Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752118Ab1COBxM (ORCPT ); Mon, 14 Mar 2011 21:53:12 -0400 Received: from mail-iy0-f174.google.com ([209.85.210.174]:37208 "EHLO mail-iy0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751278Ab1COBxJ convert rfc822-to-8bit (ORCPT ); Mon, 14 Mar 2011 21:53:09 -0400 MIME-Version: 1.0 X-Originating-IP: [76.195.161.19] In-Reply-To: References: <1299681411-9227-1-git-send-email-virtuoso@slind.org> <20110309162513.5058c824.akpm@linux-foundation.org> <20110310002534.f984f8b2.akpm@linux-foundation.org> Date: Mon, 14 Mar 2011 18:53:08 -0700 Message-ID: Subject: Re: [RFCv4] timerfd: add TFD_NOTIFY_CLOCK_SET to watch for clock changes From: Scott James Remnant To: Thomas Gleixner Cc: Andrew Morton , Alexander Shishkin , linux-kernel@vger.kernel.org, Ken MacLeod , Shaun Reich , Alexander Viro , Greg Kroah-Hartman , Feng Tang , Michael Tokarev , Marcelo Tosatti , John Stultz , Chris Friesen , Kay Sievers , "Kirill A. Shutemov" , Artem Bityutskiy , Davide Libenzi , linux-fsdevel@vger.kernel.org, linux-api@vger.kernel.org, Michael Kerrisk Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2684 Lines: 58 On Fri, Mar 11, 2011 at 11:56 AM, Thomas Gleixner wrote: > On Fri, 11 Mar 2011, Scott James Remnant wrote: >> On Thu, Mar 10, 2011 at 12:25 AM, Andrew Morton >> wrote: >> > On Wed, 9 Mar 2011 18:01:09 -0800 Scott James Remnant wrote: >> > >> >> > It would be helpful to know if the identified users of this feature >> >> > actually find it useful and adequate. __I guess the most common >> >> > application is the 1,001 desktop clock widgets. __Do you have any >> >> > feedback from any of the owners of those? >> >> > >> >> cron is another obvious one (or init systems attempting to replace >> >> cron). Having to wakeup and check the time every minute can be >> >> non-conducive to power savings, it would be better if we could just >> >> sleep until the next alarm and be woken up if the time changes in >> >> between. >> >> >> >> (That being said, we also need to poll for and/or check for timezone >> >> changes - but those are entirely userspace, so we can deal with that >> >> separately) >> > >> > Sure, there will be lots of applications. >> > >> > But what I'm asking isn't "it is a good feature". ?I'm asking "is the >> > feature implemented well". ?Ideally someone would get down and modify >> > cron to use the interface in this patch. >> > >> So I've just been thinking today - and I'm actually not sure whether >> this is needed at all for this case. >> >> A good cron implementation is going to set timers according to >> CLOCK_REALTIME; in the case where the clock changes forwards, those >> timers will fire as part of the clock changing already no? And in the >> case where the clock changes backwards, you don't want to re-run old >> ones anyway. >> >> Even the hourly/daily cases are actually at a fixed time, so would be >> triggered - and a decent implementation wouldn't trigger a given >> script more than once. > > Yeah, I was wondering about today as well. Though when you set back > your clock several days, stuff might be surprised if it's not woken up > for several days :) > I've checked the code, and more importantly, tested the setting-forward example - timers do indeed fire at the point the clock is wound forwards. This means there doesn't appear to be a utility for this patch in the cron case. In the wound back case, I believe that even current cron goes to some effort to avoid firing events that have already happened? Scott -- 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/