From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932218AbaHKUUj (ORCPT ); Mon, 11 Aug 2014 16:20:39 -0400 Received: from mx1.redhat.com ([209.132.183.28]:17165 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932083AbaHKUUi (ORCPT ); Mon, 11 Aug 2014 16:20:38 -0400 Date: Tue, 12 Aug 2014 01:48:45 +0530 From: Amit Shah To: "Paul E. McKenney" Cc: linux-kernel@vger.kernel.org, riel@redhat.com, mingo@kernel.org, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, dvhart@linux.intel.com, fweisbec@gmail.com, oleg@redhat.com, sbw@mit.edu Subject: Re: [PATCH tip/core/rcu 1/2] rcu: Parallelize and economize NOCB kthread wakeups Message-ID: <20140811201845.GG4184@grmbl.mre> References: <20140808162502.GX5821@linux.vnet.ibm.com> <20140808173710.GA13483@grmbl.mre> <20140808181835.GD5821@linux.vnet.ibm.com> <20140808183424.GB13483@grmbl.mre> <20140808214347.GH5821@linux.vnet.ibm.com> <20140808214648.GA19058@linux.vnet.ibm.com> <20140811071308.GA4184@grmbl.mre> <20140811162807.GW5821@linux.vnet.ibm.com> <20140811194126.GF4184@grmbl.mre> <20140811201102.GD5821@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140811201102.GD5821@linux.vnet.ibm.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On (Mon) 11 Aug 2014 [13:11:02], Paul E. McKenney wrote: > On Tue, Aug 12, 2014 at 01:11:26AM +0530, Amit Shah wrote: > > On (Mon) 11 Aug 2014 [09:28:07], Paul E. McKenney wrote: > > > On Mon, Aug 11, 2014 at 12:43:08PM +0530, Amit Shah wrote: > > > > On (Fri) 08 Aug 2014 [14:46:48], Paul E. McKenney wrote: > > > > > On Fri, Aug 08, 2014 at 02:43:47PM -0700, Paul E. McKenney wrote: > > > > > > On Sat, Aug 09, 2014 at 12:04:24AM +0530, Amit Shah wrote: > > > > > > > On (Fri) 08 Aug 2014 [11:18:35], Paul E. McKenney wrote: > > > > > > > > > > > > [ . . . ] > > > > > > > > > > > > > > Hmmm... What happens if you boot a7d7a143d0b4cb1914705884ca5c25e322dba693 > > > > > > > > with the kernel parameter "acpi=off"? > > > > > > > > > > > > > > That doesn't change anything - still hangs. > > > > > > > > > > > > > > I intend to look at this more on Monday, though - turning in for > > > > > > > today. In the meantime, if there's anything else you'd like me to > > > > > > > try, please let me know. > > > > > > > > > > > > OK, given that I still cannot reproduce it, I do need your help with > > > > > > the diagnostics. And so what sorts of diagnostics work for you in > > > > > > the hung state? Are you able to dump ftrace buffers? > > > > > > > > > > > > If you are able to dump ftrace buffers, please enable rcu:rcu_nocb_wake > > > > > > and send me the resulting trace. > > > > > > > > > > And another random kernel boot parameter to try is rcu_nocb_poll. > > > > > > > > Right, this gets the boot going again: > > > > > > OK, that likely indicates a lost wakeup. The event tracing enabled by > > > "rcu:rcu_nocb_wake" should help track those down. Last time, it was qemu > > > losing the wakeups, but maybe it is RCU this time. ;-) > > > > The guest goes dead pretty early; is there a trick to enabling and > > getting these traces out of the guest that I don't know of that > > doesn't involve being booted into userspace? I can perhaps try > > getting the trace output out from a virtio-serial channel; but even > > that driver isn't probed yet when the lockup happens. > > First boot with the kernel parameter "trace_event=rcu:rcu_nocb_wake". > Then when the system hangs, do "sendkey alt-sysrq-z" at the "(qemu)" > prompt to dump the ftrace buffer. This hopefully dumps the trace buffer > to dmesg. > > In addition "sendkey alt-sysrq-t" at the "(qemu)" prompt dumps all tasks' > stacks, which would also likely be useful information. Nah, this doesn't work -- the guest's totally locked up. I need a way to continuously dump buffers till the lockup happens, I suppose. Amit