Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934876Ab0HEXFX (ORCPT ); Thu, 5 Aug 2010 19:05:23 -0400 Received: from e4.ny.us.ibm.com ([32.97.182.144]:50339 "EHLO e4.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753672Ab0HEXFO (ORCPT ); Thu, 5 Aug 2010 19:05:14 -0400 Date: Thu, 5 Aug 2010 16:05:07 -0700 From: "Paul E. McKenney" To: kevin granade Cc: david@lang.hm, Arve =?iso-8859-1?B?SGr4bm5lduVn?= , Matthew Garrett , "Rafael J. Wysocki" , Arjan van de Ven , linux-pm@lists.linux-foundation.org, linux-kernel@vger.kernel.org, pavel@ucw.cz, florian@mickler.org, stern@rowland.harvard.edu, swetland@google.com, peterz@infradead.org, tglx@linutronix.de, alan@lxorguk.ukuu.org.uk Subject: Re: Attempted summary of suspend-blockers LKML thread Message-ID: <20100805230507.GR2447@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20100805001716.GO24163@linux.vnet.ibm.com> <20100805004802.GP24163@linux.vnet.ibm.com> <20100805151211.GA10080@linux.vnet.ibm.com> <20100805203102.GN2447@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 12098 Lines: 235 On Thu, Aug 05, 2010 at 03:51:35PM -0500, kevin granade wrote: > On Thu, Aug 5, 2010 at 3:31 PM, Paul E. McKenney > wrote: > > On Thu, Aug 05, 2010 at 01:13:31PM -0500, kevin granade wrote: > >> On Thu, Aug 5, 2010 at 10:46 AM, ? wrote: > >> > On Thu, 5 Aug 2010, Paul E. McKenney wrote: > >> > > >> >> On Wed, Aug 04, 2010 at 10:18:40PM -0700, david@lang.hm wrote: > >> >>> > >> >>> On Wed, 4 Aug 2010, Paul E. McKenney wrote: > >> >>>> > >> >>>> On Wed, Aug 04, 2010 at 05:25:53PM -0700, david@lang.hm wrote: > >> >>>>> > >> >>>>> On Wed, 4 Aug 2010, Paul E. McKenney wrote: > >> >> > >> >> [ . . . ] > >> >> > >> >>>>>> The music player is an interesting example. ?It would be idle most > >> >>>>>> of the time, given that audio output doesn't consume very much CPU. > >> >>>>>> So you would not want to suspend the system just because there were > >> >>>>>> no runnable processes. ?In contrast, allowing the music player to > >> >>>>>> hold a wake lock lets the system know that it would not be appropriate > >> >>>>>> to suspend. > >> >>>>>> > >> >>>>>> Or am I misunderstanding what you are proposing? > >> >>>>> > >> >>>>> the system would need to be idle for 'long enough' (configurable) > >> >>>>> before deciding to suspend, so as long as 'long enough' is longer > >> >>>>> than the music player is idle this would not be a problem. > >> >>>> > >> >>>> From a user standpoint, having the music player tell the system when > >> >>>> it is OK to suspend (e.g., when the user has paused playback) seems > >> >>>> a lot nicer than having configurable timeouts that need tweaking. > >> >>> > >> >>> every system that I have seen has a configurable "sleep if it's idle > >> >>> for this long" knob. On the iphone (work issue, I didn't want it) > >> >>> that I am currently using it can be configured from 1 min to 5 min. > >> >>> > >> >>> this is the sort of timeout I am talking about. > >> >>> > >> >>> with something in the multi-minute range for the 'do a full suspend' > >> >>> doing a wakeup every few 10s of seconds is perfectly safe. > >> >> > >> >> Ah, I was assuming -much- shorter "do full suspend" timeouts. > >> >> > >> >> My (possibly incorrect) assumption is based on the complaint that led > >> >> to my implementing RCU_FAST_NO_HZ. ?A (non-Android) embedded person was > >> >> quite annoyed (to put it mildly) at the earlier version of RCU because > >> >> it prevented the system from entering the power-saving dyntick-idle mode, > >> >> not for minutes, or even for seconds, but for a handful of -milliseconds-. > >> >> This was my first hint that "energy efficiency" means something completely > >> >> different in embedded systems than it does in the servers that I am > >> >> used to. > >> >> > >> >> But I must defer to the Android guys on this -- who knows, perhaps > >> >> multi-minute delays to enter full-suspend mode are OK for them. > >> > > >> > if the system was looking at all applications I would agree that the timeout > >> > should be much shorter. > >> > > >> > I have a couple devices that are able to have the display usable, even if > >> > the CPU is asleep (the OLPC and the Kindle, two different display > >> > technologies). With these devices I would like to see the suspend happen so > >> > fast that it can suspend between keystrokes. > >> > > >> > however, in the case of Android I think the timeouts have to end up being > >> > _much_ longer. Otherwise you have the problem of loading an untrusted book > >> > reader app on the device and the device suspends while you are reading the > >> > page. > >> > > >> > currently Android works around this by having a wakelock held whenever the > >> > display is on. This seems backwards to me, the display should be on because > >> > the system is not suspended, not the system is prevented from suspending > >> > because the display is on. > >> > > >> > Rather than having the display be on causing a wavelock to be held (with the > >> > code that is controls the display having a timeout for how long it leaves > >> > the display on), I would invert this and have the timeout be based on system > >> > activity, and when it decides the system is not active, turn off the display > >> > (along with other things as it suspends) > >> > >> IIRC, this was a major point of their (Android's) power management > >> policy. ?User input of any kind would reset the "display active" > >> timeout, which is the primary thing keeping random untrusted > >> user-facing programs from being suspended while in use. ?They seemed > >> to consider this to be a special case in their policy, but from the > >> kernel's point of view it is just another suspend blocker being held. > >> > >> I'm not sure this is the best use case to look at though, because > >> since it is user-facing, the timeout durations are on a different > >> scale than the ones they are really worried about. ?I think another > >> category of use case that they are worried about is: > >> > >> (in suspend) -> wakeup due to network -> process network activity -> suspend > >> > >> or an example that has been mentioned previously: > >> > >> (in suspend) -> wakeup due to alarm for audio processing -> process > >> batch of audio -> suspend > >> > >> In both of these cases, the display may never power on (phone might > >> beep to indicate txt message or email, audio just keeps playing), so > >> the magnitude of the "timeout" for suspending again should be very > >> small. ?Specifically, they don't want there to be a timeout at all, so > >> as little time as possible time is spent out of suspend in addition to > >> the time required to handle the event that caused wakeup. > > > > It would be good to get some sort of range for the "timeout". ?In the > > audio-output case, my understanding that the spacing between bursts of > > audio-processing activity is measured in some hundreds of milliseconds, > > in which case one would want the delays until suspend to be on the > > millisecond scale. ?But does Android really suspend between bursts of > > audio processing while playing music? ?Very cool if so! ?;-) > > Oops, yea that's actually a really bad example, that's probably > something that would be handled by low-power states. I think the > incoming text message example is a good one though. There seemed to > be a focus on user-interaction scale time scales, and I wanted to > point out that there are also very short duration time scales to > consider as well. > > *back to lurking* I really don't know the answer myself, so I was really asking the question rather than trying to catch you out. Thanx, Paul > Kevin > > > > > ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?Thanx, Paul > > > >> >>>>>>> if the backlight being on holds the wakelock, it would seem that > >> >>>>>>> almost every other use of the wakelock could (and probably should) > >> >>>>>>> be replaced by something that tickles the display to stay on longer. > >> >>>>>> > >> >>>>>> The problem with this approach is that the display consumes quite a > >> >>>>>> bit of power, so you don't want to leave it on unnecessarily. ?So if > >> >>>>>> the system is doing something (for example, playing music) that does > >> >>>>>> not require the display, you really want the display to be off. > >> >>>>> > >> >>>>> what percentage (and types) of apps are really useful with the > >> >>>>> display off. I think that there are relativly few apps that you > >> >>>>> really want to keep running if the display is off. > >> >>>> > >> >>>> The length of time those apps are running is the governing factor > >> >>>> for battery life, and not the number of such apps, right? > >> >>> > >> >>> correct, but the number of such apps indicates the scope of the problem. > >> >> > >> >> The number of such apps certainly indicates the amount of effort required > >> >> to modify them, if required. ?Is that what you are getting at? > >> > > >> > yes. > >> > > >> >>>> From another e-mail tonight it sounds like almost everything > >> >>>> already talks > >> >>> > >> >>> to a userspace daemon, so if "(the power management service in the > >> >>> system_server, possibly the media_server and the radio interface > >> >>> glue)" (plus possibly some kernel activity) are the only things > >> >>> looked at when considering if it's safe to sleep or not, all of > >> >>> these can (or already do) do 'something' every few seconds, making > >> >>> this problem sound significantly smaller than it sounded like > >> >>> before. > >> >>> > >> >>> Android could even keep it's user-space API between the system power > >> >>> daemon and the rest of userspace the same if they want to. > >> >>> > >> >>> over time, additional apps could be considered 'trusted' (or flagged > >> >>> that way by the user) and not have to interact with the power daemon > >> >>> to keep things alive. > >> >> > >> >> Hmmm... ?Isn't it the "trusted" (AKA PM-driving) apps that interact with > >> >> the power daemon via suspend blockers, rather than the other way around? > >> > > >> > I was looking at it from a kernel point of view, "trusted" (AKA PM-driving) > >> > apps are ones that have permission to grab the wakelock. Any app/daemon that > >> > is so trusted can communicate with anything else in userspace as part of > >> > making it's decision on whento take the wakelock, but those other > >> > applications would not qualify as "trusted" in my eyes. > >> > > >> >>> as for intramentation, the key tool to use to see why a system isn't > >> >>> going to sleep would be powertop, just like on other linux systems. > >> >> > >> >> Powertop is indeed an extremely valuable tool, but I am not certain > >> >> that it really provides the information that the Android guys need. > >> >> If I understand Arve's and Brian's posts, here is the scenario that they > >> >> are trying to detect: > >> >> > >> >> o ? ? ? Some PM-driving application has a bug in which it fails to > >> >> ? ? ? ?release a wakelock, thus blocking suspend indefinitely. > >> >> > >> >> o ? ? ? This PM-driving application, otherwise being a good citizen, > >> >> ? ? ? ?blocks. > >> >> > >> >> o ? ? ? There are numerous power-oblivious apps running, consuming > >> >> ? ? ? ?significant CPU. > >> >> > >> >> What the Android developers need to know is that the trusted application > >> >> is wrongly holding a wakelock. ?Won't powertop instead tell them about > >> >> all the power-oblivious apps? > >> > > >> > in my proposal (without a wakelock), powertop would tell you what > >> > applications are running and setting timers. If we can modify the > >> > kernel/suspend decision code to only look at processes in one cgroup when > >> > deciding if the system should go to sleep, a similar modification to > >> > poewrtop should let you only show stats on the "trusted" applications. > >> > > >> > If you have a userspace power management daemon that accepts requests from > >> > untrusted programs and does something to keep the system from sleeping > >> > (either taking a wakelock or setting a 'short' timer), it needs to keep the > >> > records of this itself because otherwise all the kernel will see (with > >> > either powertop or wakelock reporting) is that the power management daemon > >> > is what kept the system from sleeping. > >> > > >> > David Lang > >> > -- > >> > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > >> > the body of a message to majordomo@vger.kernel.org > >> > More majordomo info at ?http://vger.kernel.org/majordomo-info.html > >> > Please read the FAQ at ?http://www.tux.org/lkml/ > >> > > > > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/