Re: Attempted summary of suspend-blockers LKML thread

[david]

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 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 
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 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)

David Lang
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