Re: Attempted summary of suspend-blockers LKML thread

Paul E. McKenney
2010-08-07T21:11:12Z

On Sat, Aug 07, 2010 at 01:17:48PM -0700, david@lang.hm wrote:
> On Sat, 7 Aug 2010, Paul E. McKenney wrote:
>
> >On Sat, Aug 07, 2010 at 03:00:48AM -0700, david@lang.hm wrote:
> >>On Sat, 7 Aug 2010, Rafael J. Wysocki wrote:
> >>
> >>>On Saturday, August 07, 2010, david@lang.hm wrote:
> >>>>On Sat, 7 Aug 2010, Mark Brown wrote:
> >>>>
> >>>>>On Fri, Aug 06, 2010 at 04:35:59PM -0700, david@lang.hm wrote:
> >>>>>>On Fri, 6 Aug 2010, Paul E. McKenney wrote:
> >>>...
> >>>>What we want to have happen in an ideal world is
> >>>>
> >>>>when the storage isn't needed (between reads) the storage should shutdown
> >>>>to as low a power state as possible.
> >>>>
> >>>>when the CPU isn't needed (between decoding bursts) the CPU and as much of
> >>>>the system as possible (potentially including some banks of RAM) should
> >>>>shutdown to as low a power state as possible.
> >>>
> >>>Unfortunately, the criteria for "not being needed" are not really
> >>>straightforward and one of the wakelocks' roles is to work around this issue.
> >>
> >>if you can ignore the activity caused by the other "unimportant"
> >>processes in the system, why is this much different then just the
> >>one process running, in which case standard power management sleeps
> >>work pretty well.
> >
> >But isn't the whole point of wakelocks to permit developers to easily
> >and efficiently identify which processes are "unimportant" at a given
> >point in time, thereby allowing them to be ignored?
> >
> >I understand your position -- you believe that PM-driving applications
> >should be written to remain idle any time that they aren't doing something
> >"important". This is a reasonable position to take, but it is also
> >reasonable to justify your position. Exactly -why- is this better?
> >Here is my evaluation:
> >
> >o You might not need suspend blockers. This is not totally clear,
> > and won't be until you actually build a system based
> > on your design.
> >
> >o You will be requiring that developers of PM-driving applications
> > deal with more code that must be very carefully coded and
> > validated. This requirement forces the expenditure of lots
> > of people time to save a very small amount of very inexpensive
> > memory (that occupied by the suspend-blocker code).
>
> the issue isn't avoiding the memory useage, the issue is avoiding
> the special API requirement that make the userspace code no longer
> be portable.
>
> note that there are a lot of battery powered embedded devices out
> there that work just fine without wakelocks. They are able to use
> the existing idle/sleep and suspend options to get good battery
> life.

There certainly are such devices, but their power-optimized software
is highly non-portable, so I fail to see how this example can possibly
support your position. In addition, there are quite a few non-portable
Linux extensions to the user-mode API, so your point would not carry in
any case.

> The key difference is that Android allows other programs to be
> loaded on the system, and the current idle/sleep/suspend triggers
> can't tell the difference between the important software and the
> other software.

But the suspend blockers can, to the Android guys' point. And something
needs to tell the difference. It is not helpful for you to try to hide
from this issue.

> >Keep in mind that there was a similar decision in the -rt kernel.
> >One choice was similar to your proposal: all code paths must call
> >schedule() sufficiently frequently. The other choice was to allow
> >almost all code paths to be preempted, which resembles suspend blockers
> >(preempt_disable() being analogous to acquiring a suspend blocker,
> >and preempt_enable() being analogous to releasing a suspend blocker).
> >
> >Then as now, there was much debate. The choice then was preemption.
> >One big reason was that the choice of preemption reduced the amount of
> >real-time-aware code from the entire kernel to only that part of the
> >kernel that disabled preemption, which turned out to greatly simplify
> >the job of meeting aggressive scheduling-latency goals. This experience
> >does add some serious precedent against your position. So, what do you
> >believe is different in the energy-efficiency case?
>
> for one thing, there was never any thought that any code that would
> have to have preempt written would ever run anywhere else other than
> inside the linux kernel.

Portability is for the common-case power-oblivious applications.
Even on Android, the power-oblivious applications do not need to use
suspend blockers. Suspend blockers are instead used by PM-driving and
power-optimized applications. And as you yourself pointed out in an
earlier email, the PM-driving and power-optimized applications are
in the minority. Furthermore, the suspend-blocker approach allows
the bulk of the code in a PM-driving application to be written in a
power-oblivious manner, greatly easing its implementation while still
providing power-efficient operation.

So again, your point does not carry.

> If you had proposed that userspace be allowed to do
> preempt_enable/disable calls, it would have been a very different
> discussion.

There have been proposals that userspace be allowed to disable preemption,
and the proposals indeed did not get far. However, the Linux user-kernel
API -was- extended to accommodate the underlying need, namely with
futexes. And futexes provide an excellent example of a non-portable
extension to the Linux user-kernel API.

So, sorry, but again, your point does not carry.

> In the case of real-time applications, we require that things that
> are given real-time priority be carefully coded to behave well, and
> that if they depend on things that are not given real-time priority
> they may not behave as expected.

And therefore real-world applications often are designed to minimize the
amount of code that needs real-time privileges, exactly because real-time
code is harder to develop than is non-realtime code. In addition, a
number of Linux-specific facilities have been used to mitigate the
effects of bad behavior by real-time applications.

A similar effect is making itself felt in the power-efficiency arena --
apps minimize the amount of code that must obey the PM-driving rules.
Android suspend blockers are one mechanism to carry this minimization
further.

Easing development of code is something you would do well to take more
seriously.

> Priority Inheritance is a way to
> avoid complete system lockup in many cases, but it would still be
> possible for a badly written real-time app to kill the system if it
> does something like go into a busy-loop waiting for a file to be
> created by a non-real-time process.

Just as it is possible for a badly-designed PM-driving app to kill
the battery. And this is exactly why most apps are power-oblivious, and
further why Android allows PM-driving apps that are not currently holding
suspend blockers to be written in a power-oblivious manner. Doing this
reduces the opportunity for even PM-driving apps to kill the battery.

> wakelocks are like implementing real-time by allowing userspace to
> issue preempt_disable() calls to tell the scheduler not to take the
> CPU away from them until they make a preempt_enable() call.

Not so. Suspend blockers are actually less dangerous than a thread
raising its priority to a real-time level, let alone than the ability
to disable preemption at user level.

> In addition wakelocks cannot replace the need to write efficient
> code. all that wakelocks do is to prevent the system from doing a
> suspend, you still want to have the code written to not do
> unneccessary wakeups that would prevent you from using the low-power
> modes other than suspend.

If you had said "wakelocks cannot -completely- replace the need to write
efficient code", I might agree with you. Just as I would agree that a
compiler cannot -completely- replace the need to write assembly from
time to time. This is no more an argument against suspend blockers
(or something like them) than the occasional need to write assembly is
an argument against compilers.

> On the other hand, it _is_ possible for
> the idle/sleep states to be extended to also cover suspend.

Nice try, but please take a look at any of the prior discussion of how
idle and suspend differ, and then feel free to try again.

> >>>>today there are two ways of this happening, via the idle approach (on
> >>>>everything except Android), or via suspend (on Android)
> >>>>
> >>>>Given that many platforms cannot go to into suspend while still playing
> >>>>audio, the idle approach is not going to be able to be eliminated (and in
> >>>>fact will be the most common approach to be used/deugged in terms of the
> >>>>types of platforms), it seems to me that there may be a significant amount
> >>>>of value in seeing if there is a way to change Android to use this
> >>>>approach as well instead of having two different systems competing to do
> >>>>the same job.
> >>>
> >>>There is a fundamental obstacle to that, though. Namely, the Android
> >>>developers say that the idle-based approach doesn't lead to sufficient energy
> >>>savings due to periodic timers and "polling applications".
> >>
> >>polling applications can be solved by deciding that they aren't
> >>going to be allowed to affect the power management decision (don't
> >>consider their CPU useage when deciding to go to sleep, don't
> >>consider their timers when deciding when to wake back up)
> >
> >Agreed, and the focus is on how one decides which applications need
> >to be considered. After all, the activity of a highly optimized
> >audio-playback application looks exactly like that of a stupid polling
> >application -- they both periodically consume some CPU. But this is
> >something that you and the Android guys are actually agreeing about.
> >You are only arguing about exactly what mechanism should be used to
> >make this determination. The Android guys want suspend blockers, and
> >you want to extend cgroups.
>
> I want the kernel to be explicitly told that this application is
> important (or alternativly that these other applications are not). I
> suggested cgroups as a possible way to do this, but anything that
> could tell the kernel what processes to care about and what ones to
> not care about would work. My initial thought had actually been to
> do something like echo the pid of important processes into a /proc
> or /sys file, but I was under the impression that there were a lot
> of processes that would get this state and therefore a more general
> tool like cgroups (which as I understand it automatically puts
> children of a process into the same cgroup as the parent) seemed
> moreuseful

Again, the Android guys just use a different mechanism. So, as I said
in my earlier email, that the next step for you is to implement your
approach so that it can be compared in terms of energy efficiency, code
size, intrusiveness, performance, and compatibility with existing code.

Please keep in mind that the Android guys really do have code that
works in production, which, despite real and perceived imperfections,
carries serious weight. Rafael and Alan have some code that meets some
but apparently not all of the Android guys' requirements. You have some
serious work to do if you want to catch up to them.

I am sorry, but until you actually have something credible running, I
have to place much more weight on Arve's, Brian's, Rafael's, and Alan's
opinions than on yours.

> >So I believe that the next step for you is to implement your approach
> >so that it can be compared in terms of energy efficiency, code size,
> >intrusiveness, performance, and compatibility with existing code.
> >
> >>>Technically that
> >>>boils down to the interrupt sources that remain active in the idle-based case
> >>>and that are shut down during suspend. If you found a way to deactivate all of
> >>>them from the idle context in a non-racy fashion, that would probably satisfy
> >>>the Android's needs too.
> >>
> >>well, we already have similar capibility for other peripherals (I
> >>keep pointing to drive spin down as an example), the key to avoiding
> >>the races seems to be in the drivers supporting this.
> >
> >The difference is that the CPU stays active in the drive spin down
> >case -- if the drive turns out to be needed, the CPU can spin it up.
> >The added complication in the suspend case is that the CPU goes away,
> >so that you must more carefully plan for all of the power-up cases.
>
> I agree tha the power down and restart needs to be planned, but it's
> not like you are going to wake up the drive (or the audio hardware0
> without waking up the CPU first.

On servers, desktops, and laptops, agreed. In contrast, the embedded
guys have facilities that allows hardware to activate without the CPU
being active at the time. So there is a difference.

> even with idle sleep modes and drive spin-down there is no provision
> for the drive to be restarted if the CPU is asleep, you first have
> something happen that wakes up the CPU and it then wakes up the
> drive. This same approach should work for other things.

It will indeed work, but it can be quite energy inefficient, given the
capabilities of embedded hardware.

> >>the fact that Android is making it possible for suspend to
> >>selectivly avoid disabling them makes me think that a lot of the
> >>work needed to make this happen has probably been done. look at what
> >>would happen in a suspend if it decided to leave everything else on
> >>and just disable the one thing, that should e the same thing that
> >>happens if you are just disabling that one thing for idle sleep.
> >
> >We already covered the differences between suspend and idle, now
> >didn't we? ;-)
>
> we did, however at the time suspend was to stop everything, now we
> are finding that Android has multiple flavors of suspend, one of
> which stops everything, the others leave some things running.

Suspend never has stopped the time-of-day clock, so Android's approach
is nothing new. Besides, in the embedded world, the ability to leave
other types of hardware running during a suspend long predates Android.
It might well be new to you, but it is not at all new.

Thanx, Paul