From mboxrd@z Thu Jan 1 00:00:00 1970 Received: with ECARTIS (v1.0.0; list linux-mips); Mon, 16 Nov 2009 16:08:12 +0100 (CET) Received: from www.tglx.de ([62.245.132.106]:51757 "EHLO www.tglx.de" rhost-flags-OK-OK-OK-OK) by ftp.linux-mips.org with ESMTP id S1493394AbZKPPIE (ORCPT ); Mon, 16 Nov 2009 16:08:04 +0100 Received: from localhost (www.tglx.de [127.0.0.1]) by www.tglx.de (8.13.8/8.13.8/TGLX-2007100201) with ESMTP id nAGF7e8b011796 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=NO); Mon, 16 Nov 2009 16:07:40 +0100 Date: Mon, 16 Nov 2009 16:07:40 +0100 (CET) From: Thomas Gleixner To: Wu Zhangjin cc: rostedt@goodmis.org, Ralf Baechle , Frederic Weisbecker , Ingo Molnar , Nicholas Mc Guire , David Daney , Richard Sandiford , Patrik Kluba , Michal Simek , "Maciej W . Rozycki" , linux-mips@linux-mips.org, linux-kernel@vger.kernel.org, zhangfx@lemote.com, zhouqg@gmail.com Subject: Re: [PATCH v8 01/16] tracing: convert trace_clock_local() as weak function In-Reply-To: <9dc81a7a9e5a292cccdf465c533a2b08d19d6021.1258177321.git.wuzhangjin@gmail.com> Message-ID: References: <9dc81a7a9e5a292cccdf465c533a2b08d19d6021.1258177321.git.wuzhangjin@gmail.com> User-Agent: Alpine 2.00 (LFD 1167 2008-08-23) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-Virus-Scanned: clamav-milter 0.95.1 at www.tglx.de X-Virus-Status: Clean Return-Path: X-Envelope-To: <"|/home/ecartis/ecartis -s linux-mips"> (uid 0) X-Orcpt: rfc822;linux-mips@linux-mips.org Original-Recipient: rfc822;linux-mips@linux-mips.org X-archive-position: 24915 X-ecartis-version: Ecartis v1.0.0 Sender: linux-mips-bounce@linux-mips.org Errors-to: linux-mips-bounce@linux-mips.org X-original-sender: tglx@linutronix.de Precedence: bulk X-list: linux-mips On Sat, 14 Nov 2009, Wu Zhangjin wrote: > From: Wu Zhangjin > > trace_clock_local() is based on the arch-specific sched_clock(), in X86, > it is tsc(64bit) based, which can give very high precision(about 1ns > with 1GHz). but in MIPS, the sched_clock() is jiffies based, which can > give only 10ms precison with 1000 HZ. which is not enough for tracing, > especially for Real Time system. > > so, we need to implement a MIPS specific sched_clock() to get higher > precision. There is a tsc like clock counter register in MIPS, whose > frequency is half of the processor, so, if the cpu frequency is 800MHz, > the time precision reaches 2.5ns, which is very good for tracing, even > for Real Time system. > > but 'Cause it is only 32bit long, which will rollover quickly, so, such > a sched_clock() will bring with extra load, which is not good for the > whole system. so, we only need to implement a arch-specific > trace_clock_local() for tracing. as a preparation, we convert it as a > weak function. Hmm, I'm not convinced that this is really a huge overhead. First of all the rollover happens once every 10 seconds on a 800MHz machine. Secondly we have a lockless implementation of extending 32bit counters to 63 bit which is used at least by ARM to provide a high resolution sched_clock implementation. See include/linux/cnt32_63.h and the users in arch/ But that's a problem which can be discussed seperately and does not affect the rest of the tracing infrastructure. I really would recommend that you implement a sched_clock for the r4k machines based on cnt32_63 and measure the overhead. Having a fine granular sched_clock in general is probably not a bad thing. Thanks, tglx