From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1761665AbYHAVN6 (ORCPT ); Fri, 1 Aug 2008 17:13:58 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1760412AbYHAVKJ (ORCPT ); Fri, 1 Aug 2008 17:10:09 -0400 Received: from e31.co.us.ibm.com ([32.97.110.149]:48990 "EHLO e31.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1760895AbYHAVKH (ORCPT ); Fri, 1 Aug 2008 17:10:07 -0400 Date: Fri, 1 Aug 2008 14:09:58 -0700 From: "Paul E. McKenney" To: Nick Piggin Cc: Ingo Molnar , Linus Torvalds , Alexey Dobriyan , "Rafael J. Wysocki" , Kamalesh Babulal , Linux Kernel Mailing List , Adrian Bunk , Andrew Morton , Natalie Protasevich , Kernel Testers List Subject: Re: 2.6.26-rc9-git4: Reported regressions from 2.6.25 Message-ID: <20080801210958.GL14851@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <4yrA-dRoNlD.A.jcB.ny-cIB@albercik> <20080710173459.GA11648@elte.hu> <20080710180620.GA30844@elte.hu> <200807111412.00084.nickpiggin@yahoo.com.au> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <200807111412.00084.nickpiggin@yahoo.com.au> User-Agent: Mutt/1.5.15+20070412 (2007-04-11) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jul 11, 2008 at 02:11:59PM +1000, Nick Piggin wrote: > On Friday 11 July 2008 04:06, Ingo Molnar wrote: > > > i'm wondering why rcutorture didnt trigger it. I do run !HOTPLUG + > > RCU_PREEMPT kernels and never saw this. Nor did Paul. That aspect is > > weird. > > It basically requires an active rcu reader to be preempted (preferably > by something doing a lot of call_rcu or other activity ie. the writer > so it can tick along the different states quickly). > > I found just 2 threads (reader and writer) bound to the same CPU would > trigger it fastest, my reader has quite a long rcu read section. > > I'm not sure why rcutorture doesn't trigger for everyone. I'm surprised > it does not have much longer maximum read delays -- several ms I would > have thought should be useful to have a crticial section open while the > rcu engine can run through a number of states... Hit it in 10 seconds once I actually got HOTPLUG_CPU disabled. The theory behind the default settings for rcutorture are as follows: o Having two reader threads for each CPU helps ensure interactions between those threads. o The writer is normally going to have to share a CPU with a reader or two, maybe three. This should force reader-writer interactions. o The read-hold time needs to be long enough to ensure interactions with the writer, but if it is too long, there are too few rcu_read_lock() and rcu_read_unlock() events to really stress the read-side processing. o The four fakewriters ensure interaction between multiple writers. To Nick's point, I did use a hacked-up rcutorture with millisecond read-side delays when debugging preemptable RCU, but I also used stock rcutorture. I will give this some thought and see if the defaults should change or if more knobs are needed. Thanx, Paul