From mboxrd@z Thu Jan 1 00:00:00 1970 Received: with ECARTIS (v1.0.0; list linux-mips); Mon, 16 Nov 2009 16:21:39 +0100 (CET) Received: from mail-pw0-f45.google.com ([209.85.160.45]:58502 "EHLO mail-pw0-f45.google.com" rhost-flags-OK-OK-OK-OK) by ftp.linux-mips.org with ESMTP id S1492872AbZKPPVc (ORCPT ); Mon, 16 Nov 2009 16:21:32 +0100 Received: by pwi15 with SMTP id 15so3438975pwi.24 for ; Mon, 16 Nov 2009 07:21:23 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=gamma; h=domainkey-signature:received:received:subject:from:reply-to:to:cc :in-reply-to:references:content-type:organization:date:message-id :mime-version:x-mailer:content-transfer-encoding; bh=qD2AajUsmqhtUTCf0p4LDlpx6y4SRgj+Z78WP62cjJA=; b=oR3iN7QMQvcKs0Y5QQ6b7nh3UpRiY4sNNhi6oXCICBiVIEcWQxhGytAnXspOyhp4BR VULkd8YlLPhEdnmP5enlz11GU4zXXIqk4qsV/r6KcYcxYL4xIp3qjt9EIwTi79X7z2o5 JQfY8sE1ZWvq7p8Bz9QIPeC21iktKidbhbI5c= DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=subject:from:reply-to:to:cc:in-reply-to:references:content-type :organization:date:message-id:mime-version:x-mailer :content-transfer-encoding; b=R5JEBSf3aZ0SChe1IcUpEcCX6b79HYiJ2qeqveam8KKOJIF1dY7lm6OGx/KnTXUD9/ YffOBsWfhtO7YF1gI1McUSAyWy6KYpEND5mm0YDuPJVAo7sqljoiHxp0+DeDqe2w4HIM vxviFnV0pFB6RBWtRZ2b5ysGsYcYcH0qLUbfU= Received: by 10.114.54.34 with SMTP id c34mr6755443waa.47.1258384883505; Mon, 16 Nov 2009 07:21:23 -0800 (PST) Received: from ?172.16.2.101? ([222.92.8.142]) by mx.google.com with ESMTPS id 21sm1254858pxi.8.2009.11.16.07.21.15 (version=TLSv1/SSLv3 cipher=RC4-MD5); Mon, 16 Nov 2009 07:21:22 -0800 (PST) Subject: Re: [PATCH v8 01/16] tracing: convert trace_clock_local() as weak function From: Wu Zhangjin Reply-To: wuzhangjin@gmail.com To: Thomas Gleixner 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 In-Reply-To: References: <9dc81a7a9e5a292cccdf465c533a2b08d19d6021.1258177321.git.wuzhangjin@gmail.com> Content-Type: text/plain; charset="UTF-8" Organization: DSLab, Lanzhou University, China Date: Mon, 16 Nov 2009 23:21:08 +0800 Message-ID: <1258384868.15821.9.camel@falcon.domain.org> Mime-Version: 1.0 X-Mailer: Evolution 2.28.1 Content-Transfer-Encoding: 7bit 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: 24917 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: wuzhangjin@gmail.com Precedence: bulk X-list: linux-mips On Mon, 2009-11-16 at 16:07 +0100, Thomas Gleixner wrote: > 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 very much for your prompt, Will have a look at cnt32_to_63() in include/linux/cnt32_to_63.h later. Thanks & Regards, Wu Zhangjin