Re: Attempted summary of suspend-blockers LKML thread

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Tue, Aug 03, 2010 at 08:39:22PM -0700, Arve Hjønnevåg wrote:
> On Tue, Aug 3, 2010 at 5:51 PM,  <david@lang.hm> wrote:
> > On Tue, 3 Aug 2010, Paul E. McKenney wrote:
> >
> >> On Tue, Aug 03, 2010 at 04:19:25PM -0700, david@lang.hm wrote:
> >>>
> >>> On Tue, 3 Aug 2010, Arve Hj?nnev?g wrote:
> >>>
> >>>> 2010/8/2  <david@lang.hm>:
> >>>>>
> >>>>> so what is the fundamental difference between deciding to go into
> >>>>> low-power
> >>>>> idle modes to wake up back up on a given point in the future and
> >>>>> deciding
> >>>>> that you are going to be idle for so long that you may as well suspend
> >>>>> until
> >>>>> 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).
> 
> >> 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?

Hmmm...  The bit about the monotonic clock not advancing could help
explain at least some of the heartburn from the scheduler and real-time
folks.  ;-)

My guess is that this is not a problem for Android workloads, which
probably do not contain aggressive real-time components.  (With the
possible exception of interactions with the cellphone network, which
I believe are handled by a separate core with separate OS.)  However,
pulling this into the Linux kernel would require that interactions with
aggressive real-time workloads be handled, one way or another.

I can see a couple possible resolutions:

1.	Make OPPORTUNISTIC_SUSPEND depend on !PREEMPT_RT, so that
	opportunistic suspend simply doesn't happen on systems that
	support aggressive real-time workloads.

2.	Allow OPPORTUNISTIC_SUSPEND and PREEMPT_RT, but suppress
	opportunistic suspend when there is a user-created real-time
	process.  One way to handle this would be with a variation
	on a tongue-in-cheek suggestion from Peter Zijlstra, namely
	to have every real-time process hold a wakelock.  Note that
	such a wakelock would need to be held even if the real-time
	process in question was not runnable, in order to meet
	possible real-time deadlines when the real-time process was
	awakened.

3.	Your proposal here.  ;-)

Thoughts?

							Thanx, Paul

> >> 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.
> 
> >
> > 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 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)
> >
> 
> 
> 
> -- 
> Arve Hjønnevåg