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SubjectRe: Attempted summary of suspend-blockers LKML thread
On Sat, Jul 31, 2010 at 10:58 AM, Paul E. McKenney
<> wrote:
> o       Reduce the system's power consumption in order to (1) extend
>        battery life and (2) preserve state until AC power can be obtained.
> o       It is necessary to be able to use power-naive 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 AC power or (2)
>        relatively undemanding laptop battery-lifetime expectations.  The
>        system must be capable of running these power-naive applications
>        without requiring that these applications be modified, and must
>        be capable of reasonable power efficiency even when power-naive
>        applications are available.
> 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-naive applications that do not use blocking system calls.
> o       In order to avoid overrunning hardware and/or kernel buffers,
>        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 low-power state is received while the system is
>        transitioning into a low-power state, the system must transition
>        back out of the low-power state so that it can hand the user
>        input off to the corresponding application.
> o       If a power-aware application receives user input, then that
>        application must be given the opportunity to process that
>        input.
> o       A power-aware application must be able to efficiently communicate
>        its needs to the system, so that such communication can be
>        performed on hot code paths.  Communication via open() and
>        close() is considered too slow, but communication via ioctl()
>        is acceptable.

The problem with using open and close to prevent an allow suspend is
not that it is too slow but that it interferes with collecting stats.
The wakelock code has a sysfs interface that allow you to use a
open/write/close sequence to block or unblock suspend. There is no
limit to the amount of kernel memory that a process can consume with
this interface, so the suspend blocker patchset uses a /dev interface
with ioctls to block or unblock suspend and it destroys the kernel
object when the file descriptor is closed.

> o       Power-naive applications must be prohibited from controlling
>        the system power state.  One acceptable approach is through
>        use of group permissions on a special power-control device.
> o       Statistics of the power-control actions taken by power-aware
>        applications must be provided, and must be keyed off of program
>        name.

We don't key the stats off the program name, but having useful
statistics is critical too us. The current code in linux-next does not
appear to allow this (I'm referring to pm_stay_awake here, etc not

> o       Power-aware applications can make use of power-naive infrastructure.
>        This means that a power-aware application must have some way,
>        whether explicit or implicit, to ensure that any power-naive
>        infrastructure is permitted to run when a power-aware application
>        needs it to run.
> o       When a power-aware application is preventing the system from
>        shutting down, and is also waiting on a power-naive application,
>        the power-aware application must set a timeout to handle
>        the possibility that the power-naive application might halt
>        or otherwise fail.  (Such timeouts are also used to limit the
>        number of kernel modifications required.)

wake-lock/suspend-blocker timeouts have nothing to do with the timeout
used by applications when waiting for a response from a less trusted

> o       If no power-aware or power-optimized application are indicating
>        a need for the system to remain operating, the system is permitted
>        (even encouraged!) to suspend all execution, even if power-naive
>        applications are runnable.  (This requirement did appear to be
>        somewhat controversial.)

I would say it should suspend even if power aware applications are
runnable. Most applications do not exclusively perform critical work.

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

It must be power-efficient. Repeated attempts to suspend will kill the
idle battery life.

> 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 power 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       Any initialization of the API that controls the system power
>        state must be unconditional, so as to be free from failure.
>        (I don't currently understand how this relates, probably due to
>        my current insufficient understanding of the proposed patch set.)

Unconditional initialization makes it easier to add suspend blockers
to existing kernel code since you don't have to add new failure exit
paths. It is not a strong requirement.

> o       The API that controls the system power 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 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.

Arve Hjønnevåg
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