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    SubjectRe: Attempted summary of suspend-blockers LKML thread
    On Thu, Aug 05, 2010 at 03:51:35PM -0500, kevin granade wrote:
    > On Thu, Aug 5, 2010 at 3:31 PM, Paul E. McKenney
    > <paulmck@linux.vnet.ibm.com> wrote:
    > > On Thu, Aug 05, 2010 at 01:13:31PM -0500, kevin granade wrote:
    > >> On Thu, Aug 5, 2010 at 10:46 AM,  <david@lang.hm> wrote:
    > >> > On Thu, 5 Aug 2010, Paul E. McKenney wrote:
    > >> >
    > >> >> On Wed, Aug 04, 2010 at 10:18:40PM -0700, david@lang.hm wrote:
    > >> >>>
    > >> >>> On Wed, 4 Aug 2010, Paul E. McKenney wrote:
    > >> >>>>
    > >> >>>> On Wed, Aug 04, 2010 at 05:25:53PM -0700, david@lang.hm wrote:
    > >> >>>>>
    > >> >>>>> On Wed, 4 Aug 2010, Paul E. McKenney wrote:
    > >> >>
    > >> >> [ . . . ]
    > >> >>
    > >> >>>>>> The music player is an interesting example.  It would be idle most
    > >> >>>>>> of the time, given that audio output doesn't consume very much CPU.
    > >> >>>>>> So you would not want to suspend the system just because there were
    > >> >>>>>> no runnable processes.  In contrast, allowing the music player to
    > >> >>>>>> hold a wake lock lets the system know that it would not be appropriate
    > >> >>>>>> to suspend.
    > >> >>>>>>
    > >> >>>>>> Or am I misunderstanding what you are proposing?
    > >> >>>>>
    > >> >>>>> the system would need to be idle for 'long enough' (configurable)
    > >> >>>>> before deciding to suspend, so as long as 'long enough' is longer
    > >> >>>>> than the music player is idle this would not be a problem.
    > >> >>>>
    > >> >>>> From a user standpoint, having the music player tell the system when
    > >> >>>> it is OK to suspend (e.g., when the user has paused playback) seems
    > >> >>>> a lot nicer than having configurable timeouts that need tweaking.
    > >> >>>
    > >> >>> every system that I have seen has a configurable "sleep if it's idle
    > >> >>> for this long" knob. On the iphone (work issue, I didn't want it)
    > >> >>> that I am currently using it can be configured from 1 min to 5 min.
    > >> >>>
    > >> >>> this is the sort of timeout I am talking about.
    > >> >>>
    > >> >>> with something in the multi-minute range for the 'do a full suspend'
    > >> >>> doing a wakeup every few 10s of seconds is perfectly safe.
    > >> >>
    > >> >> Ah, I was assuming -much- shorter "do full suspend" timeouts.
    > >> >>
    > >> >> My (possibly incorrect) assumption is based on the complaint that led
    > >> >> to my implementing RCU_FAST_NO_HZ.  A (non-Android) embedded person was
    > >> >> quite annoyed (to put it mildly) at the earlier version of RCU because
    > >> >> it prevented the system from entering the power-saving dyntick-idle mode,
    > >> >> not for minutes, or even for seconds, but for a handful of -milliseconds-.
    > >> >> This was my first hint that "energy efficiency" means something completely
    > >> >> different in embedded systems than it does in the servers that I am
    > >> >> used to.
    > >> >>
    > >> >> But I must defer to the Android guys on this -- who knows, perhaps
    > >> >> multi-minute delays to enter full-suspend mode are OK for them.
    > >> >
    > >> > if the system was looking at all applications I would agree that the timeout
    > >> > should be much shorter.
    > >> >
    > >> > I have a couple devices that are able to have the display usable, even if
    > >> > the CPU is asleep (the OLPC and the Kindle, two different display
    > >> > technologies). With these devices I would like to see the suspend happen so
    > >> > fast that it can suspend between keystrokes.
    > >> >
    > >> > however, in the case of Android I think the timeouts have to end up being
    > >> > _much_ longer. Otherwise you have the problem of loading an untrusted book
    > >> > reader app on the device and the device suspends while you are reading the
    > >> > page.
    > >> >
    > >> > currently Android works around this by having a wakelock held whenever the
    > >> > display is on. This seems backwards to me, the display should be on because
    > >> > the system is not suspended, not the system is prevented from suspending
    > >> > because the display is on.
    > >> >
    > >> > Rather than having the display be on causing a wavelock to be held (with the
    > >> > code that is controls the display having a timeout for how long it leaves
    > >> > the display on), I would invert this and have the timeout be based on system
    > >> > activity, and when it decides the system is not active, turn off the display
    > >> > (along with other things as it suspends)
    > >>
    > >> IIRC, this was a major point of their (Android's) power management
    > >> policy.  User input of any kind would reset the "display active"
    > >> timeout, which is the primary thing keeping random untrusted
    > >> user-facing programs from being suspended while in use.  They seemed
    > >> to consider this to be a special case in their policy, but from the
    > >> kernel's point of view it is just another suspend blocker being held.
    > >>
    > >> I'm not sure this is the best use case to look at though, because
    > >> since it is user-facing, the timeout durations are on a different
    > >> scale than the ones they are really worried about.  I think another
    > >> category of use case that they are worried about is:
    > >>
    > >> (in suspend) -> wakeup due to network -> process network activity -> suspend
    > >>
    > >> or an example that has been mentioned previously:
    > >>
    > >> (in suspend) -> wakeup due to alarm for audio processing -> process
    > >> batch of audio -> suspend
    > >>
    > >> In both of these cases, the display may never power on (phone might
    > >> beep to indicate txt message or email, audio just keeps playing), so
    > >> the magnitude of the "timeout" for suspending again should be very
    > >> small.  Specifically, they don't want there to be a timeout at all, so
    > >> as little time as possible time is spent out of suspend in addition to
    > >> the time required to handle the event that caused wakeup.
    > >
    > > It would be good to get some sort of range for the "timeout".  In the
    > > audio-output case, my understanding that the spacing between bursts of
    > > audio-processing activity is measured in some hundreds of milliseconds,
    > > in which case one would want the delays until suspend to be on the
    > > millisecond scale.  But does Android really suspend between bursts of
    > > audio processing while playing music?  Very cool if so!  ;-)
    >
    > Oops, yea that's actually a really bad example, that's probably
    > something that would be handled by low-power states. I think the
    > incoming text message example is a good one though. There seemed to
    > be a focus on user-interaction scale time scales, and I wanted to
    > point out that there are also very short duration time scales to
    > consider as well.
    >
    > *back to lurking*

    I really don't know the answer myself, so I was really asking the
    question rather than trying to catch you out.

    Thanx, Paul

    > Kevin
    >
    > >
    > >                                                        Thanx, Paul
    > >
    > >> >>>>>>> if the backlight being on holds the wakelock, it would seem that
    > >> >>>>>>> almost every other use of the wakelock could (and probably should)
    > >> >>>>>>> be replaced by something that tickles the display to stay on longer.
    > >> >>>>>>
    > >> >>>>>> The problem with this approach is that the display consumes quite a
    > >> >>>>>> bit of power, so you don't want to leave it on unnecessarily.  So if
    > >> >>>>>> the system is doing something (for example, playing music) that does
    > >> >>>>>> not require the display, you really want the display to be off.
    > >> >>>>>
    > >> >>>>> what percentage (and types) of apps are really useful with the
    > >> >>>>> display off. I think that there are relativly few apps that you
    > >> >>>>> really want to keep running if the display is off.
    > >> >>>>
    > >> >>>> The length of time those apps are running is the governing factor
    > >> >>>> for battery life, and not the number of such apps, right?
    > >> >>>
    > >> >>> correct, but the number of such apps indicates the scope of the problem.
    > >> >>
    > >> >> The number of such apps certainly indicates the amount of effort required
    > >> >> to modify them, if required.  Is that what you are getting at?
    > >> >
    > >> > yes.
    > >> >
    > >> >>>> From another e-mail tonight it sounds like almost everything
    > >> >>>> already talks
    > >> >>>
    > >> >>> to a userspace daemon, so if "(the power management service in the
    > >> >>> system_server, possibly the media_server and the radio interface
    > >> >>> glue)" (plus possibly some kernel activity) are the only things
    > >> >>> looked at when considering if it's safe to sleep or not, all of
    > >> >>> these can (or already do) do 'something' every few seconds, making
    > >> >>> this problem sound significantly smaller than it sounded like
    > >> >>> before.
    > >> >>>
    > >> >>> Android could even keep it's user-space API between the system power
    > >> >>> daemon and the rest of userspace the same if they want to.
    > >> >>>
    > >> >>> over time, additional apps could be considered 'trusted' (or flagged
    > >> >>> that way by the user) and not have to interact with the power daemon
    > >> >>> to keep things alive.
    > >> >>
    > >> >> Hmmm...  Isn't it the "trusted" (AKA PM-driving) apps that interact with
    > >> >> the power daemon via suspend blockers, rather than the other way around?
    > >> >
    > >> > I was looking at it from a kernel point of view, "trusted" (AKA PM-driving)
    > >> > apps are ones that have permission to grab the wakelock. Any app/daemon that
    > >> > is so trusted can communicate with anything else in userspace as part of
    > >> > making it's decision on whento take the wakelock, but those other
    > >> > applications would not qualify as "trusted" in my eyes.
    > >> >
    > >> >>> as for intramentation, the key tool to use to see why a system isn't
    > >> >>> going to sleep would be powertop, just like on other linux systems.
    > >> >>
    > >> >> Powertop is indeed an extremely valuable tool, but I am not certain
    > >> >> that it really provides the information that the Android guys need.
    > >> >> If I understand Arve's and Brian's posts, here is the scenario that they
    > >> >> are trying to detect:
    > >> >>
    > >> >> o       Some PM-driving application has a bug in which it fails to
    > >> >>        release a wakelock, thus blocking suspend indefinitely.
    > >> >>
    > >> >> o       This PM-driving application, otherwise being a good citizen,
    > >> >>        blocks.
    > >> >>
    > >> >> o       There are numerous power-oblivious apps running, consuming
    > >> >>        significant CPU.
    > >> >>
    > >> >> What the Android developers need to know is that the trusted application
    > >> >> is wrongly holding a wakelock.  Won't powertop instead tell them about
    > >> >> all the power-oblivious apps?
    > >> >
    > >> > in my proposal (without a wakelock), powertop would tell you what
    > >> > applications are running and setting timers. If we can modify the
    > >> > kernel/suspend decision code to only look at processes in one cgroup when
    > >> > deciding if the system should go to sleep, a similar modification to
    > >> > poewrtop should let you only show stats on the "trusted" applications.
    > >> >
    > >> > If you have a userspace power management daemon that accepts requests from
    > >> > untrusted programs and does something to keep the system from sleeping
    > >> > (either taking a wakelock or setting a 'short' timer), it needs to keep the
    > >> > records of this itself because otherwise all the kernel will see (with
    > >> > either powertop or wakelock reporting) is that the power management daemon
    > >> > is what kept the system from sleeping.
    > >> >
    > >> > David Lang
    > >> > --
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    > >
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