From mboxrd@z Thu Jan 1 00:00:00 1970 From: Luotao Fu Subject: Re: 2.6.24-rc8-rt1: Strange latencies on mpc5200 powerpc Date: Mon, 28 Jan 2008 16:11:29 +0100 Message-ID: <479DF0A1.3000906@pengutronix.de> References: <1200544050.318.18.camel@localhost.localdomain> <478F2A46.5070506@grandegger.com> <20080123145340.GE6200@pengutronix.de> <47976D06.1010900@grandegger.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Luotao Fu , Steven Rostedt , LKML , RT , Ingo Molnar , Thomas Gleixner To: Wolfgang Grandegger Return-path: Received: from metis.extern.pengutronix.de ([83.236.181.26]:56831 "EHLO metis.extern.pengutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753605AbYA1PLd (ORCPT ); Mon, 28 Jan 2008 10:11:33 -0500 In-Reply-To: <47976D06.1010900@grandegger.com> Sender: linux-rt-users-owner@vger.kernel.org List-ID: Hi, I took some time today and went through Wolfgangs scenarios partly. Now some results from my side. I ran my tests on a 2.6.24-rt1 Wolfgang Grandegger wrote: > I also did some more measurements and made, by chance, interesting > observations. I will summarize in more detail later on. Here are some > preliminary results. My high latencies of up to 570us (without latency > tracer) seem to be caused mainly by the following setting: > > CONFIG_RCU_TRACE=m > I also got high latencies without CONFIG_RCU_TRACE set at all. setting CONFIG_RCU_TRACE to y or m causes also high latency though in my test runs. Hence I doubt the rcu Tracer is really the problem. As I mentioned in my last mail. Only thing I could reproduce reliably is that the measurement results depend heavily on kind of the non-rt Workload. For with hackbench or cache calibrator I couldn't produce abnormal high latencies. Nor could I produce the hight latencies on a system booted via flash. Hence my suspects stays on the fec irq thread and filesystem access routines. > With CONFIG_NO_HZ=y or CONFIG_PPC_BESTCOMM_GEN_BD=y the latency > increases by approx. 100..150us, each. > Since I was producing high latencies independently from the rcu settings. I didn't spend a lot of time playing around with the GEN_BD and dynamic clock. All I can say is that disabling them also cause high latency. ;-) The average results don't differ significantly in my test runs. cheers Luotao Fu -- Dipl.-Ing. Luotao Fu | Phone: +49-5121-206917-3 Pengutronix - Linux Solutions for Science and Industry Entwicklungszentrum Nord http://www.pengutronix.de