Re: Attempted summary of suspend-blockers LKML thread

From: Paul E. McKenney
Date: Wed Aug 04 2010 - 19:06:56 EST

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:
> >>>On Wed, Aug 04, 2010 at 12:15:59PM -0700, david@xxxxxxx wrote:
> >>>>On Wed, 4 Aug 2010, Matthew Garrett wrote:
> >>>>>No! And that's precisely the issue. Android's existing behaviour could
> >>>>>be entirely implemented in the form of binary that manually triggers
> >>>>>suspend when (a) the screen is off and (b) no userspace applications
> >>>>>have indicated that the system shouldn't sleep, except for the wakeup
> >>>>>event race. Imagine the following:
> >>>>>
> >>>>>1) The policy timeout is about to expire. No applications are holding
> >>>>>wakelocks. The system will suspend providing nothing takes a wakelock.
> >>>>>2) A network packet arrives indicating an incoming SIP call
> >>>>>3) The VOIP application takes a wakelock and prevents the phone from
> >>>>>suspending while the call is in progress
> >>>>>
> >>>>>What stops the system going to sleep between (2) and (3)? cgroups don't,
> >>>>>because the voip app is an otherwise untrusted application that you've
> >>>>>just told the scheduler to ignore.
> >>>>
> >>>>Even in the current implementation (wakelocks), Since the VOIP
> >>>>application isn't allowed to take a wakelock, wouldn't the system go to
> >>>>sleep immediatly anyway, even if the application gets the packet and
> >>>>starts the call? What would ever raise the wakelock to keep the phone
> >>>>from sleeping in the middle of the call?
> >>>
> >>>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...

> >>>The second is that the incoming network packet causes
> >>>the kernel to take a wakelock that will be released once userspace has
> >>>processed the network packet. This ensures that at least one wakelock is
> >>>held for the entire relevant period of time.
> >>
> >>how do you determine that userspace has processed the network packet
> >>so that the kernel can release the wakelock (or is this one of the
> >>cases where there is a timer related to the wakelock)
> >
> >There are two cases:
> >
> >1. The application is permitted to acquire suspend blockers.
> > In this case, the application would acquire a suspend blocker
> > before reading the input. It would then read the input (at
> > which point the kernel releases its suspend blocker), do any
> > needed processing, and finally release the suspend blocker.
> >
> > So in this case, the system knows that the application is
> > done processing the input when that application releases
> > its suspend blocker.
> in my proposal, the application is trusted to take the wakelock, so
> 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.

> >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. ;-)

> >>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.

> 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.

> >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?

Thanx, Paul

> The primary thing that I was getting at was the other things above.
> David Lang
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