Re: Attempted summary of suspend-blockers LKML thread

Arve Hjønnevåg
2010-08-04T22:51:34Z

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.

--
Arve Hjønnevåg