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

["Rafael J. Wysocki" <rjw@sisk.pl>]

On Sunday, 15 July 2007 21:23, david@lang.hm wrote:
> On Sun, 15 Jul 2007, Rafael J. Wysocki wrote:
> 
> >>>> 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
> >
> > If the memory contents changes in step 2, then the information collected by
> > the main kernel will be inaccurate.
> 
> why would the memory use change when the new kernel is run? is it becouse 
> of whatever it does to the devices for the hand-off?

Yes, I think so, although I'm not sure, because I don't know what happens to
devices during a "normal" kexec.

> >>>> 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)
> >
> > Please don't tell me.  I've written uswsusp on the basis of these arguments,
> > but I don't cosider it as an overwhelming success ...
> 
> uswsusp has the problem that you are trying to run some userspace, but not 
> other userspace as you are doing the hibernate. with this approach you 
> don't have that problem since you are running a completely seperate 
> userspace.

You're running it from a RAM disk, because you can't touch filesystems.  You
can do the same with uswsusp, just fine.

> >>>> 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.
> >
> > Well, I don't think so.
> 
> Ok, I could easily be misunderstanding something here (and the comments 
> with the latest 'kexec back and forth' patch indicate there may still be 
> work to do here after all)
> 
> >> 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?
> >
> > Perhaps.  Currently, the hibernated kernel will run device_resume() after
> > the restore, which is not exactly compatible with what kexec does.
> 
> but kexec isn't needed during the restore process, is it?

Generally, it's not needed.  _However_, the current handling of devices is
such that:
(a) hibernated kernel uses device_suspend() to put them into low power states
    and creates the image
(b) hibernated kernel uses device_resume() to get devices back to work and
    saves the image
(c) during the restore the boot kernel loads the image and uses
    device_suspend() to prepare devices for the "old" kernel
(d) hibernated kernel gets control and uses device_resume() to get devices back
    to work.
Now, if you use kexec instead of (a) and (b), then whatever it does to devices
is generally incompatible with the device_resume() in (d) (because, for
instance, some device driver's .resume() routine may expect some data to be
saved by the corresponding .suspend() at specific locations).

Greetings,
Rafael


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