Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758938Ab0HEXoC (ORCPT ); Thu, 5 Aug 2010 19:44:02 -0400 Received: from ogre.sisk.pl ([217.79.144.158]:43413 "EHLO ogre.sisk.pl" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758370Ab0HEXoA convert rfc822-to-8bit (ORCPT ); Thu, 5 Aug 2010 19:44:00 -0400 From: "Rafael J. Wysocki" To: Arve =?iso-8859-1?q?Hj=F8nnev=E5g?= Subject: Re: Attempted summary of suspend-blockers LKML thread Date: Fri, 6 Aug 2010 01:41:45 +0200 User-Agent: KMail/1.13.5 (Linux/2.6.35-rjw+; KDE/4.4.4; x86_64; ; ) Cc: Matthew Garrett , david@lang.hm, "Paul E. McKenney" , 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 References: <20100801054816.GI2470@linux.vnet.ibm.com> <201008051734.06736.rjw@sisk.pl> In-Reply-To: MIME-Version: 1.0 Content-Type: Text/Plain; charset="iso-8859-1" Content-Transfer-Encoding: 8BIT Message-Id: <201008060141.46109.rjw@sisk.pl> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 6909 Lines: 153 On Friday, August 06, 2010, Arve Hj?nnev?g wrote: > 2010/8/5 Rafael J. Wysocki : > > On Thursday, August 05, 2010, Arve Hj?nnev?g wrote: > >> 2010/8/4 Rafael J. Wysocki : > >> > On Thursday, August 05, 2010, Arve Hj?nnev?g 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 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 -- 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/