Re: [linux-pm] suspend blockers & Android integration

[Rafael J. Wysocki]

On Thursday, June 10, 2010, Neil Brown wrote:
> On Thu, 10 Jun 2010 10:59:43 +0200
> "Rafael J. Wysocki" <rjw@xxxxxxx> wrote:
> 
> > On Thursday, June 10, 2010, Neil Brown wrote:
> > > On Wed, 9 Jun 2010 11:40:27 +0200
> > > "Rafael J. Wysocki" <rjw@xxxxxxx> wrote:
> > > 
> > > > On Wednesday 09 June 2010, Felipe Contreras wrote:
> > > > > On Wed, Jun 9, 2010 at 6:46 AM, Linus Torvalds
> > > > > <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> > > > > > On Tue, 8 Jun 2010, david@xxxxxxx wrote:
> > > > > >>
> > > > > >> having suspend blockers inside the kernel adds significant complexity, it's
> > > > > >> worth it only if the complexity buys you enough. In this case the question is
> > > > > >> if the suspend blockers would extend the sleep time enough more to matter. As
> > > > > >> per my other e-mail, this is an area with rapidly diminishing returns as the
> > > > > >> sleep times get longer.
> > > > > >
> > > > > > Well, the counter-argument that nobody seems to have brought up is that
> > > > > > suspend blockers exist, are real code, and end up being shipped in a lot
> > > > > > of machines.
> > > > > >
> > > > > > That's a _big_ argument in favour of them. Certainly much bigger than
> > > > > > arguing against them based on some complexity-arguments for an alternative
> > > > > > that hasn't seen any testing at all.
> > > > > >
> > > > > > IOW, I would seriously hope that this discussion was more about real code
> > > > > > that _exists_ and does what people need. It seems to have degenerated into
> > > > > > something else.
> > > > > >
> > > > > > Because in the end, "code talks, bullshit walks". People can complain and
> > > > > > suggest alternatives all they want, but you can't just argue. At some
> > > > > > point you need to show the code that actually solves the problem.
> > > > > 
> > > > > That's assuming there is an actual problem, which according to all the
> > > > > embedded people except android, there is not.
> > > > 
> > > > Yes, there is, but they've decided to ignore it.
> > > > 
> > > > > And if there is indeed such a problem (probably not big), it might be
> > > > > solved properly by the time suspend blockers are merged, or few
> > > > > releases after.
> > > > 
> > > > Not quite.  Have you followed all of the discussion, actually?
> > > > 
> > > > > Whatever the solution (or workaround) is, it would be nice if it could
> > > > > be used by more than just android people, and it would also be nice to
> > > > > do it without introducing user-space API that *nobody* likes and might
> > > > > be quickly deprecated.
> > > > 
> > > > I agree with Linus and I don't have that much of a problem with the API that
> > > > people seem to have.  In fact the much-hated user space API is just a char
> > > > device driver with 3 ioctls (that can be extended in future if need be) and
> > > > the kernel API is acceptable to me. 
> > > 
> > > I think there is a little bit more to it than that.  It seems there is a new
> > > ioctl for input/event devices to say "Any events queued here should be
> > > treated as wake-up events".  There may be similar additions to other devices,
> > > but I know of no details.
> > 
> > That's not in the patchset as in my pull request.
> > 
> > It is used on Android, though, and it would have been submitted separately, had
> > the first patchset been merged.
> 
> Very true.  But as the one cannot be used without the other, they really need
> to be considered as a package.

You can use suspend blockers as in the pull request without the input patch
in principle.

> > 
> > > I wonder if we can get a complete statement of changes to the user-space
> > > API...
> > > 
> > > >                                       Yes, there is some overlap between it
> > > > and PM QoS, but IMhO that overlap may be reduced over time (eg. by
> > > > using PM QoS requirements to implement suspend blockers). 
> > > > 
> > > > To me, the question boils down to whether or not we're able to persuade the
> > > > Android people to use any other approach (eg. by demonstrating that something
> > > > else is actually better), because even if we invent a brilliant new approach,
> > > > but Android ends up using its old one anyway, the net result will be as though
> > > > we haven't done anything useful.
> > > 
> > > Yes.  There is no point unless we can meet somewhere in the middle.  I think
> > > that would have to include a full suspend that freezes all processes.
> > > Solutions which reject that - while quite clever - would require too much
> > > change to Android user-space to be acceptable.
> > 
> > Moreover, having thought a bit more about the "power manager in user space"
> > concept I'm not sure if it really is that better than the original wakelocks
> > idea.  Namely, it only repaces a kernel-based mechanism with a user space
> > task doing basically the same thing, but the communication between that task
> > and the other cooperating user space tasks is arguably more complicated (it
> > also uses the kernel resources, although indirectly).
> > 
> > So, for a phone-like system, where you'd generally want to simplify user space,
> > having a "power manager" in the kernel seems to make sense to me.
> >
> 
> Following that logic would we end up putting everything in the kernel?

No, I don't think so.

> To my mind the advantage of having something in user-space is flexibility -
> you can refine the interfaces and behaviours without bothering the kernel.
> The reasons for putting things in the kernel are:
>  - tight integration with VM or processes (the two main abstractions that the
>    kernel has to manage)
>  - privileged access to devices
>  - arbitration between processes with different privilege levels

I'd add inter-process communication to this list as well.

> Need-for-speed, on the other hand, is not necessarily a justification for
> going in the kernel - experience shows that we can make user-space
> interactions quite fast enough.

Still, if a kernel-based approach ends up being simpler than a user-space-only
alternative, I tend to prefer the former.  And please remember that in this
particular case all user-space-only alternatives in fact _have_ _to_ use
facilities provided by the kernel anyway, although they can use the existing
ones.

> The "power manager" itself is quite trivial.  It just needs something like:
>     while true
>           wait for all wake-locks to be dropped
>           activate suspend
> and for 'activate suspend' to be interruptible by something taking out a
> wake-lock.

Where the last thing is kind of complicated.

> To my mind, the most interesting part of this is interruptibility.  There is
> essentially only one way to do that in Unix/Linux: signals.

Well, I don't follow. :-)

> So it makes sense to have a task - with a pid - performing this loop so that
> it can be sent a signal.
> And we already have mechanisms for sending signals on all sorts of different
> events, both kernel events (fcntl(F_OWNER)), file events (FNOTIFY), and
> normal process operations (kill).
> 
> If you are going to have a process (which I think you should)

There's no such process in the original patchset and it seemed to get away with
that.

> it makes sense for it to spend some time in user-space to use ipc-of-your-choice to
> communicate with other processes, and spend some time in kernel-space to be
> able to wait for in-kernel locks to be dropped, and to use signals to get out
> of kernel-space, and system calls to get into kernel space.
> 
> Yes, the proposed user-space interface for suspend blocks may not be all that
> horrible.  But the fact is that you can get all the required functionality by
> just using currently existing interfaces.  Against that background, adding
> anything new should be avoided.

Again, if things can be simplified overall by adding something new to the
kernel, I don't see a problem with that in principle.  Especially if that new
stuff doesn't really interfere with anything already existing.

There also is the "real code" argument you seem to totally neglect.  Whatever
you suggest, you should actually implement it and show that it works and that
it really is better than the proposed approach.  Till then, it's just pure
speculation.

So there's real code that I don't really have a problem with and I haven't seen
any alternative implemented and working yet.

Rafael
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