From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934487AbWK1CUm (ORCPT ); Mon, 27 Nov 2006 21:20:42 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S934492AbWK1CUm (ORCPT ); Mon, 27 Nov 2006 21:20:42 -0500 Received: from smtp.osdl.org ([65.172.181.25]:29863 "EHLO smtp.osdl.org") by vger.kernel.org with ESMTP id S934488AbWK1CUl convert rfc822-to-8bit (ORCPT ); Mon, 27 Nov 2006 21:20:41 -0500 Date: Mon, 27 Nov 2006 18:09:45 -0800 From: Andrew Morton To: John Cc: linux-kernel@vger.kernel.org, tglx@timesys.com, mingo@elte.hu, johnstul@us.ibm.com Subject: Re: Incorrect behavior of timer_settime() for absolute dates in the past Message-Id: <20061127180945.15b37668.akpm@osdl.org> In-Reply-To: <456AF6F5.7050201@privacy.net> References: <455D7CDD.9000202@privacy.net> <456AF6F5.7050201@privacy.net> X-Mailer: Sylpheed version 2.2.7 (GTK+ 2.8.17; x86_64-unknown-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 27 Nov 2006 15:32:21 +0100 John wrote: > John wrote: > > > I'm playing with the POSIX timers API. My platform is x86 running Linux > > 2.6.18.1 patched with the high-resolution timer subsystem. > > > > http://www.tglx.de/hrtimers.html > > > > I'm seeing unexpected behavior from timer_settime(). > > > > int timer_settime(timer_t timerid, int flags, > > const struct itimerspec *value, struct itimerspec *ovalue); > > > > timer_settime() is used to arm a timer. If the TIMER_ABSTIME flag is > > set, then the timer should fire when the appropriate clock reaches the > > date specified by value. If that date is in the past, the timer should > > fire immediately. > > > > The Open Group Base Specifications Issue 6 states: > > http://www.opengroup.org/onlinepubs/009695399/functions/timer_getoverrun.html > > > > > > "If the flag TIMER_ABSTIME is set in the argument flags, timer_settime() > > shall behave as if the time until next expiration is set to be equal to > > the difference between the absolute time specified by the it_value > > member of value and the current value of the clock associated with > > timerid. That is, the timer shall expire when the clock reaches the > > value specified by the it_value member of value. If the specified time > > has already passed, the function shall succeed and the expiration > > notification shall be made." > > > > In my tests, when timer_settime() is called with an expiration date in > > the past, the timer still takes some time to fire. > > > > Here's a run-down of the code provided as an attachment: > > > > I switch to a SCHED_RR scheduling policy. In other words, whenever my > > process wants the CPU, it gets it. (No other SCHED_RR or SCHED_FIFO > > processes on the system.) I mask the signal that will be delivered on > > timer expiration. I then arm a timer with an expiration date in the > > past, check whether the signal is pending, and block waiting for the > > signal. I then print how long I've had to wait. > > > > # ./a.out > > RESOLUTION=1 ns > > NOW=969.735545919 > > SLEEPING 1 SECOND... > > NOW=970.735581398 > > NOW=970.735613525 > > NOW=970.735749017 > > nsdiff=135492 ns i.e. 135.5 µs > > > > Any ideas? > > Is there a better forum to discuss this matter? > It hasn't been forgotten about. This problem, plus the dynticks-makes-us-disable-the-TSC problem, plus dynticks-breaks-teunis@wintersgift.com's-synaptics driver are all (IMO) blocking a merge.