From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755939Ab2E2XLl (ORCPT ); Tue, 29 May 2012 19:11:41 -0400 Received: from relay3.sgi.com ([192.48.152.1]:47475 "EHLO relay.sgi.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1755840Ab2E2XLk (ORCPT ); Tue, 29 May 2012 19:11:40 -0400 Date: Tue, 29 May 2012 18:11:35 -0500 From: Russ Anderson To: Don Zickus Cc: linux-kernel@vger.kernel.org, x86@kernel.org, Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , Andrew Morton , Greg Kroah-Hartman , rja@americas.sgi.com Subject: Re: [PATCH] x86: Avoid intermixing cpu dump_stack output on multi-processor systems Message-ID: <20120529231135.GD4592@sgi.com> Reply-To: Russ Anderson References: <20120524144229.GA27713@sgi.com> <20120529175352.GA31524@redhat.com> <20120529191935.GB29726@sgi.com> <20120529223923.GF32472@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20120529223923.GF32472@redhat.com> User-Agent: Mutt/1.4.2.2i Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, May 29, 2012 at 06:39:23PM -0400, Don Zickus wrote: > On Tue, May 29, 2012 at 02:19:35PM -0500, Russ Anderson wrote: > > On Tue, May 29, 2012 at 01:53:53PM -0400, Don Zickus wrote: > > > On Thu, May 24, 2012 at 09:42:29AM -0500, Russ Anderson wrote: > > > > When multiple cpus on a multi-processor system call dump_stack() > > > > at the same time, the backtrace lines get intermixed, making > > > > the output worthless. Add a lock so each cpu stack dump comes > > > > out as a coherent set. > > > > > > > > For example, when a multi-processor system is NMIed, all of the > > > > cpus call dump_stack() at the same time, resulting in output for > > > > all of cpus getting intermixed, making it impossible to tell what > > > > any individual cpu was doing. With this patch each cpu prints > > > > its stack lines as a coherent set, so one can see what each cpu > > > > was doing. > > > > > > For this particular test case, it sounds like you are doing what > > > trigger_all_cpu_backtrace() is doing? It doesn't solve the general > > > problem, but probably your particular usage? > > > > In this case, I am just using the hardware NMI, which sends the NMI > > signal to each logical cpu. Since each cpu receives the NMI at nearly > > the exact same time, they end up in dump_stack() at the same time. > > Without some form of locking, trace lines from different cpus end > > up intermixed, making it impossible to tell what any individual > > cpu was doing. > > I forgot the original reasons for having the NMI go to each CPU instead of > just the boot CPU (commit 78c06176), but it seems like if you revert that > patch and have the nmi handler just call trigger_all_cpu_backtrace() > instead (which does stack trace locking for pretty output), that would > solve your problem, no? That locking is safe because it is only called in > the NMI context. We want NMI to hit all the cpus at the same time to get a coherent snapshot of what is happening in the system at one point in time. Sending an IPI one cpu at a time skews the results, and doesn't really solve the problem of multiple cpus going into dump_stack() at the same time. NMI isn't the only possible caller of dump_stack(). FWIW, "Wait for up to 10 seconds for all CPUs to do the backtrace" on a 4096 cpu system isn't long enough. :-) > Whereas the lock you are proposing can be called in a mixture of NMI and > IRQ which could cause deadlocks I believe. Since this is a lock just around the dump_stack printk, would checking for forward progress and a timeout to catch any possible deadlock be sufficient? In the unlikely case of a deadlock the lock gets broken and some of the cpu backtraces get intermixed. That is still a huge improvement over the current case where all of the backtraces get intermixed. > Cheers, > Don -- Russ Anderson, OS RAS/Partitioning Project Lead SGI - Silicon Graphics Inc rja@sgi.com