From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755417Ab3HaU7P (ORCPT ); Sat, 31 Aug 2013 16:59:15 -0400 Received: from e39.co.us.ibm.com ([32.97.110.160]:50518 "EHLO e39.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753950Ab3HaU7O (ORCPT ); Sat, 31 Aug 2013 16:59:14 -0400 Date: Sat, 31 Aug 2013 13:58:31 -0700 From: "Paul E. McKenney" To: Steven Rostedt Cc: linux-kernel@vger.kernel.org, Ingo Molnar , Andrew Morton , Peter Zijlstra , Frederic Weisbecker , Jiri Olsa Subject: Re: [RFC][PATCH 12/18 v2] ftrace: Disable RCU unsafe checker when function graph is enabled Message-ID: <20130831205831.GC3871@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20130831051117.884125230@goodmis.org> <20130831051702.436132384@goodmis.org> <20130831195550.GR3871@linux.vnet.ibm.com> <1377981737.26624.6.camel@pippen.local.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1377981737.26624.6.camel@pippen.local.home> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: No X-Content-Scanned: Fidelis XPS MAILER x-cbid: 13083120-9332-0000-0000-000001449AC0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Aug 31, 2013 at 04:42:17PM -0400, Steven Rostedt wrote: > On Sat, 2013-08-31 at 12:55 -0700, Paul E. McKenney wrote: > > On Sat, Aug 31, 2013 at 01:11:29AM -0400, Steven Rostedt wrote: > > > From: "Steven Rostedt (Red Hat)" > > > > > > Having the RCU unsafe checker running when function graph is enabled > > > can cause a live lock. That's because the RCU unsafe checker enables > > > full lockdep debugging on RCU which does a lot of interal calls that > > > may be traced by the function graph tacer. This adds quite a bit of > > > > s/tacer/tracer/ (Yeah, yeah, picky, picky!) > > > > > overhead and can possibly live lock the system. > > > > > > Just do not do the RCU unsafe checks when function graph tracer is > > > enabled. > > > > > > Signed-off-by: Steven Rostedt > > > > One question: How does the user/tester/developer know that RCU-unsafe > > checks have been disabled by function-graph tracing? Would it make > > sense to print something to dmesg calling this out? Or do the transitions > > happen too often? > > Ideally, (for 3.13) I plan on having this be a sysctl switch. That will > be 1 or 0, depending on if it is enabled or not. But yeah, I can add a > printk to show that the kernel changed it. It doesn't happen often, and > mainly by a user (or selftest). It would be good. Sooner or later someone is going to miss the fact that they added an unsafe RCU read-side code sequence because they had the checks suppressed during testing. It would be good to have a way to see that this happened. ;-) Thanx, Paul