Re: Attempted summary of suspend-blockers LKML thread

Arve Hjønnevåg
2010-08-05T22:02:50Z

2010/8/5 Rafael J. Wysocki <rjw@sisk.pl>:
> On Thursday, August 05, 2010, Arve Hjønnevåg wrote:
>> 2010/8/4 Rafael J. Wysocki <rjw@sisk.pl>:
>> > On Thursday, August 05, 2010, Arve Hjønnevåg wrote:
>> >> On Wed, Aug 4, 2010 at 1:56 PM, Matthew Garrett <mjg59@srcf.ucam.org> 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.
>

Our wakelock stats currently have
(name,)count,expire_count,wake_count,active_since,total_time,sleep_time,max_time
and last_change. Not all of these are equally important (total_time is
most important followed by active_since), but you only have count.
Also as discussed before, many wakelocks/suspendblockers are not
associated with a struct device.

>> 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

We need more than that (see above).

> 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.
>

What about adding a handle that is passed to all three functions?

>> >> 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.
>

Not having clear rules for what the drivers should do is a problem.
The comments in your code seem to advocate using timeouts instead of
overlapping pm_stay_awake/pm_relax sections. I find this
recommendation strange given all the opposition to
wakelock/suspendblocker timeouts. But more importantly, calling
pm_wakeup_event with a timeout of 0 is incompatible with the android
user space code, and I would prefer that the kernel interfaces would
encourage drivers to block suspend until user space has consumed the
event, which works for the android user space, instead of just long
enough to work with a hypothetical user space power manager.

--
Arve Hjønnevåg