Re: Attempted summary of suspend-blockers LKML thread

From: david
Date: Wed Aug 04 2010 - 20:02:00 EST

On Wed, 4 Aug 2010, Arve Hj?nnev?g wrote:

2010/8/4 <david@xxxxxxx>:
On Wed, 4 Aug 2010, Arve Hj?nnev?g wrote:

2010/8/4  <david@xxxxxxx>:

On Wed, 4 Aug 2010, Arve Hj?nnev?g wrote:

On Wed, Aug 4, 2010 at 3:31 PM,  <david@xxxxxxx> wrote:

On Wed, 4 Aug 2010, Matthew Garrett wrote:

On Wed, Aug 04, 2010 at 10:51:07PM +0200, Rafael J. Wysocki wrote:

On Wednesday, August 04, 2010, Matthew Garrett wrote:

No! And that's precisely the issue. Android's existing behaviour
be entirely implemented in the form of binary that manually triggers
suspend when (a) the screen is off and (b) no userspace applications
have indicated that the system shouldn't sleep, except for the
event race. Imagine the following:

1) The policy timeout is about to expire. No applications are
wakelocks. The system will suspend providing nothing takes a
2) A network packet arrives indicating an incoming SIP call
3) The VOIP application takes a wakelock and prevents the phone from
suspending while the call is in progress

What stops the system going to sleep between (2) and (3)? cgroups
because the voip app is an otherwise untrusted application that
just told the scheduler to ignore.

I _think_ you can use the just-merged /sys/power/wakeup_count
avoid the race (if pm_wakeup_event() is called at 2)).

Yes, I think that solves the problem. The only question then is
it's preferable to use cgroups or suspend fully, which is pretty much
to the implementation. In other words, is there a reason we're still
having this conversation? :) It'd be good to have some feedback from
Google as to whether this satisfies their functional requirements.

the proposal that I nade was not to use cgroups to freeze some
not others, but to use cgroups to decide to ignore some processes when
deciding if the system is idle, stop everything or nothing. cgroups are
a way of easily grouping processes (and their children) into different

That does not avoid the dependency problem. A process may be waiting
on a resource that a process you ignore owns. I you ignore the process
that owns the resource and enter idle when it is ready to run (or
waiting on a timer), you are still effectively blocking the other

and if you don't have a wakelock the same thing will happen. If you

Not the same thing. If you don't hold a wakelock the entire system
will suspend and when it wakes up it continues where it left off.
Timeout still have time left before they expire.

in what I'm proposing, if the 'privilaged/trusted" processes are idle long
enough the entire system will suspend, and when it wakes up everything will
continue to process normally

If you are triggering a system suspend from idle (I assume all cpus
idle), you also have to consider when to resume. You cannot abort
suspend just because a cpu is not idle anymore, since suspend itself
will wake up threads.

since the premise is that we only consider the activity of some processes when we are deciding if the system is idle, it's very possible that not all CPUs will be idle when we decide to suspend.

will suspend wake up the application threads? or will it create it's own processes (and/or use processes that can be tagged ahead of time)?

the process to take a while you can set a timeout to wake up every 30
seconds or so and wait again, this would be enough to prevent you from

I don't think polling is an acceptable solution to this problem. You
user space code know needs to know what "idle" timeout you have
selected so it can choose a faster poll rate. When is it safe to stop

I think the timeouts are of such an order of magnatude that the polling can
be infrequent enough to not be a significant amount of load, but be faster
than any timeout

How do you ever enter suspend in this system? Currently timers in the
kernel and trusted user space code causes a significant power draw and
you want insignificant timers to prevent suspend.

no, I want recent activity from a privilaged/trusted process to prevent suspend. I'm saying that such a process can wake up infrequently enough that the wakeups themselves are not a significant amount of processing if it wants to keep the system awake so that other things can keep running.

you say below that the system will never suspend while the backlight is on, well the process that monitors/controlls the backlight would be one of these trusted processes.

to sleep (or am I misunderstanding how long before you go into suspend
without a wakelock set, see my other e-mail for the full question)

We suspend as soon as no wakelocks are held. There is no delay.

So, if I have a bookreader app that is not allowed to get the wakelock, and
nothing else is running, the system will suspend immediatly after I click a
button to go to the next page? it will not stay awake to give me a chance to
read the page at all?

how can any application run without wakelock privilages?

A wakelock is active when the screen is on.

so what controls when the screen is on? what still needs to run (and keep the system awake) if the screen is off?

David Lang