Message-Id: <200707201317.58025.rjw@sisk.pl>
Date:	Fri, 20 Jul 2007 13:17:57 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	david@...g.hm
Cc:	Milton Miller <miltonm@....com>,
	linux-pm <linux-pm@...ts.linuxfoundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Alan Stern <stern@...land.harvard.edu>,
	"Huang, Ying" <ying.huang@...el.com>,
	Jeremy Maitin-Shepard <jbms@....edu>
Subject: Re: [linux-pm] Re: Hibernation considerations

On Friday, 20 July 2007 01:07, david@...g.hm wrote:
> On Thu, 19 Jul 2007, Rafael J. Wysocki wrote:
> 
> > On Thursday, 19 July 2007 17:46, Milton Miller wrote:
> >>
> >> The currently identified problems under discussion include:
> >> (1) how to interact with acpi to enter into S4.
> >> (2) how to identify which memory needs to be saved
> >> (3) how to communicate where to save the memory
> >> (4) what state should devices be in when switching kernels
> >> (5) the complicated setup required with the current patch
> >> (6) what code restores the image
> >
> > (7) how to avoid corrupting filesystems mounted by the hibernated kernel
> 
> I didn't realize this was a discussion item. I thought the options were 
> clear, for some filesystem types you can mount them read-only, but for 
> ext3 (and possilby other less common ones) you just plain cannot touch 
> them.

That's correct.  And since you cannot thouch ext3, you need either to assume
that you won't touch filesystems at all, or to have a code to recognize the
filesystem you're dealing with.

> >>> (2) Upon start-up (by which I mean what happens after the user has
> >>> pressed
> >>>     the power button or something like that):
> >>>   * check if the image is present (and valid) _without_ enabling ACPI
> >>> (we don't
> >>>     do that now, but I see no reason for not doing it in the new
> >>> framework)
> >>>   * if the image is present (and valid), load it
> >>>   * turn on ACPI (unless already turned on by the BIOS, that is)
> >>>   * execute the _BFS global control method
> >>>   * execute the _WAK global control method
> >>>   * continue
> >>>   Here, the first two things should be done by the image-loading
> >>> kernel, but
> >>>   the remaining operations have to be carried out by the restored
> >>> kernel.
> >>
> >> Here I agree.
> >>
> >> Here is my proposal.  Instead of trying to both write the image and
> >> suspend, I think this all becomes much simpler if we limit the scope
> >> the work of the second kernel.  Its purpose is to write the image.
> >> After that its done.   The platform can be powered off if we are going
> >> to S5.   However, to support suspend to ram and suspend to disk, we
> >> return to the first kernel.
> >
> > We can't do this unless we have frozen tasks (this way, or another) before
> > carrying out the entire operation.  In that case, however, the kexec-based
> > approach would have only one advantage over the current one.  Namely, it
> > would allow us to create bigger images.
> 
> we all agree that tasks cannot run during the suspend-to-ram state, but 
> the disagreement is over what this means
> 
> at one extreme it could mean that you would need the full freezer as per 
> the current suspend projects.
> 
> at the other extreme it could mean that all that's needed is to invoke the 
> suspend-to-ram routine before anything else on the suspended kernel on the 
> return from the save and restore kernel.
> 
> we just need to figure out which it is (or if it's somewhere in between).

Well, I think that the "invoke the suspend-to-ram routine before anything else
on the suspended kernel" thing won't be easy to implement in practice.

> >>> It's selectively stopping kernel threads, which is just about right.
> >>> If you
> >>> that _this_ is a main problem with the freezer, then think again.
> >>>
> >>>> with kexec you don't need to let any portion of the origional kernel
> >>>> or
> >>>> userspace operate so you don't have a problem.
> >>>
> >>> In fact, the main problem with the freezer is that it is a
> >>> coarse-grained
> >>> solution.  Therefore, what I believe we should do is to evolve in the
> >>> directoin
> >>> of more fine-grained solutions and gradually phase out the freezer.
> >>>
> >>> The kexec-based approach is an attempt to replace one coarse-grained
> >>> solution
> >>> (the freezer) with even more coarse-grained solution (stopping the
> >>> entire
> >>> kernel with everything), which IMO doesn't address the main problem.
> >>>
> >>
> >> I think this addresses teh problem.   Its probably a bit harder than
> >> powermac because we have to fully quiesce devices; we can't cheat by
> >> leaving interrupts off.   But once the drivers save the state of their
> >> devices and stop their queues, it should be easy to audit the paths to
> >> powerdown devices and call the platform suspend and ram wakeup paths.
> >>
> >>
> >> Going back to the requirements document that started this thread:
> >>
> >> Message-ID: <200707151433.34625.rjw@...k.pl>
> >> On Sun Jul 15 05:27:03 2007, Rafael J. Wysocki wrote:
> >>> (1) Filesystems mounted before the hibernation are untouchable
> >>
> >> This is because some file systems do a fsck or other activity even when
> >> mounted read only.  For the kexec case, however, this should be "file
> >> systems mounted by the hibernated system must not be written".   As has
> >> been mentioned in the past, we should be able to use something like dm
> >> snapshot to allow fsck and the file system to see the cleaned copy
> >> while not actually writing the media.
> >
> > We can't _require_ users to use the dm snapshot in order for the hibernation
> > to work, sorry.
> >
> > And by _reading_ from a filesystem you generally update metadata.
> 
> not if the filesystem is mounted read-only (except on ext3)

Well, if the filesystem in question is a journaling one and the hibernated
kernel has mounted this fs read-write, this seems to be tricky anyway.
 
> >> The kjump kernel must not have any knowledge retained if we reuse it.
> >>
> >>> (2) Swap space in use before the hibernation must be handled with care
> >>
> >> Yes.  Actually, even though they have been used by the write-in-the
> >> kernel users, they will be among the most difficult devices to use for
> >> snapshots by a userspace second kernel.
> >>
> >>> (3) There are memory regions that must not be saved or restored
> >>
> >> because they may not exist.   This means that we must identify the
> >> memory to be saved and restored in a format to be passed between the
> >> kernel.
> >>
> >>> (4) The user should be able to limit the size of a hibernation image
> >>
> >> This means the suspending kernel must arrange to reduce its active
> >> memory.  The limited save can be done by providing a limited list in
> >> (3).
> >
> > It seems to me that you don't understand the problem here.
> >
> > Assume you have 90% of RAM allocated before the hibernation and the user has
> > requested the image to be not greater than 50% of RAM.  In that case you have
> > to free some memory _before_ identifying memory to save and you must not
> > race with applications that attempt to allocate memory while you're doing it.
> 
> I disagree a little bit.
> 
> first off, only the suspending kernel can know what can be freed and what 
> is needed to do so (remember this is kernel internals, it can change from 
> patch to patch, let alone version to version)
> 
> second, if you have a lot of memory to free, and you can't just throw away 
> caches to do so, you don't know what is going to be involved in freeing 
> the memory, it's very possilbe that it is going to involve userspace, so 
> you can't freeze any significant portion of the system, so you can't 
> eliminate all chance of races
> 
> what you can do is
> 
> 1. try to free stuff
> 2. stop the system and account for memory, is enough free
> if not goto 1
> 
> if userspace is dirtying memory fast enough, or is just useing enough 
> memory that you can't meet your limit you just won't be able to suspend.

This means unreliable hibernation for some workloads.  While I agree that
shouldn't be a problem in a common case, there are users who will complain. ;-)

> but under any other conditions you will eventually get enough memory free.
> 
> so try several times and if you still fail tell the user they have too 
> much stuff running and they need to kill something.

Well, with the freezer that's much simpler (and more reliable, I'd say): you
freeze tasks and _then_ you shrink memory.

> >>> (6) State of devices from before hibernation should be restored, if
> >>> possible
> >>
> >> related to suspend should be transparent ... yes.
> >>
> >>> (7) On ACPI systems special platform-related actions have to be
> >>> carried out at
> >>>     the right points, so that the platform works correctly after the
> >>> restore
> >>
> >> I believe I have explained my suggestion.
> >>
> >>> (8) Hibernation and restore should not be too slow
> >>
> >> We control the added code.   We are using full runtime drivers and will
> >> run at hardware speeds.
> >
> > That may not be enough.  If you're going to save, say, 80% of RAM on a 2 GB
> > machine, then you'll have to be using image compression.
> 
> this doesn't make sense, 20% of 2G is 400M, if you can't make a kernel and 
> userspace that can run in 400M you have a serious problem.

I was talking about the _speed_ of writing and reading.

> even if you wanted to save 99% of RAM on a 2G system, you have 20M of ram 
> to play with, which should easily be enough.
> 
> remember, linux runs on really small systems as well, and while you do 
> have to load some drivers for the big system, there are a lot of other 
> things that aren't needed.
> 
> > All in all, we have three different and working implementation of the
> > image-writing and image-reading code at our disposal.  Why would you want to
> > break the open doors?
> 
> becouse you say that the current methods won't work without ACPI support.

I didn't say that.  [Or if I did, please point me to this message.]

Anyway, this wouldn't be true even if I did.

What I've been trying to say from the very beginning is that the current
frameworks _support_ hibernation a la ACPI S4 (although that's not exactly
ACPI S4) and if we are going to introduce a new framework, then it should
be designed to _support_ ACPI S4 fully _from_ _the_ _start_.

This DOESN'T mean that the non-ACPI hibernation should be unsupported and
it DOESN"T mean that the non-ACPI hibernation is not supported currently.
IT IS SUPPORTED.

Greetings,
Rafael


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