Re: Attempted summary of suspend-blockers LKML thread

[david]

On Wed, 4 Aug 2010, Paul E. McKenney wrote:

> On Wed, Aug 04, 2010 at 03:29:25PM -0700, david@xxxxxxx wrote:
> > On Wed, 4 Aug 2010, Paul E. McKenney wrote:
> > > On Wed, Aug 04, 2010 at 12:29:36PM -0700, david@xxxxxxx wrote:
> > > > On Wed, 4 Aug 2010, Matthew Garrett wrote:
> > > > > There's two parts of that. The first is that the voip application is
> > > > > allowed to take a wakelock - but that doesn't mean that you trust it the
> > > > > rest of the time.
> > > >
> > > > why would you trust it to take a wakelock, but not trust it the rest
> > > > of the time?
> > > >
> > > > in my proposal I'm saying that if you would trust the application to
> > > > take a wakelock, you instead trust it to be sane in the rest of it's
> > > > power activity (avoiding polling, etc) and so you consider it for
> > > > sleep decisions.
> > >
> > > The word "trust" does not appear to be helping here.  ;-)
> > >
> > > The VOIP application acquires a suspend blocker when it needs to prevent
> > > the system from suspending, and releases that suspend blocker when it
> > > can tolerate the system suspending.  It is important to note that while
> > > the VOIP application holds the suspend blocker, the system won't suspend
> > > even if it is completely idle (for example, if the VOIP application uses
> > > blocking system calls, during the time that the VOIP application is
> > > waiting for its next event).
> >
> > In the terminology I have been using, the VOIP sofware is then
> > trusted to take the wakelock appropriately, and I'm then saying it
> > would be in the trusted cgroup
>
> Understood, but...
>
> > it would be trusted to not use the CPU wildly inappropriatly and so
> > it running would make the system active and so it would not sleep.
>
> ... here you seem to be assuming that "trusted to properly use a wakelock"
> implies "coded to optimize power usage when not holding a wakelock."
> But this does not necessarily follow.

I am saying that 'trusted to use a wakelock' does imply 'trusted to not 
waste power'

I am sure that there are apps that do not manage power effectivly, but I 
don't expect that giving those same developers wakelock power will make 
them do much better. I expect that any time they discover the system going 
to sleep on them, they will just add another wakelock to keep it awake. If 
they were the type to carefully consider what was really important and 
what wasn't, I would expect them to write fairly power efficiant code in 
the first place.

> > > 2.	The application is prohibited from acquiring suspend blockers.
> > > 	In this case, the system might well be suspended before the
> > > 	application has a chance to do more than read the input.
> > >
> > > 	But the application will get a chance to process the input
> > > 	when the next input event is directed to it.
> >
> > In this case  the system would go ahead and suspend, but the next
> > time the sustem wakes up for any reason, this application would
> > continue to run and process the input
>
> Yep, that is in fact what I said.  ;-)

so the systems are pretty much equivalent from this point of view.

> > > > two things here,
> > > >
> > > > on the dirty networks that I see as common, refusing to sleep if
> > > > network packets are arriving will mean that you never go to sleep.
> > > >
> > > > secondly, nothing stops the code doing the idle/suspend decision
> > > > from considering network activity. I would be surprised if there
> > > > weren't already options to support this today.
> > >
> > > I don't know about the general networking case for Android, but the
> > > example of downloading was discussed some time back.  The application
> > > doing the download acquires a suspend blocker, which it releases once
> > > the download is complete (or once a timeout expires, if I remember
> > > correctly).  In this particular case, the network packets were not
> > > bringing the device out of suspend.
> >
> > it would seem reasonable to say that if a packet arrives for an
> > existing connection (which the kernel does know) it is considered
> > activity for purposes of sleeping.
>
> That would be up to the people creating the system in question.  In
> some cases, they might (as you say) want every packet arriving to
> wake up the system, in other cases they might not.  We should not
> be taking that design decision away from them.

well, if you have the kernel take a wakelock when the packet arrives, you 
are doing the equivalent thing.

> > I don't know if you would care enough to try and say that packets
> > for untrusted apps  network connections don't keep the system awake,
> > or just allow them to (after all, keypresses going to untrusted apps
> > do keep the system awake)
>
> Again, this is up to the people creating the system in question.  On
> some Android systems, there is a particular button you have to press
> to wake the system up after it has suspended itself.

note that here I am talking about thigns that will keep the system awake, 
not wake it up after it suspends.

> > > There might well be other cases where networking packets -do- bring
> > > the system out of suspend, but I must leave this to someone who knows
> > > more about Android than do I.
> >
> > this would be the normal wake-on-lan type of functionality that
> > exists without Android.
>
> Although this wake-on-LAN functionality applies only to special
> wake-up packets, not to normal packets, right?

that's not my understanding. my understanding (which could be flawed) is 
that wake-on-lan programs your IP into the NIC and if the NIC sees traffic 
for you it will wake you up. I've never had a reason to use it, so I could 
easily be mistaken.

David Lang
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