On Wed, Aug 04, 2010 at 12:15:59PM -0700, david@xxxxxxx wrote:On Wed, 4 Aug 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.
Even in the current implementation (wakelocks), Since the VOIP
application isn't allowed to take a wakelock, wouldn't the system go to
sleep immediatly anyway, even if the application gets the packet and
starts the call? What would ever raise the wakelock to keep the phone
from sleeping in the middle of the call?
There's two parts of that. The first is that the voip application is
allowed to take a wakelock - but that doesn't mean that you trust it the
rest of the time.
The second is that the incoming network packet causes
the kernel to take a wakelock that will be released once userspace has
processed the network packet. This ensures that at least one wakelock is
held for the entire relevant period of time.