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SubjectRe: Attempted summary of suspend-blockers LKML thread
On Wed, 4 Aug 2010, Arve Hj?nnev?g wrote:

> 2010/8/4 <david@lang.hm>:
>> On Wed, 4 Aug 2010, Arve Hj?nnev?g wrote:
>>
>>> 2010/8/4  <david@lang.hm>:
>>>>
>>>> On Wed, 4 Aug 2010, Arve Hj?nnev?g wrote:
>>>>
>>>>> On Wed, Aug 4, 2010 at 3:31 PM,  <david@lang.hm> 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
>>>>>>>>> could
>>>>>>>>> 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
>>>>>>>>> wakeup
>>>>>>>>> event race. Imagine the following:
>>>>>>>>>
>>>>>>>>> 1) The policy timeout is about to expire. No applications are
>>>>>>>>> holding
>>>>>>>>> wakelocks. The system will suspend providing nothing takes a
>>>>>>>>> wakelock.
>>>>>>>>> 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
>>>>>>>>> don't,
>>>>>>>>> because the voip app is an otherwise untrusted application that
>>>>>>>>> you've
>>>>>>>>> just told the scheduler to ignore.
>>>>>>>>
>>>>>>>> I _think_ you can use the just-merged /sys/power/wakeup_count
>>>>>>>> mechanism
>>>>>>>> to
>>>>>>>> avoid the race (if pm_wakeup_event() is called at 2)).
>>>>>>>
>>>>>>> Yes, I think that solves the problem. The only question then is
>>>>>>> whether
>>>>>>> it's preferable to use cgroups or suspend fully, which is pretty much
>>>>>>> up
>>>>>>> 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
>>>>>> processes
>>>>>> and
>>>>>> 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
>>>>>> just
>>>>>> a way of easily grouping processes (and their children) into different
>>>>>> groups.
>>>>>>
>>>>>
>>>>> 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
>>>>> process.
>>>>
>>>> and if you don't have a wakelock the same thing will happen. If you
>>>> expect
>>>
>>> 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
>>>> going
>>>
>>> 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
>>> polling?
>>
>> 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
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