Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754756Ab0LAKod (ORCPT ); Wed, 1 Dec 2010 05:44:33 -0500 Received: from mail2.shareable.org ([80.68.89.115]:53009 "EHLO mail2.shareable.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753709Ab0LAKoc (ORCPT ); Wed, 1 Dec 2010 05:44:32 -0500 Date: Wed, 1 Dec 2010 10:43:59 +0000 From: Jamie Lokier To: Lennart Poettering Cc: Alexander Shishkin , linux-kernel@vger.kernel.org, Thomas Gleixner , Alexander Viro , Greg Kroah-Hartman , Feng Tang , Andrew Morton , Michael Tokarev , Marcelo Tosatti , John Stultz , Chris Friesen , Kay Sievers , "Kirill A. Shutemov" , Artem Bityutskiy , Davide Libenzi , linux-fsdevel@vger.kernel.org Subject: Re: [PATCH] [RFC] timerfd: add TFD_NOTIFY_CLOCK_SET to watch for clock changes Message-ID: <20101201104359.GJ22787@shareable.org> References: <1290532938-7332-1-git-send-email-virtuoso@slind.org> <20101123224346.GA19350@tango.0pointer.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20101123224346.GA19350@tango.0pointer.de> User-Agent: Mutt/1.5.13 (2006-08-11) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3065 Lines: 68 Lennart Poettering wrote: > On Tue, 23.11.10 19:22, Alexander Shishkin (virtuoso@slind.org) wrote: > > > Certain userspace applications (like "clock" desktop applets or cron or > > systemd) might want to be notified when some other application changes > > the system time. There are several known to me reasons for this: > > - avoiding periodic wakeups to poll time changes; > > - rearming CLOCK_REALTIME timers when said changes happen; > > - changing system timekeeping policy for system-wide time management > > programs; > > - keeping guest applications/operating systems running in emulators > > up to date. > > > > This is another attempt to approach notifying userspace about system > > clock changes. The other one is using an eventfd and a syscall [1]. In > > the course of discussing the necessity of a syscall for this kind of > > notifications, it was suggested that this functionality can be achieved > > via timers [2] (and timerfd in particular [3]). This idea got quite > > some support [4], [5], [6] and some vague criticism [7], so I decided > > to try and go a bit further with it. > > I agree with Kay, this is pretty much exactly what we want for > systemd. (Assuming that the time jump due to system suspend is > propagated to userspace like any other time jump with this path). I hope the time jump due to suspend is *not* propagated in the same way to userspace :-) What I'd like to see: 1. Time jump due to the system clock being stepped: Notification. This is *not* a change in real time. It means the clock was corrected/changed. No physical time passed. 2. Time jump due to suspend/resume: Different notification. This *is* a change in real time. Physical time passed. 3. Time drift corrections: As now, no notification, it's just the clock being regulated. To signal the difference between 1 and 2, there ought to be some way for userspace to determine how much of the clock delta corresponds with physical time, by reading some sort of "monotonic" clock :-) CLOCK_MONOTONIC is unsuitable because it stops at suspend. Maybe it should stay that way. But maybe not - programs using CLOCK_MONOTONIC usually want to trigger timeouts etc. based on real elapsed time, and after suspend/resume, it's quite reasonable to want to trigger all of a program's short timeouts immediately. Indeed some network protocol userspace may currently behave *incorrectly* over suspend/resume, especially those using clock times to validate their caches, *because* CLOCK_MONOTONIC doesn't count it. So maybe CLOCK_MONOTONIC should be changed to include elapsed time during suspend/resume, and CLOCK_MONOTONIC_RAW could remain as it is, for programs that want that? That, plus this proposed patch, would signal the difference between 1 and 2 above nicely. -- Jamie -- 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/