From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751645AbaHMFq3 (ORCPT ); Wed, 13 Aug 2014 01:46:29 -0400 Received: from mx1.redhat.com ([209.132.183.28]:22499 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751589AbaHMFq2 (ORCPT ); Wed, 13 Aug 2014 01:46:28 -0400 Date: Wed, 13 Aug 2014 11:14:39 +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: <20140813054439.GA29913@grmbl.mre> References: <20140811162807.GW5821@linux.vnet.ibm.com> <20140811194126.GF4184@grmbl.mre> <20140811201102.GD5821@linux.vnet.ibm.com> <20140811201845.GG4184@grmbl.mre> <20140811203421.GE5821@linux.vnet.ibm.com> <20140812034531.GA13801@linux.vnet.ibm.com> <20140812053321.GK4184@grmbl.mre> <20140812160621.GC4752@linux.vnet.ibm.com> <20140812213936.GA3106@linux.vnet.ibm.com> <20140812214151.GB3106@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140812214151.GB3106@linux.vnet.ibm.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On (Tue) 12 Aug 2014 [14:41:51], Paul E. McKenney wrote: > On Tue, Aug 12, 2014 at 02:39:36PM -0700, Paul E. McKenney wrote: > > On Tue, Aug 12, 2014 at 09:06:21AM -0700, Paul E. McKenney wrote: > > > On Tue, Aug 12, 2014 at 11:03:21AM +0530, Amit Shah wrote: > > > > [ . . . ] > > > > > > I know of only virtio-console doing this (via userspace only, > > > > though). > > > > > > As in userspace within the guest? That would not work. The userspace > > > that the qemu is running in might. There is a way to extract ftrace info > > > from crash dumps, so one approach would be "sendkey alt-sysrq-c", then > > > pull the buffer from the resulting dump. For all I know, there might also > > > be some script that uses the qemu "x" command to get at the ftrace buffer. > > > > > > Again, I cannot reproduce this, and I have been through the code several > > > times over the past few days, and am not seeing it. I could start > > > sending you random diagnostic patches, but it would be much better if > > > we could get the trace data from the failure. I think the only recourse I now have is to dump the guest state from qemu, and attempt to find the ftrace buffers by poking pages and finding some ftrace-like struct... and then dumping the buffers. > > Hearing no objections, random patch #1. The compiler could in theory > > cause trouble without this patch, so there is some possibility that > > it is a fix. > > #2... This would have been a problem without the earlier patch, but > who knows? (#1 moved from theoretically possible but not on x86 to > maybe on x86 given a sufficiently malevolent compiler with the > patch that you located with bisection.) I tried all 3 patches individually, and all 3 together, no success. My gcc is gcc-4.8.3-1.fc20.x86_64. I'm using a fairly uptodate Fedora 20 system on my laptop for these tests. Curiously, patches 1 and 3 applied fine, but this one had a conflict. > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h > index 1dc72f523c4a..1da605740e8d 100644 > --- a/kernel/rcu/tree_plugin.h > +++ b/kernel/rcu/tree_plugin.h > @@ -2137,6 +2137,17 @@ static bool __call_rcu_nocb(struct rcu_data *rdp, struct rcu_head *rhp, I have this hunk at line 2161, and... > trace_rcu_callback(rdp->rsp->name, rhp, > -atomic_long_read(&rdp->nocb_q_count_lazy), > -atomic_long_read(&rdp->nocb_q_count)); > + > + /* > + * If called from an extended quiescent state with interrupts > + * disabled, invoke the RCU core in order to allow the idle-entry > + * deferred-wakeup check to function. > + */ > + if (irqs_disabled_flags(flags) && > + !rcu_is_watching() && > + cpu_online(smp_processor_id())) > + invoke_rcu_core(); > + > return true; I have return 1; here. I'm on linux.git, c8d6637d0497d62093dbba0694c7b3a80b79bfe1. Amit