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    SubjectAttempted summary of suspend-blockers LKML thread, take two
    Continuing to rush in where angels fear to tread...

    This is an updated version of my list posted a couple of days ago at
    http://lkml.org/lkml/2010/7/31/73. Again, this email is an attempt
    to present the Android guys' requirements, based on my interpretation
    of LKML discussions.

    Please note that I am not proposing a solution that meets these
    requirements, nor am I attempting to judge the various proposed solutions.
    In fact, I am not even trying to judge whether the requirements are
    optimal, or even whether or not they make sense at all. My only goal
    at the moment is to improve my understanding of what the Android folks'
    requirements are. That said, I do discuss example mechanisms where
    needed to clarify the meaning of the requirements. This should not be
    interpreted as a preference for any given example mechanism.

    But first I am going to look at nomenclature, as it appears to me that
    at least some of the flamage was due to conflicting definitions.

    Ducking into the nearest bunker to avoid the hailstorm of frozen fish...

    Thanx, Paul

    ------------------------------------------------------------------------

    DEFINITIONS

    These have been updated based on LKML and linux-pm discussions. The names
    are probably still sub-optimal, but incremental progress is nevertheless
    a very good thing. I have also added a section entitled "CATEGORIES OF
    APPLICATION BEHAVIOR" based on a suggestion from James Bottomley.

    o "Ill-behaved application" AKA "untrusted application" AKA
    "crappy application". The Android guys seem to be thinking in
    terms of applications that are well-designed and well-implemented
    in general, but which do not take power consumption or battery
    life into account. Examples include applications designed for
    externally powered PCs. Many other people seemed to instead be
    thinking in terms of an ill-conceived or useless application,
    perhaps exemplified by "bouncing cows".

    Assuming I have correctly guessed what the Android guys were
    thinking of, perhaps "power-oblivious applications" would be a
    better description, which I will use until someone convinces
    me otherwise.

    o "PM-driving application" are applications that are permitted
    to acquire suspend blockers on Android. Verion 8 of the
    suspend-blocker patch seems to use group permissions to determine
    which applications are classified as power aware. More generally,
    PM-driving applications seem to be those that have permission
    to exert some control over the system's sleep state.

    Note that an application might be power-oblivious on one Android
    device and PM-driving on another, depending on whether the user
    allows that application to acquire suspend blockers. The
    classification might even change over time. For example, a
    user might give an application PM-driving status initially,
    but change his or her mind after some experience with that
    application.

    o Oddly enough, "power-optimized applications" were not discussed.
    See "POWER-OPTIMIZED APPLICATIONS" below for a brief introduction.
    The short version is that power-optimized applications are those
    PM-driving applications that have been aggressively tuned to
    reduce power consumption.

    o Individual devices in an embedded system can enter "device
    low-power states" when not in use.

    o The system as a whole can enter a "system sleep state" when
    the system as a whole is not in use. Suspend blockers are about
    system sleep states rather than device low-power states.

    o There was much discussion of "idle" (AKA "deep idle") and
    "suspend" (as in the the current Linux-kernel suspend operations).
    The following characteristics distinguish "idle" from "suspend":

    1. Idle states are entered by a given CPU only there are no
    runnable tasks for that CPU. In contrast, opportunistic
    suspend can halt the entire system even when there
    are tasks that are ready, willing, and able to run.
    (But please note that this might not apply to real-time
    tasks.)

    Freezing of subsets of applications is somewhat related
    to the idle/suspend discussion, but is covered in a
    later section of this document.

    2. There can be a set of input events that do not bring
    the system out of suspend, but which would bring the
    system out of idle. Exactly which events are in this
    set depends both on hardware capabilities and on the
    platform/application policy. For example, on one of
    the Android-based smartphones, touchscreen input is
    ignored when the system is suspended, but is handled
    normally when idle.

    3. The system comes out of idle when a timer expires. In
    contrast, timers might or might not bring the system
    out of suspend, depending on both hardware capabilities
    and platform/application policy.


    CATEGORIES OF APPLICATION BEHAVIOR

    There are a number of categories of application behavior with respect
    to power management and energy efficiency. These can be classified via
    the following questions: (1) What degree of control is an application
    permitted over its own behavior? (2) What degree of control is an
    application permitted over the power state of individual devices within
    the system? (3) What degree of control is an application permitted
    over the system sleep state? (4) To what degree has the application
    been tuned to reduce its power consumption, either in isolation or in
    conjunction with other applications that might be running concurrently?

    These categories are discussed below.

    o What degree of control is an application permitted over its
    own behavior?

    The Linux kernel already has many controls over application
    behavior:

    o the CAP_ capabilities from include/linux/capability.h.

    o Processes can be assigned to multiple groups, allowing
    them privileged access to portions of the filesystem.

    o The chroot() system call limits a process's access to the
    specified subtree of the filesystem.

    o The ulimit facility can limit CPU consumption, number
    of processes, memory, etc. on a per-user basis. The
    rlimit facility has similar effects on a per-process
    basis.

    o The mlockall() system call provides privileged access
    to memory, avoiding page-fault overhead.

    But more relevant to this discussion, real-time processes are
    permitted a much higher degree of control over the timing of their
    execution than are non-real-time processes. However, suspending
    the system destroys any pretense of offering real-time guarantees,
    which might explain much of the ire towards suspend blockers from
    the real-time and scheduler folks. For but one example, Peter
    Zijlstra suggested that he would merge a patch that acquired
    a suspend blocker any time that the runqueues were non-empty.
    My first reaction was amusement at this vintage Peter Zijlstra
    response, and my second reaction was that it was a futile gesture,
    as the Android guys would simply back out any such change.

    After more thought, however, a variation of Peter's approach
    might well be the key to resolving this tension between
    real-time response on the one hand and Android's desire to
    conserve power at any cost on the other. Given that suspending
    destroys real-time response, why not acquire a suspend blocker
    any time there is a user-created real-time task in the system,
    whether runnable or not? Of course, a simpler approach would
    be to make Android's OPPORTUNISTIC_SUSPEND depend on !PREEMPT_RT.

    o What degree of control is an application permitted over the power
    state of individual devices within the system?

    Is the application in question permitted to power down the
    CPU or peripheral devices? As more of the power control is
    automated based on usage, it is possible that this question will
    become less relevant. The longer the latency and the greater
    the energy consumption of a power-up/power-down sequence for
    a given device, the less suitable that device is for automatic
    power-up/power-down decisions. Cache SRAMs and main-memory
    DRAM tend to be less suitable for automation for this reason.

    o What degree of control is an application permitted over the
    system sleep state?

    Is the application permitted to suspend the device? Or in the
    case of Android, is the application permitted to acquire a
    suspend blocker, which prevents the device from being suspended?

    o To what degree has the application been tuned to reduce its
    power consumption, either in isolation or in conjunction with
    other applications that might be running concurrently?

    See the "POWER-OPTIMIZED APPLICATIONS" section below for more
    detail on the lengths that embedded developers go to in order
    to conserve power -- or, more accurately, to extend battery life.


    REQUIREMENTS

    o Reduce the system's power consumption in order to (1) extend
    battery life and (2) preserve state until external power can
    be obtained.

    o It is necessary to be able to use power-oblivious applications.
    Many of these applications were designed for use in PC platforms
    where power consumption has historically not been of great
    concern, due to either (1) the availability of external power or
    (2) relatively undemanding laptop battery-lifetime expectations.
    The system must be capable of running these power-oblivious
    applications without requiring that these applications be
    modified, and must be capable of reasonable power efficiency
    even when power-oblivious applications are in use.

    o If the display is powered off, there is no need to run any
    application whose only effect is to update the display.

    Although one could simply block such an application when it
    next tries to access the display, it appears that it is highly
    desirable that the application also be prevented from consuming
    power computing anything that will not be displayed. Furthermore,
    whatever mechanism is used must operate on power-oblivious
    applications that do not use blocking system calls.

    There might well be similar requirements for other output-only
    devices, as suggested by Alan Stern.

    o In order to avoid overrunning hardware and/or kernel buffers,
    and to minimize response latencies, designated input events
    must be delivered to the corresponding application in a timely
    fashion. The application might or might not be required to
    actually process the events in a timely fashion, depending on
    the specific application.

    In particular, if user input that would prevent the system
    from entering a sleep state is received while the system is
    transitioning into a sleep state, the system must transition
    back out of the sleep state so that it can hand the user
    input off to the corresponding application.

    Other input events do not force a wakeup, and such input events
    -can- be lost due to buffer overflow in hardware or the kernel.
    Of course, the response latency to such input events can be
    unbounded.

    o The API must provide a way for PM-driving applications that
    receive events to keep themselves running until they have been
    able to process those events.

    o Statistics of the power-control actions taken by PM-driving
    applications must be provided. Given the current Android
    implementation, the suspend blockers are manipulated via
    ioctl(), so that a given application's activity can be tracked
    via the suspend-blocker device, which remains open throughout
    the application's lifetime. Statistics are aggregated by
    name, which is passed by the application in through the
    suspend-blocker interface.

    o PM-driving applications can make use of power-oblivious
    infrastructure. This means that a PM-driving application must
    have some way, whether explicit or implicit, to ensure that
    any power-oblivious infrastructure is permitted to run when a
    PM-driving application needs it to run.

    o If no PM-driving or power-optimized application are indicating
    a need for the system to remain operating, the system is permitted
    (even encouraged!) to suspend all execution, regardless of the
    state of power-oblivious applications. (This requirement did
    appear to be somewhat controversial, both in terms of what is
    meant by "runnable" and in terms of what constitutes "execution".)

    In Android, this is implemented by suspending even while
    PM-driving or power-optimized applications are active, -unless-
    a suspend blocker is held.

    o Transition to system sleep state must be power-efficient.
    In particular, methods based on repeated attempts to suspend
    are considered to be too inefficient to be useful.

    o Individual peripherals and CPUs must still use standard
    power-conservation measures, for example, transitioning CPUs into
    low-power states on idle and powering down peripheral devices
    and hardware accelerators that have not been recently used.

    o The API that controls the system sleep state must be accessible
    both from Android's Java replacement, from userland C code,
    and from kernel C code (both process level and irq code, but
    not NMI handlers).

    o The API that controls the system sleep state must operate
    correctly on SMP systems of modest size. (My guess is that
    "modest" means up to four CPUs, maybe up to eight CPUs.)

    o Any QoS-based solution must take display and user-input
    state into account. In other words, the QoS must be expressed
    as a function of the display and the user-input states.

    o Transitioning to extremely low-power sleep states requires saving
    and restoring DRAM and/or cache SRAM state, which in itself
    consumes significant energy. The power savings must therefore
    be balanced against the energy consumed in the state transitions.

    o The current Android userspace API must be supported in order
    to support existing device software.

    o Any mechanism that freezes some subset of the applications must
    ensure that none of the frozen applications hold any user-level
    resources, such as pthread mutexes. The reason for this is that
    freezing an application that holds a shared pthread mutex will
    result in an application-level hang should some unfrozen process
    attempt to acquire that same pthread mutex. Note that although
    the current cgroup freezer ensures that frozen applications do not
    hold any kernel-level mutexes (at least assuming these mutexes
    are not wrongly held when returning to user-level execution),
    it currently does nothing to prevent freezing processes holding
    pthread mutexes. (There are some proposals to address this issue.)


    NICE-TO-HAVES

    o It would be nice to be able to identify power-oblivious
    applications that never were depended on by PM-driving
    applications. This particular class of power-oblivious
    applications could be shut down when the screen blanks even
    if some PM-driving application was preventing the system from
    powering down.

    There are two obstacles to meeting this requirement:

    1. There must be a reliable way to identify such
    applications. This should be doable, for example the
    application might be tagged by its developer.

    2. There must be a reliable way to freeze them such
    that no frozen application holds a resource that
    might be contended by a non-frozen application.

    Although the cgroup freezer does ensure that frozen
    tasks hold no kernel-level resources, it currently does
    nothing to ensure that no user-level resources are held.
    There are some alternative proposals, which might or
    might not be more successful.

    o Any initialization of the API that controls the system power
    state should be unconditional, so as to be free from failure.
    Such unconditional initialization reduces the intrusiveness of
    the Android patchset.


    APPARENT NON-REQUIREMENTS

    o Transitioning to system sleep states need not be highly scalable,
    as evidenced by the global locks. (If you believe that this
    will in fact be required, please provide a use case. But please
    understand that I do know something about scalability trends,
    but also about uses for transistors beyond more cores.)

    That said, it should not be hard to provide a highly scalable
    implementation of suspend blockers, especially if large systems
    are allowed to take their time suspending themselves.

    o Conserving power in the WiFi and cellular telephony networks.
    At the moment, the focus is on increased battery life in the
    handheld device, perhaps even at the expense of additional
    power consumed by the externally powered WiFi and cell-telephony
    equipment.

    o Synchronizing wakeups of unrelated applications. This is of
    course an important requirement for power savings overall, but
    seems to be left to other mechanisms (e.g., timer aggregation)
    by the Android folks. (One can argue that suspend blockers will
    aggregate timers after a sufficiently long suspension, but they
    would not necessarily stay aggregated during the wakeup period
    without some other mechanism helping out.)


    SUGGESTED USAGE

    These are constraints that the developer is expected to abide by,
    "for best results" and all that.

    o When a PM-driving application is preventing the system from
    shutting down, and is also waiting on a power-oblivious
    application, the PM-driving application should set a timeout
    to handle the possibility that the power-oblivious application
    might halt or otherwise fail.


    POWER-OPTIMIZED APPLICATIONS

    A typical power-optimized application manually controls the power state
    of many separately controlled hardware subsystems to minimize power
    consumption. Such optimization normally requires an understanding
    of the hardware and of the full system's workload: strangely enough,
    concurrently running two separately power-optimized applications often
    does -not- result in a power-optimized system. Such optimization also
    requires knowledge of what the application will be doing in the future,
    so that needed hardware subsystems can be proactively powered up just
    when the application will need them. This is especially important when
    powering down cache SRAMS or banks of main memory, because such components
    take significant time (and consume significant energy) when preparing them
    to be powered off and when restoring their state after powering them on.

    Consider an MP3 player as an example. Such a player will periodically
    read MP3-encoded data from flash memory, decode it (possibly using
    hardware acceleration), and place the resulting audio data into main
    memory. Different systems have different ways of getting the data from
    main memory to the audio output device, but let's assume that the audio
    output device consumes data at a predictable rate such that the software
    can use timers to schedule refilling of the device's output buffer.
    The timer duration will of course need to allow for the time required to
    power up the CPU and L2 cache. The timer can be allowed to happen too
    soon, albeit with a battery-lifetime penalty, but cannot be permitted
    to happen too late, as this will cause "skips" in the playback.

    If MP3 playback is the only application running in the system, things
    are quite easy. We calculate when the audio output device will empty
    its buffer, allow a few milliseconds to power up the needed hardware,
    and set a timer accordingly. Because modern audio output devices have
    buffers that can handle roughly a second's worth of output, it is well
    worthwhile to spend the few milliseconds required to flush the cache
    SRAMS in order to put the system into an extremely low-power sleep state
    over the several hundred milliseconds of playback.

    Now suppose that this device is also recording audio -- perhaps the device
    is being used to monitor an area for noise pollution, and the user is also
    using the device to play music via earphones. The audio input process
    will be the inverse of the audio output process: the microphone data
    will fill a data buffer, which must be collected into DRAM, then encoded
    (perhaps again via MP3) and stored into flash. It would be easy to create
    an optimal application for audio input, but running this optimal audio
    input program concurrently with the optimal audio playback program would
    not necessarily result in a power-optimized combination. This lack of
    optimality is due to the fact that the input and output programs would
    each burn power separately powering down and up. In contrast, an optimal
    solution would align the input and output programs' timers so that a
    single power-down/power-up event would cover both programs' processing.
    This would trade off optimal processing of each (for example, by draining
    the input buffer before it was full) in order to attain global optimality
    (by sharing power-down/power-up overhead).

    There are a number of ways to achieve this:

    1. Making the kernel group timers that occur at roughly the same
    time, as has been discussed on this list many times. This can
    work in many cases, but can be problematic in the audio example,
    due to the presence of hard deadlines.

    2. Write the programs to be aware of each other, so that each
    adjusts its behavior when the other is present. This seems
    to be current practice in the battery-powered embedded arena,
    but is quite complex, sensitive to both hardware configuration
    and software behavior, and requires that all combinations of
    programs be anticipated by the designer -- which can be a serious
    disadvantage given today's app stores.

    3. Use new features such as range timers, so that each program
    can indicate both its preference and the degree of flexibility
    that it can tolerate. This also works in some cases, but as
    far as I know, current proposals do not allow the kernel to take
    power-consumption penalties into account.

    4. Provide "heartbeat" services that allow applications to
    synchronize with each other. This seems most applicable for
    applications that run infrequently, such as email-checking and
    location-service applications.

    5. Use of hardware facilities that allow DMA to be scheduled across
    time. This would allow the CPU to be turned on only for
    decode/encode operations. I am under the impression that this
    sort of time-based DMA hardware does exist in the embedded space
    and that it is actually used for this purpose.

    6. Your favorite solution here.

    Whatever solution is chosen, the key point to keep in mind is that
    running power-optimized applications in combination does -not- result
    in optimal system behavior.


    OTHER EXAMPLE APPLICATIONS

    GPS application that silently displays position.

    There is no point in this application consuming CPU cycles
    or in powering up the GPS hardware unless the display is
    active. Such an application could be handled by the Android
    suspend-blocker proposal. Of course, such an application could
    also periodically poll the display, shutting itself down if the
    display is inactive. In this case, it would also need to have
    some way to be reactivated when the display comes back on.

    GPS application that alerts the user when a given location is reached.

    This application should presumably run even when the display
    is powered down due to input timeout. The question of whether
    or not it should continue running when the device is powered
    off is an interesting one that would be likely to spark much
    spirited discussion. Regardless of the answer to this question,
    the GPS application would hopefully run very intermittently,
    adjusting the delay interval based on the device's velocity and
    distance from the location in question.

    I don't know enough about GPS hardware to say under what
    circumstances the GPS hardware itself should be powered off.
    However, my experience indicates that it takes significant
    time for the GPS hardware to get a position fix after being
    powered on, so presumably this decision would also be based
    on device velocity and distance from the location in question.

    Assuming that the application can run only intermittently,
    suspend blockers would work reasonably well for this use case.
    If the application needed to run continuously, battery life
    would be quite short regardless of the approach used.

    MP3 playback.

    This requires a PM-driving (and preferably a power-optimized)
    application. Because the CPU need only run intermittently,
    suspend blockers can handle this use case. Presumably switching
    the device off would halt playback.

    Bouncing cows.

    This can work with a power-oblivious application that is shut down
    whenever the display is powered off or the device is switched off,
    similar to the GPS application that silently displays position.


    ACKNOWLEDGMENTS

    Of course, just because I acknowledge their contributions does
    not necessarily mean that I think they agree with my assessment
    of the requirements behind suspend blockers. ;-)

    Nevertheless, I am grateful for any and all feedback, whatever
    the form of that feedback might be. I am new to this area, and
    have much to learn.

    Alan Stern
    Arjan van de Ven
    Arve Hjønnevåg
    David Brownell
    David Lang
    Florian Mickler
    James Bottomley
    Mikael Abrahamsson
    Olivier Galibert
    Paul Menage
    Rafael J. Wysocki
    Ted Ts'o
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