From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758189Ab0KOTs5 (ORCPT ); Mon, 15 Nov 2010 14:48:57 -0500 Received: from e3.ny.us.ibm.com ([32.97.182.143]:56159 "EHLO e3.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757936Ab0KOTs4 (ORCPT ); Mon, 15 Nov 2010 14:48:56 -0500 Subject: Re: [PATCH] clocksource: document some basic concepts From: john stultz To: Peter Zijlstra Cc: Linus Walleij , linux-kernel@vger.kernel.org, Thomas Gleixner , Nicolas Pitre , Colin Cross , Ingo Molnar , Rabin Vincent In-Reply-To: <1289818094.2109.487.camel@laptop> References: <1289817228-14838-1-git-send-email-linus.walleij@stericsson.com> <1289818094.2109.487.camel@laptop> Content-Type: text/plain; charset="UTF-8" Date: Mon, 15 Nov 2010 11:48:37 -0800 Message-ID: <1289850517.3004.21.camel@localhost.localdomain> Mime-Version: 1.0 X-Mailer: Evolution 2.30.3 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 2010-11-15 at 11:48 +0100, Peter Zijlstra wrote: > On Mon, 2010-11-15 at 11:33 +0100, Linus Walleij wrote: > > +sched_clock() > > +------------- > > + > > +In addition to the clock sources and clock events there is a special weak > > +function in the kernel called sched_clock(). This function shall return the > > +number of nanoseconds since the system was started. An architecture may or > > +may not provide an implementation of sched_clock() on its own. > > + > > +As the name suggests, sched_clock() is used for scheduling the system, > > +determining the absolute timeslice for a certain process in the CFS scheduler > > +for example. It is also used for printk timestamps when you have selected to > > +include time information in printk for things like bootcharts. > > + > > +Compared to clock sources, sched_clock() has to be very fast: it is called > > +much more often, especially by the scheduler. If you have to do trade-offs > > +between accuracy compared to the clock source, you may sacrifice accuracy > > +for speed in sched_clock(). It however require the same basic characteristics > > +as the clock source, i.e. it has to be monotonic. > > Not so, we prefer it be synchronized and monotonic, but we don't require > so, see below. > > > +The sched_clock() function may wrap only on unsigned long long boundaries, > > +i.e. after 64 bits. Since this is a nanosecond value this will mean it wraps > > +after circa 585 years. (For most practical systems this means "never".) > > Currently true, John Stultz was going to look into ammending this by > teaching the kernel/sched_clock.c bits about early wraps (and a way for > architectures to specify this) I'd like to, although at the moment I don't have much space on my plate to do this, so in the mean time, if someone has time and interest into looking at this, ping me and I can lay out the basics of what likely should be done. thanks -john