On Friday, August 06, 2010, Arve Hjønnevåg wrote: > 2010/8/5 Rafael J. Wysocki <rjw@xxxxxxx>: > > 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. > > > > 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. OK How much of that is used in practice and what for exactly? Do you _really_ have to debug the wakelocks in drivers that much? > >> 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? I don't think that will fly at this point. > >> >> 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. There's no recommendation either way. > But more importantly, calling > pm_wakeup_event with a timeout of 0 is incompatible with the android > user space code, Which I don't find really relevant, sorry. > 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. Well, that are your personal preferences, which I respect. I also have some personal preferences that are not necessarily followed by the kernel code. Thanks, Rafael _______________________________________________ linux-pm mailing list linux-pm@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/linux-pm
