc
>
> I don't agree.  You need to think of many limitations in advance, because
> they need to be taken into consideration in the design.
>
> Otherwise we'll end up with something that will need to be bandaided like the
> freezer. :-)

on the other hand, worrying about all the possible ways to do things can 
paralize you.

the big advantage of the kexec approach is that the new userspace that's 
setup with the new kernel can do _anything_. if/when this works you will 
see people doing things that you probably never imagined (a simple one is 
to suspend a machine at work, send the image over the network and resume 
on a different machine at home). and all these strange things are 
encapsulated so that you don't have to worry about how they will be done 
now.

it's good to try and find the places where you have fundamental changes to 
make to support them, but there are a lot of things that boil down to 
implementation details, everyone agrees that it can be done reliably, the 
decisions on how just need to be made.

David Lang
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