Message-Id: <200707142306.33783.rjw@sisk.pl>
Date:	Sat, 14 Jul 2007 23:06:32 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	david@...g.hm
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 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.

> 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.
 
> 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.

Greetings,
Rafael


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