2010/8/5 Rafael J. Wysocki <rjw@xxxxxxx>: > 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? count, tells you how many times the wakelock was activated. If a wakelock prevented suspend for a long time a large count tells you it handled a lot of events while a small count tells you it took a long time to process the events, or the wakelock was not released properly. expire_count, tells you how many times the timeout expired. For the input event wakelock in the android kernel (which has a timeout) an expire count that matches the count tells you that someone opened an input device but is not reading from it (this has happened several times). wake_count, tells you that this is the first wakelock that was acquired in the resume path. This is currently less useful than I would like on the Nexus One since it is usually "SMD_RPCCALL" which does not tell me a lot. active_since, tells you how long a a still active wakelock has been active. If someone activated a wakelock and never released it, it will be obvious here. total_time, total time the wake lock has been active. This one should be obvious. sleep_time, total time the wake lock has been active when the screen was off. max_time, longest time the wakelock was active uninterrupted. This used less often, but the battery on a device was draining fast, but the problem went away before looking at the stats this will show if a wakelock was active for a long time. > > Do you _really_ have to debug the wakelocks in drivers that much? > Wake locks in drivers sometimes need to be debugged. If the api has no accountability, then these problems would take forever to fix. >> >> 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. > Why not? I think allowing drivers to modify a global reference count with no accountability is a terrible idea. >> >> >> 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. I'm referring to this paragraph: * Second, a wakeup event may be detected by one functional unit and processed * by another one. In that case the unit that has detected it cannot really * "close" the "no suspend" period associated with it, unless it knows in * advance what's going to happen to the event during processing. This * knowledge, however, may not be available to it, so it can simply specify time * to wait before the system can be suspended and pass it as the second * argument of pm_wakeup_event(). > >> 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 > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ > -- Arve Hjønnevåg _______________________________________________ linux-pm mailing list linux-pm@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/linux-pm
