Re: Attempted summary of suspend-blockers LKML thread

Paul E. McKenney
2010-08-01T19:56:17Z

On Sun, Aug 01, 2010 at 05:41:30PM +0200, Rafael J. Wysocki wrote:
> On Saturday, July 31, 2010, Alan Stern wrote:
> > On Sat, 31 Jul 2010, Paul E. McKenney wrote:
> >
> > > Rushing in where angels fear to tread...
> > >
> > > I had been quite happily ignoring the suspend-blockers controversy.
> > > However, now that I have signed up for the Linaro project that involves
> > > embedded battery-powered devices, I find that ignorance is no longer
> > > bliss. I have therefore reviewed much of the suspend-blocker/wakelock
> > > material, but have not yet seen a clear exposition of the requirements
> > > that suspend blockers are supposed to meet. This email is a attempt
> > > to present the requirements, based on my interpretation of the 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 include example mechanisms as 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. Following
> > > that, the requirements, nice-to-haves, apparent non-requirements,
> > > an example power-optimized applications, and finally a brief look
> > > at other applications.
> > >
> > > Donning the asbestos suit, the one with the tungsten pinstripes...
> > >
> > > Thanx, Paul
> >
> > At the risk of sticking my neck out, I think a few of your statements
> > don't fully capture the important ideas.
> >
> > > ------------------------------------------------------------------------
> > >
> > > DEFINITIONS
> > >
> > > 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
> > > AC-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-naive applications" would be a
> > > better description, which I will use until someone convinces
> > > me otherwise.
>
> I'd slightly prefer these to be called "power-oblvious applications", to
> reflect the fact that their authors might not take power management into
> consideration in any form.

I am fine with "power-oblivious applications".

> > > o "Power-aware 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, power-aware applications seem to be those that
> > > have permission to exert some control over the system's
> > > power state.
> >
> > Notice that these definitions allow a program to be both power-naive
> > and power-aware. In addition, "power-awareness" isn't an inherent
> > property of the application itself, since users are allowed to decide
> > which programs may exert control over the system's power state. The
> > same application could be power-aware on one system and non-power-aware
> > on another.
>
> Also, there is another type of "power-awareness", related to the ability to
> react to power management events signaled, for example, by pm-utils using
> dbus protocol (NetworkManager is one such application). However, the
> applications having that ability don't really participate in making a decision
> to change the state of the system, while the applications using wakelocks do.

Perhaps this group is best named "power-aware applications"?

> In the wakelocks (or suspend blockers, whatever you prefer to call them) world
> no single entity is powerful enough to make the system go into a sleep state,
> but some applications and device drivers collectively can make that happen.
> The applications using wakelocks not only are aware of system power
> management, but also are components of a "collective power manager", so
> perhaps it's better to call them "PM-driving applications" or something like
> this.

Right, any PM-driving application can -prevent- the system from entering
a deep sleep state, but no single application can force this -- aside
from using the traditional non-opportunistic suspend facility, that is.

> > > 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
> > > power-aware applications that have been aggressively tuned to
> > > reduce power consumption.
> >
> > This would be essentially the same as power-aware && !power_naive,
> > right?
>
> Not really, IMO. !power_naive means "doesn't use wakelocks" in this context,
> while "power-optimized" would mean something like "not only uses wakelocks,
> but also tries to reduce energy consumption by as much as possible".

I agree with this. A power-optimized application is something that
goes to lengths to minimize its power consumption, regardless of whether
something like wakelocks is in the picture.

> > > REQUIREMENTS
> > >
> > > o Reduce the system's power consumption in order to (1) extend
> > > battery life and (2) preserve state until AC power can be obtained.
> >
> > External power, not necessarily AC power (a very minor point).
> >
> > > 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.
> >
> > On Android this goes somewhat farther. IIUC, they want hardly anything
> > to run while the display is powered off. (But my understanding could
> > be wrong.)
>
> Not really. Quite a lot of things happen on these systems while the display
> is off (let alone the periodic battery monitoring on Nexus One :-)). They
> can send things over the network and do similar stuff in that state.
>
> I think the opposite is true, ie. the display is aggressively turned off
> whenever it appears not to be used, because it draws a lot of power.

Fair enough. It appears to me that Android won't suspend if the display
is on, but I could easily be confused here.

> > For computers in general, of course, this statement is correct. The
> > same is true for any output-only device. For example, if the audio
> > speakers are powered off, there is no need to run any application whose
> > only effect is to play sounds through the speakers.
>
> Agreed.
>
> > > 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.
> >
> > This goes well beyond overrunning buffers! Events must be delivered in
> > a timely fashion so that the system isn't perceived to be inoperative.
>
> That's correct, although it doesn't seem to apply to any kind of input
> events. For example, on Nexus One the touchscreen doesn't generate wakeup
> events (ie. events that wake the system up from a sleep states), so I'm not
> sure to what extent they are supposed to block (automatic) suspends.

Good point, I forgot that not all events do wakeups. I updated accordingly.

> > > 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.
>
> Side note. I'd like to avoid confusing device states with system-as-a-whole
> states, so I always prefer to refer to the system-as-a-whole-low-power states
> as "system sleep states", while term "low-power state" is reserved for
> individual devices.
>
> Also in some cases (ACPI mostly) a "system sleep state" is more than a
> "system low-power state", because you can put the system into a low-power
> state by putting a number of devices into low-power states, which is not
> sufficient to put the system into a sleep state (the platform has to be
> programmed in a special way to carry out that operation). Now, wakelocks
> are about "system sleep states", not about "system low-power states" in
> general.

Good point, noted.

> > > o If a power-aware application receives user input, then that
> > > application must be given the opportunity to process that
> > > input.
> >
> > A better way to put this is: The API must provide a means for
> > power-aware applications receiving user input to keep themselves
> > running until they have been able to process the input.
> >
> > This is probably also true for power-aware applications having other
> > needs (e.g., non-input-driven computation). In general, power-aware
> > applications must have a mechanism to prevent themselves from being
> > stopped for power-related reasons.
>
> Agreed.
>
> > > 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.
> > >
> > > 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.
> >
> > You mean non-power-aware applications, not power-naive applications.
> > But then the statement is redundant; it follows directly from the
> > definition of "power-aware".
>
> Agreed.

OK, but I still believe that an enforcement mechanism is required.

> > > o Statistics of the power-control actions taken by power-aware
> > > applications must be provided, and must be keyed off of program
> > > name.
> > >
> > > 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.)
> >
> > No, this is not a requirement. A power-optimized application would do
> > this, of course, by definition. But a power-aware application doesn't
> > have to.
>
> Agreed.

Again, this requirement was explicitly called out by the Android folks.

> > > 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.)
> >
> > The controversy was not over the basic point but rather over the
> > detailed meaning of "runnable". A technical matter, related to the
> > implementation of the scheduler.
>
> Well, I _think_ it was about the basic point too, since "all execution" means
> periodic timers in particular and involves shutting down clock sources (except
> for the RTC).
>
> Arguably, suspending "all execution" is not necessary to achieve a satisfactory
> level of energy saving, at least on a number of systems.

Indeed, some embedded systems are capable of doing quite a lot even when
almost everything, including the CPU and cache, is powered down.

Thanx, Paul