Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932836Ab0FUPXh (ORCPT ); Mon, 21 Jun 2010 11:23:37 -0400 Received: from iolanthe.rowland.org ([192.131.102.54]:34932 "HELO iolanthe.rowland.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S932811Ab0FUPXf (ORCPT ); Mon, 21 Jun 2010 11:23:35 -0400 Date: Mon, 21 Jun 2010 11:23:33 -0400 (EDT) From: Alan Stern X-X-Sender: stern@iolanthe.rowland.org To: Florian Mickler cc: "Rafael J. Wysocki" , Linux-pm mailing list , Matthew Garrett , Linux Kernel Mailing List , Dmitry Torokhov , Arve =?ISO-8859-15?B?SGr4bm5lduVn?= , Neil Brown , mark gross <640e9920@gmail.com> Subject: Re: [RFC][PATCH] PM: Avoid losing wakeup events during suspend In-Reply-To: <20100621073233.3f874ad0@schatten.dmk.lab> Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2882 Lines: 64 On Mon, 21 Jun 2010, Florian Mickler wrote: > On Sun, 20 Jun 2010 22:23:38 -0400 (EDT) > Alan Stern wrote: > > This is the race I was talking about: > > > > > > What happens if an event arrives just before you read > > > > /sys/power/wakeup_count, but the userspace consumer doesn't realize > > > > there is a new unprocessed event until after the power manager checks > > > > it? > > > > > I think this is not the kernel's problem. In this approach the kernel makes it > > > possible for the user space to avoid the race. Whether or not the user space > > > will use this opportunity is a different matter. > > > > It is _not_ possible for userspace to avoid this race. Help from the > > kernel is needed. > > It is possible if every (relevant) userspace program implements a > callback for the powermanager to check if one of it's wakeup-sources > got activated. > > That way the powermanager would read /sys/power/wakeup_count, then do > the roundtrip to all it's registered users and only then suspend. > > This turns the suspend_blockers concept around. Instead of actively > signaling the suspend_blockers, the userspace programs only answer > "yes/no" when asked. (i.e. polling?) In the end you would want to have communication in both directions: suspend blockers _and_ callbacks. Polling is bad if done too often. But I think the idea is a good one. In fact, you don't need a "yes/no" response. Programs merely need a chance to activate a new suspend blocker if a wakeup source was recently activated before they acknowledge the poll. > You _can not_ implement userspace suspend blockers with this approach, > as it is vital for every userspace program to get scheduled and check > it's wakeup-source (if even possible) before you know that the right > parties have won the race. I'm not sure what you mean. Certainly you can take a userspace suspend-blocker implementation of the sort discussed before (where programs communicate their needs to a central power-manager process) and add this callback mechanism on top. There is still at least one loophole to be closed: Android's timer-based wakelocks. These include cases where the Android developers didn't add enough wakelocks to cover the entire path from kernel-event to userspace-handler, so they punted and relied on a timer to decide when the wakelock should be deactivated. (There may be other cases too; I didn't follow the original discussion very closely.) It's not clear whether these things can be handled already in Rafael's scheme with your addition, or whether something new is needed. Alan Stern -- 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/