Message-ID: <alpine.DEB.2.00.1008031723530.6545@asgard.lang.hm>
Date:	Tue, 3 Aug 2010 17:51:06 -0700 (PDT)
From:	david@...g.hm
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
cc:	Arve Hjønnevåg <arve@...roid.com>,
	Arjan van de Ven <arjan@...radead.org>,
	"Ted Ts'o" <tytso@....edu>, linux-pm@...ts.linux-foundation.org,
	linux-kernel <linux-kernel@...r.kernel.org>, mjg59@...f.ucam.org,
	pavel@....cz, florian@...kler.org, rjw@...k.pl,
	stern@...land.harvard.edu, swetland@...gle.com,
	peterz@...radead.org, tglx@...utronix.de, alan@...rguk.ukuu.org.uk
Subject: Re: Attempted summary of suspend-blockers LKML thread

On Tue, 3 Aug 2010, Paul E. McKenney wrote:

> On Tue, Aug 03, 2010 at 04:19:25PM -0700, david@...g.hm wrote:
>> On Tue, 3 Aug 2010, Arve Hj?nnev?g wrote:
>>
>>> 2010/8/2  <david@...g.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)

> 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)

> 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.

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)

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)

David Lang
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