Re: Attempted summary of suspend-blockers LKML thread

From: Arve Hjønnevåg
Date: Wed Aug 04 2010 - 18:08:42 EST

On Wed, Aug 4, 2010 at 1:56 PM, Matthew Garrett <mjg59@xxxxxxxxxxxxx> 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

How? By passing a timeout to pm_wakeup_event when the network driver
gets the packet or by passing 0. If you pass a timeout it is the same
as using a wakelock with a timeout and should work (assuming the
timeout you picked is long enough). If you don't pass a timeout it
does not work, since the packet may not be visible to user-space yet.

> 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

I have seen no proposed way to use cgroups that will work. If you
leave some processes running while other processes are frozen you run
into problems when a frozen process holds a resource that a running
process needs.

> having this conversation? :) It'd be good to have some feedback from
> Google as to whether this satisfies their functional requirements.

That is "this"? The merged code? If so, no it does not satisfy our
requirements. The in kernel api, while offering similar functionality
to the wakelock interface, does not use any handles which makes it
impossible to get reasonable stats (You don't know which pm_stay_awake
request pm_relax is reverting). The proposed in user-space interface
of calling into every process that receives wakeup events before every
suspend call is also not compatible with existing apps.

Arve Hjønnevåg
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