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

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Date:	Wed, 4 Aug 2010 16:15:39 -0700
From:	Arve Hjønnevåg <arve@...roid.com>
To:	david@...g.hm
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

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.

> 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?

> 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.

-- 
Arve Hjønnevåg
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