Re: Attempted summary of suspend-blockers LKML thread

From: david
Date: Wed Aug 04 2010 - 00:49:24 EST

On Tue, 3 Aug 2010, Arve Hj?nnev?g wrote:

On Tue, Aug 3, 2010 at 5:51 PM, <david@xxxxxxx> wrote:
On Tue, 3 Aug 2010, Paul E. McKenney wrote:

On Tue, Aug 03, 2010 at 04:19:25PM -0700, david@xxxxxxx wrote:

On Tue, 3 Aug 2010, Arve Hj?nnev?g wrote:

2010/8/2  <david@xxxxxxx>:

so what is the fundamental difference between deciding to go into
idle modes to wake up back up on a given point in the future and
that you are going to be idle for so long that you may as well suspend
there is user input?

Low power idle modes are supposed to be transparent. Suspend stops the
monotonic clock, ignores ready threads and switches over to a separate
set of wakeup events/interrupts. We don't suspend until there is user
input, we suspend until there is a wakeup event (user-input, incoming
network data/phone-calls, alarms etc..).

s/user input/wakeup event/ and my question still stands.

low power modes are not transparent to the user in all cases (if the
screen backlight dimms/shuts off a user reading something will
notice, if the system switches to a lower clock speed it can impact
user response time, etc) The system is making it's best guess as to
how to best srve the user by sacraficing some capibilities to save
power now so that the power can be available later.

as I see it, suspending until a wakeup event (button press, incoming
call, alarm, etc) is just another datapoint along the same path.

If the system could not wake itself up to respond to user input,
phone call, alarm, etc and needed the power button pressed to wake
up (or shut down to the point where the battery could be removed and
reinstalled a long time later), I would see things moving into a
different category, but as long as the system has the ability to
wake itself up later (and is still consuming power) I see the
suspend as being in the same category as the other low-power modes
(it's just more expensive to go in and out of)

why should the suspend be put into a different category from the
other low-power states?

OK, I'll bite...

thanks, this is not intended to be a trap.

From an Android perspective, the differences are as follows:

1.      Deep idle states are entered only if there are no runnable tasks.
       In contrast, opportunistic suspend can happen even when there
       are tasks that are ready, willing, and able to run.

Ok, this is a complication to what I'm proposing (and seems a little odd,
but I can see how it can work), but not neccessarily a major problem. it
depends on exactly how the decision is made to go into low power states
and/or suspend. If this is done by an application that is able to look at
either all activity or ignore one cgroup of processes at different times in
it's calculations than this would work.

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 a deep
       idle state.  For example, I believe that it was stated that one
       of the Android-based smartphones ignores touchscreen input while
       suspended, but pays attention to it while in deep idle states.

I see this as simply being a matter of what devices are still enabled at the
different power savings levels. At one level the touchscreen is still
powered, while at another level it isn't, and at yet another level you have
to hit the power soft-button. This isn't fundamentally different from
powering off a USB peripheral that the system decides is idle (and then not
seeing input from it until something else wakes the system)

The touchscreen on android devices is powered down long before we
suspend, so that is not a good example. There is still a significant
difference between suspend and idle though. In idle all interrupts
work, in suspend only interrupts that the driver has called
enable_irq_wake on will work (on platforms that support it).

are you talking about Android here or are you talking genricly across all platforms?

3.      The system comes out of a deep idle state when a timer
       expires.  In contrast, timers cannot expire while the
       system is suspended.  (This one is debatable: some people
       argue that timers are subject to jitter, and the suspend
       case for timers is the same as that for deep idle states,
       but with unbounded timer jitter.  Others disagree.  The
       resulting discussions have produced much heat, but little
       light.  Such is life.)

if you have the ability to wake for an alarm, you have the ability to wake
for a timer (if from no other method than to set the alarm to when the timer
tick would go off)

If you just program the alarm you will wake up see that the monotonic
clock has not advanced and set the alarm another n seconds into the
future. Or are proposing that suspend should be changed to keep the
monotonic clock running? If you are, why? We can enter the same
hardware states from idle, and modifying suspend to wake up more often
would increase the average power consumption in suspend, not improve
it for idle. In other words, if suspend wakes up as often as idle, why
use suspend?

no, I was thinking that you set the alarm to go off, and when it goes off and wakes you up, you correct other local clocks before doing anything else.

even if they wake up at the same time, you would use suspend instead of idle if it saved more power (allowing for the power to get in and out of suspend vs the power to get in and out of idle)

in this case, another reason you would consider using suspend over idle is that you can suspend until the next timer that your privilaged applications have set, and skip timers set by the non-privilaged applications, resulting in more time asleep.

There may well be others.

Whether these distinctions are a good thing or a bad thing is one of
the topics of this discussion.  But the distinctions themselves are
certainly very real, from what I can see.

Or am I missing your point?

these big distinction that I see as significant seem to be in the decision
of when to go into the different states, and the difference between the
states  themselves seem to be less significant (and either very close to, or
within the variation that already exists for power saving modes)

If I'm right bout this, then it would seem to simplify the concept and
change it from some really foreign android-only thing into a special case
variation of existing core concepts.

Suspend is not an android only concept. The android extensions just
allow us to aggressively use suspend without loosing (or delaying)
wakeup events. On the hardware that we shipped we can enter the same
power mode from idle as we do in suspend, but we still use suspend
primarily because it stops the monotonic clock and all the timers that
use it. Changing suspend to behave more like an idle mode, which seems
to be what you are suggesting, would not buy us anything.

Ok, If I am understanding you correctly I think this is an important point.

What Android calls suspend is not what other linux distros call suspend, it's just a low-power mode with different wakeup rules.

Is this correct?

If this is the case it seems even more so that the android suspend should be addressed by tieing into the power management/idle stuff rather than the suspend/hibernate side of things

is the reason you want to stop the onotonic clock and the timers because the applications need to be fooled into thinking no time has passed?

or is it to prevent extranious wakeups?

or is it to save additional power?

you have many different power saving modes, the daemon (or kernel code) that
is determining which mode to go into would need different logic (including,
but not limited to the ability to be able to ignore one or more cgroups of
processes). different power saving modes have different trade-offs, and some
of them power down different peripherals (which is always a platform
specific, if not system specific set of trade-offs)

The hardware specific idle hook can (and does) decide to go into any
power state from idle that does not disrupt any active devices.

This I know is an Andoid specific thing. On other platforms power states very definantly can make user visible changes.

This all depends on the ability for the code that decides to switch power
modes (including to trigger suspend) to be able to see things in sufficient
detail to be able to do different things depending on the class of programs.
I don't know enough about this code to know if this is the case or not, I
really wish that someone familiar with the power saving code could either
confirm that this is possible, or state that it's not possible (or at least,
not without major surgery)