From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932876AbaHVRP3 (ORCPT ); Fri, 22 Aug 2014 13:15:29 -0400 Received: from mx1.redhat.com ([209.132.183.28]:23981 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932812AbaHVRP0 (ORCPT ); Fri, 22 Aug 2014 13:15:26 -0400 Date: Fri, 22 Aug 2014 22:44:05 +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, 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: <20140822171405.GJ16198@grmbl.mre> References: <20140813054439.GA29913@grmbl.mre> <20140813130049.GS4752@linux.vnet.ibm.com> <20140815052411.GF1934@grmbl.mre> <20140815150402.GD4752@linux.vnet.ibm.com> <20140818175345.GD31856@grmbl.mre> <20140819040149.GJ4752@linux.vnet.ibm.com> <20140822122453.GG16198@grmbl.mre> <20140822123651.GH16198@grmbl.mre> <20140822125649.GI16198@grmbl.mre> <20140822144819.GG2663@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140822144819.GG2663@linux.vnet.ibm.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On (Fri) 22 Aug 2014 [07:48:19], Paul E. McKenney wrote: > On Fri, Aug 22, 2014 at 06:26:49PM +0530, Amit Shah wrote: > > On (Fri) 22 Aug 2014 [18:06:51], Amit Shah wrote: > > > On (Fri) 22 Aug 2014 [17:54:53], Amit Shah wrote: > > > > On (Mon) 18 Aug 2014 [21:01:49], Paul E. McKenney wrote: > > > > > > > > > The odds are low over the next few days. I am adding nastier rcutorture > > > > > testing, however. It would still be very good to get debug information > > > > > from your setup. One approach would be to convert the trace function > > > > > calls into printk(), if that would help. > > > > > > > > I added a few printks on the lines of the traces in cases where > > > > rcu_nocb_poll was checked -- since that reproduces the hang. Are the > > > > following traces sufficient, or should I keep adding more printks? > > > > > > > > In the case of rcu-trace-nopoll.txt, the messages stop after a while > > > > (when the guest locks up hard). That's when I kill the qemu process. > > > > > > And this is bt from gdb when the endless > > > > > > RCUDEBUG __call_rcu_nocb_enqueue 2146 rcu_preempt 0 WakeNot > > > > > > messages are being spewed. > > > > > > I can't time it, but hope it gives some indication along with the printks. > > > > ... and after the system 'locks up', this is the state it's in: > > > > ^C > > Program received signal SIGINT, Interrupt. > > native_safe_halt () at ./arch/x86/include/asm/irqflags.h:50 > > 50 } > > (gdb) bt > > #0 native_safe_halt () at ./arch/x86/include/asm/irqflags.h:50 > > #1 0xffffffff8100b9c1 in arch_safe_halt () at ./arch/x86/include/asm/paravirt.h:111 > > #2 default_idle () at arch/x86/kernel/process.c:311 > > #3 0xffffffff8100c107 in arch_cpu_idle () at arch/x86/kernel/process.c:302 > > #4 0xffffffff8106a25a in cpuidle_idle_call () at kernel/sched/idle.c:120 > > #5 cpu_idle_loop () at kernel/sched/idle.c:220 > > #6 cpu_startup_entry (state=) at kernel/sched/idle.c:268 > > #7 0xffffffff813e068b in rest_init () at init/main.c:418 > > #8 0xffffffff81a8cf5a in start_kernel () at init/main.c:680 > > #9 0xffffffff81a8c4ba in x86_64_start_reservations (real_mode_data=) at arch/x86/kernel/head64.c:193 > > #10 0xffffffff81a8c607 in x86_64_start_kernel (real_mode_data=0x13f90 ) > > at arch/x86/kernel/head64.c:182 > > #11 0x0000000000000000 in ?? () > > > > > > Wondering why it's doing this. Am stepping through > > cpu_startup_entry() to see if I get any clues. > > This looks to me like normal behavior in the x86 ACPI idle loop. > My guess is that the lockup is caused by indefinite blocking, in > which case we would expect all the CPUs to be in the idle loop. Hm, found it: The stall happens in do_initcalls(). pm_sysrq_init() is the function that causes the hang. When I #if 0 the line register_sysrq_key('o', &sysrq_poweroff_op); in pm_sysrq_init(), the boot proceeds normally. Now what this is, and what relation this has to rcu and that patch in particular is next... Amit