Re: Attempted summary of suspend-blockers LKML thread

From: david
Date: Wed Aug 04 2010 - 18:31:58 EST

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.

David Lang
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