From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: Attempted summary of suspend-blockers LKML thread Date: Wed, 4 Aug 2010 20:59:34 -0700 Message-ID: <20100805035934.GA2491__5808.48649046874$1280980833$gmane$org@linux.vnet.ibm.com> References: <20100801054816.GI2470@linux.vnet.ibm.com> <20100804205654.GA4986@srcf.ucam.org> <201008050220.51805.rjw@sisk.pl> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Return-path: Content-Disposition: inline In-Reply-To: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: linux-pm-bounces@lists.linux-foundation.org Errors-To: linux-pm-bounces@lists.linux-foundation.org To: Arve =?iso-8859-1?B?SGr4bm5lduVn?= Cc: david@lang.hm, peterz@infradead.org, swetland@google.com, linux-kernel@vger.kernel.org, florian@mickler.org, linux-pm@lists.linux-foundation.org, tglx@linutronix.de, alan@lxorguk.ukuu.org.uk, Arjan van de Ven List-Id: linux-pm@vger.kernel.org On Wed, Aug 04, 2010 at 06:02:28PM -0700, Arve Hj=F8nnev=E5g wrote: > 2010/8/4 Rafael J. Wysocki : > > On Thursday, August 05, 2010, Arve Hj=F8nnev=E5g wrote: > >> On Wed, Aug 4, 2010 at 1:56 PM, 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 trigg= ers > >> >> > suspend when (a) the screen is off and (b) no userspace applicati= ons > >> >> > have indicated that the system shouldn't sleep, except for the wa= keup > >> >> > event race. Imagine the following: > >> >> > > >> >> > 1) The policy timeout is about to expire. No applications are hol= ding > >> >> > wakelocks. The system will suspend providing nothing takes a wake= lock. > >> >> > 2) A network packet arrives indicating an incoming SIP call > >> >> > 3) The VOIP application takes a wakelock and prevents the phone f= rom > >> >> > 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 y= ou've > >> >> > just told the scheduler to ignore. > >> >> > >> >> I _think_ you can use the just-merged /sys/power/wakeup_count mecha= nism to > >> >> avoid the race (if pm_wakeup_event() is called at 2)). > >> > > >> > Yes, I think that solves the problem. The only question then is whet= her > >> > >> 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? Arve, you say that like it is a bad thing. ;-) Seriously, the hope is that Rafael's implementation is useful to other projects in addition to Android. And, all else being equal, the more people who need a given facility, the more likely that facility is to make it to mainline, right? And yes, I see you call out some additional things that Android needs, but hopefully this gap can be closed one way or another. Thanx, Paul > >> > it's preferable to use cgroups or suspend fully, which is pretty muc= h 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. 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. > = > >> The proposed in user-space interface > >> of calling into every process that receives wakeup events before every > >> suspend call > > > > Well, =A0you 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. > = > >> is also not compatible with existing apps. > > > > Thanks, > > Rafael > > -- > > 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 =A0http://vger.kernel.org/majordomo-info.html > > Please read the FAQ at =A0http://www.tux.org/lkml/ > > > = > = > = > -- = > Arve Hj=F8nnev=E5g