Message-ID: <20111024104444.09337fe6@notabene.brown>
Date:	Mon, 24 Oct 2011 10:44:44 +1100
From:	NeilBrown <neilb@...e.de>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	Linux PM list <linux-pm@...r.kernel.org>,
	mark gross <markgross@...gnar.org>,
	LKML <linux-kernel@...r.kernel.org>,
	John Stultz <john.stultz@...aro.org>,
	Alan Stern <stern@...land.harvard.edu>
Subject: Re: [RFC][PATCH 0/2] PM / Sleep: Extended control of
 suspend/hibernate interfaces

On Sun, 23 Oct 2011 15:16:36 +0200 "Rafael J. Wysocki" <rjw@...k.pl> wrote:

> On Sunday, October 23, 2011, NeilBrown wrote:
> > On Sun, 23 Oct 2011 00:07:33 +0200 "Rafael J. Wysocki" <rjw@...k.pl> wrote:
> > 
> > > On Tuesday, October 18, 2011, NeilBrown wrote:

> > > > > 
> > > > > > With that problem solved, experimenting is much easier in user-space than in
> > > > > > the kernel.
> > > > > 
> > > > > Somehow, I'm not exactly sure if we should throw all kernel-based solutions away
> > > > > just yet.
> > > > 
> > > > My rule-of-thumb is that we should reserve kernel space for when
> > > >   a/ it cannot be done in user space
> > > >   b/ it cannot be done efficient in user space
> > > >   c/ it cannot be done securely in user space
> > > > 
> > > > I don't think any of those have been demonstrated yet.  If/when they are it
> > > > would be good to get those kernel-based solutions out of the draw (so yes:
> > > > keep them out of the rubbish bin).
> > > 
> > > I have one more rule.  If my would-be user space solution has the following
> > > properties:
> > > 
> > > * It is supposed to be used by all of the existing variants of user space
> > >   (i.e. all existing variants of user space are expected to use the very same
> > >   thing).
> > > 
> > > * It requires all of those user space variants to be modified to work with it
> > >   correctly.
> > > 
> > > * It includes a daemon process having to be started on boot and run permanently.
> > > 
> > > then it likely is better to handle the problem in the kernel.
> > 
> > By that set or rules, upowerd, dbus, pulse audio, bluez, and probably systemd
> > all need to go in the kernel.  My guess is that you might not find wide
> > acceptance for these rules.
> 
> Well, that's not what I thought.  Perhaps I didn't express that precisely
> enough.  Take systemd, for example.  You still can design and use a Linux-based
> system without systemd, so there's no requirement that _all_ variants of user
> space use the given approach.  The choice of whether or not to use systemd
> is not a choice between a working and non-working system.
> 
> However, this is not the case with the system daemon, becuase it's supposed
> to handle problems that aren't possible to address without it.  So either you
> use it, or you end up with a (slightly) broken system.

I think you are seeing a distinction that isn't there.

Every system needs a process to run as 'init' - as PID == 1.
It might be systemd, it might be sysv-init, it might be /bin/sh, but there
are tasks that process much perform and there must be exactly one process
performing those tasks and the test of the systems need to be able to work
with that task (or ignore if it it is wholely independent).

Similarly every system need one process to manage suspend.  It can be my
daemon or your daemon or Alan's daemon but it cannot be 2 or more of them
running at the same time as that doesn't make any more sense than having
systemd and init running at the same time.


>  
> > > > So I'd respond with "I'm not at all sure that we should throw away an
> > > > all-userspace solution just yet".  Particularly because many of us seem to
> > > > still be working to understand what all the issues really are.
> > > 
> > > OK, so perhaps we should try to implement two concurrent solutions, one
> > > kernel-based and one purely in user space and decide which one is better
> > > afterwards?
> > 
> > Absolutely.
> > 
> > My primary reason for entering this discussion is eloquently presented in
> >        http://xkcd.com/386/
> > 
> > Someone said "We need to change the kernel to get race-free suspend" and this
> > simply is not true.  I wanted to present a way to use the existing
> > functionality to provide race-free suspend - and now even have code to do it.
> > 
> > If someone else wants to write a different implementation, either in
> > userspace or kernel that is fine.
> > 
> > They can then present it as "I know this can be implemented in userspace, but
> > I don't like that solution for reasons X, Y, Z and so here is my better
> > kernel-space implementation" then that is cool.  We can examine X, Y, Z and
> > the code and see if the argument holds up.  Maybe it will, maybe not.
> > 
> > So far the only arguments I've seen for putting the code in the kernel are:
> > 
> >  1/ it cannot be done in userspace - demonstrably wrong
> 
> I'm not sure if that's correct.  If you meant "it can be done in user space
> without _any_ kernel modifications", I probably wouldn't agree.

I have code to do it correctly today with no kernel modifications.  It is
called "lsusd".   Proof by example.  Or can you show that lsusd doesn't work
correctly?


> 
> >  2/ it is more efficient in the kernel - not demonstrated or even
> >     convincingly argued
> 
> I don't agree with that, but let's see.

If you don't agree, then you presumably have a demonstration or a convincing
argument.  Can you share it?

> 
> >  3/ doing it in user-space is too confusing - we would need a clear
> >     demonstration that a kernel interface is less confusing - and still
> >     correct.  Also the best way to remove confusion is with clear
> >     documentation and sample code, not by making up new interfaces.
> 
> The user space solution makes up new interfaces too, although they are
> confined to user space.
> 
> To me, it all boils down to two factors: (1) the complexity and efficiency
> of the code needed to implement the feature and (2) the complexity of the
> resulting framework (be it in the kernel or in user space).
> 
> >  4/ doing it in the kernel makes it more accessible to multiple desktops.
> >     The success of freedesktop.org seems to contradict that.
> 
> I don't agree here too.  Is Android a member of freedesktop.org?
>

This is completely irrelevant.

The "multiple desktops" issue that you brought up is (as I understand it)
multiple desktops running on the same computer, whether concurrently or
sequentially.
Android simply does not face that issue - it is the only "desktop" and is in
complete control of the machine it runs on.
So it doesn't need to solve the issue, so it doesn't need to be a member of
freedesktop.org.


> > So if you can do it a "better" way, please do.  But also please make sure
> > you can quantify "better".   I claim that user-space solutions are "better"
> > because they are more flexible and easier to experiment with.  The "no
> > regressions" rule actively discourages experimentation in the kernel so
> > people should only do it if there is a clear benefit.
> 
> You seem to suppose that every kernel modification necessarily has a potential
> to lead to some regressions.  I'm not exactly use if that's correct
> (e.g. adding a new driver usually doesn't affect people who don't need it).

I think that experimenting in the kernel (or at least in the upstream kernel)
is likely to result in creating functionality that ultimately will
not get used - the whole point of experimenting is that you probably get it
wrong the first time.
If this happens we either:
  - remove the unwanted functionality, which could be considered a regression
    and so must be done very carefully
  - leave the unwanted functionality there thus creating clutter and a
    maintenance burden.

i.e. the point of the "no-regressions" reference is that it tends to make it
harder to remove mistakes.  Not impossible of course, but it requires a lot
more care and time.

So I am against adding code to the kernel until the problem is really well
understood.  From the sorts of discussion that has been going on both in
this thread and elsewhere I'm not convinced the problem really is well
understood at all.
I think we are very much at the stage where people should be experimenting
with solutions, sharing the results, and learning.

So please feel free to publish sample code - whether for the kernel or for
user-space.  But it will only be credible if it is a fairly complete
proposal - e.g. with sample code demonstrating how the kernel features are
used.

(my lsusd really needs a 'plugin' for pm_utils to get it to communicate with
lsusd rather than writing to /sys/power/state ... I should probably add
that.  Then it would be complete and usable on current desktops).



> 
> > User-space solutions are much easier to introduce and then deprecate.
> 
> That's demonstrably incorrect and the counter example is the hibernation user
> space interface.  The sheer amount of work needed to implement user
> space-driven hibernation and maintain that code shows that it's not exactly
> easy and it would be more difficult to deprecate than many existing kernel
> interfaces at this point.
> 
> So, even if you have implemented something in user space, the "no regressions"
> rule and deprecation difficulties will apply to it as well as to the kernel as
> soon as you make a sufficient number of people use it.

Can we agree then that we shouldn't impose any part of a possible solution on
anyone until it has been sensibly tested and reviewed in a variety of
different use cases and found to be reliable and usable?

I think that addresses my main concern with kernel-space additions - I fear
that parts of them will end up unnecessary and unused but we will be stuck
with them.

Thanks,
NeilBrown


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