Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934363Ab0HFJva (ORCPT ); Fri, 6 Aug 2010 05:51:30 -0400 Received: from casper.infradead.org ([85.118.1.10]:53117 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759021Ab0HFJvZ convert rfc822-to-8bit (ORCPT ); Fri, 6 Aug 2010 05:51:25 -0400 Subject: Re: [patch 1/2] x86_64 page fault NMI-safe From: Peter Zijlstra To: Masami Hiramatsu Cc: Mathieu Desnoyers , Frederic Weisbecker , Linus Torvalds , Ingo Molnar , LKML , Andrew Morton , Steven Rostedt , Steven Rostedt , Thomas Gleixner , Christoph Hellwig , Li Zefan , Lai Jiangshan , Johannes Berg , Arnaldo Carvalho de Melo , Tom Zanussi , KOSAKI Motohiro , Andi Kleen , "H. Peter Anvin" , Jeremy Fitzhardinge , "Frank Ch. Eigler" , Tejun Heo , 2nddept-manager@sdl.hitachi.co.jp In-Reply-To: <4C5BA937.5010504@hitachi.com> References: <20100714221418.GA14533@nowhere> <20100714223107.GA2350@Krystal> <20100714224853.GC14533@nowhere> <20100714231117.GA22341@Krystal> <20100714233843.GD14533@nowhere> <20100715162631.GB30989@Krystal> <1280855904.1923.675.camel@laptop> <20100803182556.GA13798@Krystal> <1280904410.1923.700.camel@laptop> <20100804144539.GA4617@Krystal> <1280933788.1923.1281.camel@laptop> <4C5BA937.5010504@hitachi.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8BIT Date: Fri, 06 Aug 2010 11:50:40 +0200 Message-ID: <1281088240.1947.357.camel@laptop> Mime-Version: 1.0 X-Mailer: Evolution 2.28.3 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2042 Lines: 46 On Fri, 2010-08-06 at 15:18 +0900, Masami Hiramatsu wrote: > Peter Zijlstra wrote: > > On Wed, 2010-08-04 at 10:45 -0400, Mathieu Desnoyers wrote: > > > >> How do you plan to read the data concurrently with the writer overwriting the > >> data while you are reading it without corruption ? > > > > I don't consider reading while writing (in overwrite mode) a valid case. > > > > If you want to use overwrite, stop the writer before reading it. > > For example, would you like to read system audit log always after > stop the audit? > > NO, that's a most important requirement for tracers, especially for > system admins (they're the most important users of Linux) to check > the system health and catch system troubles. > > For performance measurement and checking hotspot, one-shot tracing > is enough. But it's just for developers. But for the real world > computing, Linux is just an OS, users want to run their system, > middleware and applications, without troubles. But when they hit > a trouble, they wanna shoot it ASAP. > The flight recorder mode is mainly for those users. You cannot over-write and consistently read the buffer, that's plain impossible. With sub-buffers you can swivel a sub-buffer and consistently read that, but there is no guarantee the next sub-buffer you steal was indeed adjacent to the previous buffer you stole as that might have gotten over-written by the active writer while you were stealing the previous one. If you want to snapshot buffers, do that, simply swivel the whole trace buffer, and continue tracing in a new one, then consume the old trace in a consistent manner. I really see no value in being able to read unrelated bits and pieces of a buffer. So no, I will _not_ support reading an over-write buffer while there is an active reader. -- 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/