Attempted summary of suspend-blockers LKML thread
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From: Pavel Machek on 4 Aug 2010 17:20 Hi! > > > > If this doesn't work for the Android folks for whatever reason, another > > > > approach would be to do the freeze in user code, which could track > > > > whether any user-level resources (pthread mutexes, SysV semas, whatever) > > > > where held, and do the freeze on a thread-by-thread basis within each > > > > "victim" application as the threads reach safe points. > > > > > > The main problem I see with the cgroups solution is that it doesn't seem > > > to do anything to handle avoiding loss of wakeup events. > > > > In different message, Arve said they are actually using low-power idle > > to emulate suspend on Android. > > Hello, Pavel, > > Could you please point me at this message? AFAICT, this tells us that idle and suspend is the same hardware state on current Android hardware: Pavel Message-ID: <AANLkTinjH0C0bSK=Y2wKASnbJFsR2BN303xBXkaHbmRC(a)mail.gmail.com> Arve said: If you just program the alarm you will wake up see that the monotonic clock has not advanced and set the alarm another n seconds into the future. Or are proposing that suspend should be changed to keep the monotonic clock running? If you are, why? We can enter the same hardware states from idle, and modifying suspend to wake up more often would increase the average power consumption in suspend, not improve it for idle. In other words, if suspend wakes up as often as idle, why use suspend? -- (english) http://www.livejournal.com/~pavelmachek (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: Paul E. McKenney on 4 Aug 2010 17:20 On Wed, Aug 04, 2010 at 09:56:55PM +0100, Matthew Garrett 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 > 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 > having this conversation? :) It'd be good to have some feedback from > Google as to whether this satisfies their functional requirements. The issue with cgroup freezer as currently defined is that it can freeze processes that hold user-level resources (pthread mutexes, SysV semas, ....). If some non-frozen process attempts to acquire that resource, you get a hang. There might be some ways to work around this, for example, Arjan suggests momentarily unfreezing periodically, and I suggested doing the freeze in user-space code, but we don't know if either of these will really do what is required. Also, I believe that Android's use of cgroups would be in addition to suspending fully rather than instead of. Freezing a subset of the applications allows cutting power drain from output-only apps when the screen blanks but where some app such as a download needs to keep the system active. They still would need to suspend once the download completes. But we do need to hear from the Android guys on these points. Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: Florian Mickler on 4 Aug 2010 17:40 On Wed, 4 Aug 2010 23:15:13 +0200 Pavel Machek <pavel(a)ucw.cz> wrote: > Hi! > > > > > > If this doesn't work for the Android folks for whatever reason, another > > > > > approach would be to do the freeze in user code, which could track > > > > > whether any user-level resources (pthread mutexes, SysV semas, whatever) > > > > > where held, and do the freeze on a thread-by-thread basis within each > > > > > "victim" application as the threads reach safe points. > > > > > > > > The main problem I see with the cgroups solution is that it doesn't seem > > > > to do anything to handle avoiding loss of wakeup events. > > > > > > In different message, Arve said they are actually using low-power idle > > > to emulate suspend on Android. > > > > Hello, Pavel, > > > > Could you please point me at this message? > > AFAICT, this tells us that idle and suspend is the same hardware state > on current Android hardware: > Pavel > > Message-ID: <AANLkTinjH0C0bSK=Y2wKASnbJFsR2BN303xBXkaHbmRC(a)mail.gmail.com> > > Arve said: > > If you just program the alarm you will wake up see that the monotonic > clock has not advanced and set the alarm another n seconds into the > future. Or are proposing that suspend should be changed to keep the > monotonic clock running? If you are, why? We can enter the same > hardware states from idle, and modifying suspend to wake up more often > would increase the average power consumption in suspend, not improve > it for idle. In other words, if suspend wakes up as often as idle, why > use suspend? > > They always told us from the beginning, that on the msm platform they reach the same powerlevel from suspend and idle. They still get gains from using opportunistic suspend. Cheers, Flo -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: Rafael J. Wysocki on 4 Aug 2010 17:40 On Wednesday, August 04, 2010, Paul E. McKenney wrote: > On Wed, Aug 04, 2010 at 09:56:55PM +0100, Matthew Garrett 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 > > 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 > > having this conversation? :) It'd be good to have some feedback from > > Google as to whether this satisfies their functional requirements. > > The issue with cgroup freezer as currently defined is that it can freeze > processes that hold user-level resources (pthread mutexes, SysV semas, > ...). If some non-frozen process attempts to acquire that resource, you > get a hang. There might be some ways to work around this, for example, > Arjan suggests momentarily unfreezing periodically, and I suggested doing > the freeze in user-space code, but we don't know if either of these will > really do what is required. > > Also, I believe that Android's use of cgroups would be in addition to > suspending fully rather than instead of. Freezing a subset of the > applications allows cutting power drain from output-only apps when > the screen blanks but where some app such as a download needs to keep > the system active. They still would need to suspend once the download > completes. One can argue that once the download has been completed, the cpuidle framework should make the system reduce its energy consumption to the level achievable by using suspend. However, the problem is the cpuidle framework only deals with CPUs right now (at least generally) and there's a problem of the interrupt sources that allow the monotonic clock to advance and are deactivated during suspend (which allows more energy to be saved, because periodic timers are then effectively disabled). So, it seems, system suspend is necessary to maximize energy savings as long as the cpuidle framework cannot take care of I/O devices and interrupt sources in general. Thanks, Rafael -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: Mark Brown on 4 Aug 2010 17:50
On Wed, Aug 04, 2010 at 11:15:13PM +0200, Pavel Machek wrote: > > > In different message, Arve said they are actually using low-power idle > > > to emulate suspend on Android. > > Could you please point me at this message? > AFAICT, this tells us that idle and suspend is the same hardware state > on current Android hardware: This is all massively system dependant. On some systems when the system is in the same idle state in the lowest power idle mode as in suspend however as a result of not doing the suspend (which causes Linux to quiesce most of the hardware) many more things will be able to generate wake events. On other systems you will achieve a lower power state by using suspend for various reasons, some fixable and some not. I rather suspect Arve was talking about the former case. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ |