Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933150Ab0HDXKM (ORCPT ); Wed, 4 Aug 2010 19:10:12 -0400 Received: from e2.ny.us.ibm.com ([32.97.182.142]:55670 "EHLO e2.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933132Ab0HDXKH (ORCPT ); Wed, 4 Aug 2010 19:10:07 -0400 Date: Wed, 4 Aug 2010 16:10:03 -0700 From: "Paul E. McKenney" To: david@lang.hm Cc: 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: <20100804231003.GL24163@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20100801054816.GI2470@linux.vnet.ibm.com> <20100804185520.GA2417@srcf.ucam.org> <201008042251.08266.rjw@sisk.pl> <20100804205654.GA4986@srcf.ucam.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline 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: 3574 Lines: 69 On Wed, Aug 04, 2010 at 03:56:42PM -0700, david@lang.hm wrote: > On Wed, 4 Aug 2010, Arve Hj?nnev?g wrote: > > >On Wed, Aug 4, 2010 at 3:31 PM, wrote: > >>On Wed, 4 Aug 2010, 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 proposal that I nade was not to use cgroups to freeze some processes and > >>not others, but to use cgroups to decide to ignore some processes when > >>deciding if the system is idle, stop everything or nothing. cgroups are just > >>a way of easily grouping processes (and their children) into different > >>groups. > > > >That does not avoid the dependency problem. A process may be waiting > >on a resource that a process you ignore owns. I you ignore the process > >that owns the resource and enter idle when it is ready to run (or > >waiting on a timer), you are still effectively blocking the other > >process. > > and if you don't have a wakelock the same thing will happen. If you > expect the process to take a while you can set a timeout to wake up > every 30 seconds or so and wait again, this would be enough to > prevent you from going to sleep (or am I misunderstanding how long > before you go into suspend without a wakelock set, see my other > e-mail for the full question) The difference between the Android scheme and your proposal is that the Android scheme freezes -all- the processes, not just a subset of them. Therefore, in the Android scheme, the case of one process attempting to acquire a resource held by a frozen process. In contrast, any scheme that attempts to freeze only a subset of the processes must somehow either avoid or properly handle the situation where a frozen process is holding a resource that a running process is trying to acquire. Thanx, Paul -- 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/