Re: Attempted summary of suspend-blockers LKML thread

[Paul E. McKenney]

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
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