linux-kernel - Re: Attempted summary of suspend-blockers LKML thread

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Date:	Wed, 4 Aug 2010 16:23:43 -0700 (PDT)
From:	david@...g.hm
To:	Arve Hjønnevåg <arve@...roid.com>
cc:	Matthew Garrett <mjg59@...f.ucam.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Arjan van de Ven <arjan@...radead.org>,
	linux-pm@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
	pavel@....cz, florian@...kler.org, 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 Wed, 4 Aug 2010, Arve Hj?nnev?g wrote:

> 2010/8/4  <david@...g.hm>:
>> On Wed, 4 Aug 2010, Arve Hj?nnev?g wrote:
>>
>>> On Wed, Aug 4, 2010 at 3:31 PM,  <david@...g.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

>> 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

>> 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?

David Lang