Re: Attempted summary of suspend-blockers LKML thread

[Rafael J. Wysocki]

On Thursday, August 05, 2010, Arve Hjønnevåg wrote:
> 2010/8/4 Rafael J. Wysocki <rjw@xxxxxxx>:
> > On Thursday, August 05, 2010, Arve Hjønnevåg wrote:
> >> 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.
> >
> > Alternatively, pm_stay_awake() / pm_relax() can be used.
> >
> 
> Which makes the driver and/or network stack changes identical to using
> wakelocks, right?

Please refer to the Matthew's response.

> >> > 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).
> >
> > Why is that a problem (out of curiosity)?
> >
> 
> Not having stats or not knowing what pm_relax is undoing? We need
> stats to be able to debug the system.

You have the stats in struct device and they are available via sysfs.
I suppose they are insufficient, but I'd like to know why exactly.

> If the system does not suspend
> at all or is awake for too long, the wakelock stats tells us which
> component is at fault. Since pm_stay_awake and pm_relax does not
> operate on a handle, you cannot determine how long it prevented
> suspend for.

Well, if you need that, you can add a counter of "completed events" into
struct dev_pm_info and a function similar to pm_relax() that
will update that counter.  I don't think anyone will object to that change.

> >> The proposed in user-space interface
> >> of calling into every process that receives wakeup events before every
> >> suspend call
> >
> > Well,  you don't really need to do that.
> >
> 
> Only if the driver blocks suspend until user-space has read the event.
> This means that for android to work we need to block suspend when
> input events are not processed, but a system using your scheme needs a
> pm_wakeup_event call when the input event is queued. How to you switch
> between them? Do we add separate ioctls in the input device to enable
> each scheme? If someone has a single threaded user space power manager
> that also reads input event it will deadlock if you block suspend
> until it reads the input events since you block when reading the wake
> count.

Well, until someone actually tries to implement a power manager in user space
it's a bit vague.

Thanks,
Rafael
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