From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932990AbcAHSfi (ORCPT ); Fri, 8 Jan 2016 13:35:38 -0500 Received: from e36.co.us.ibm.com ([32.97.110.154]:57016 "EHLO e36.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932967AbcAHSfc (ORCPT ); Fri, 8 Jan 2016 13:35:32 -0500 X-IBM-Helo: d03dlp03.boulder.ibm.com X-IBM-MailFrom: paulmck@linux.vnet.ibm.com X-IBM-RcptTo: linux-kernel@vger.kernel.org Date: Fri, 8 Jan 2016 10:35:27 -0800 From: "Paul E. McKenney" To: Ross Green Cc: linux-kernel@vger.kernel.org, mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, Mathieu Desnoyers , josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com, Eric Dumazet , dvhart@linux.intel.com, =?iso-8859-1?Q?Fr=E9d=E9ric?= Weisbecker , oleg@redhat.com, pranith kumar Subject: Re: rcu_preempt self-detected stall on CPU from 4.4-rc4, since 3.17 Message-ID: <20160108183527.GF3818@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20160103061720.GT4054@linux.vnet.ibm.com> <20160103181538.GB32217@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 16010818-0021-0000-0000-000015E763B5 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jan 07, 2016 at 10:10:54AM +1100, Ross Green wrote: > Just for completeness I have attached another rcu_preempt stall > warning from a kinux-3.17.1 kernel. > > So it looks like these stall warning go back that far. > > It could be that the kernel instrumentation improved enough to detect > this situation around the 3.17 kernel. I can't find any earlier > records of these stall warnings before these kernels. I guess someone > must have suspected there was a potential for these stalls and hence > the detection facility. Thank you for testing this, Ross! For whatever it is worth, the stall-detection code was added early on, back in the 2.6 days. On the workaround end of things, I am having limited success by forcing RCU's grace-period kthread off of outgoing CPUs at down-prepare time. I really don't like this workaround because I would rather give the sysadm full control of where these kthreads run, but workarounds are workarounds... See 3263d1f49276 (rcu: Migrate grace-period kthread off of outgoing CPU) for the current version in -rcu. Thanx, Paul > Regards, > > Ross > > On Tue, Jan 5, 2016 at 1:21 AM, Ross Green wrote: > > Well with the release of 4.4-rc8 I have built and > > started testing the kernel. > > > > With some luck I managed to get a rcu_preempt stall within a few hours > > of testing. > > > > Upon booting, I ran a small series of bench marks to make sure > > everything is running as expected. limited regression testing and then > > just left the system to idle away, with periodic monitoring from the > > network. > > > > > > please find attached two stack traces from linux-4.4-rc8 and also linux-4.4-rc7. > > > > The interesting thing with the rc7 trace is that there are multiple > > stalls that have occurred over a 6 day period. > > > > I realise Paul you have a number of changes pending for the the RCU > > code. It would be good to try and establish what is happening with > > these stalls before the impact of those changes given that timings and > > dynamics might change whats happening in the current environment. > > > > As reported earlier, i have never been able to induce these stalls > > with heavy loading of the system. The only method I can be sure of is > > to leave the system quiet and let time go by till a stall occurs. > > > > Regards, > > > > Ross > > > > On Mon, Jan 4, 2016 at 9:00 AM, Ross Green wrote: > >> Thanks Paul for your analysis and investigation, > >> > >> I understand your patches are designed not to "fix" the problem, but > >> more to move the problem ahead. > >> > >> I will include a few more stack traces from various kernels. I can go > >> back to around 3.17, with similar trace results. > >> > >> My observation is that the problem can occur at various times and with > >> out any "bad" effect other than more stalls could happen afterwards. > >> > >> At first I wondered if they might actually be a false positive as the > >> kernel seemed to carry on and run quite happily. It is rare that I > >> find a kernel just locks up after observing such a stall, or a > >> complete kernel splat! > >> > >> Unfortunately with my testing I have never been able to induce the > >> problem under any heavy load that would immediately trigger the > >> problem. Indeed most heavy cpu utilisation seemed to just sail on > >> quite nicely. > >> > >> The time for a fault, seems so far, to be non-deterministic with > >> quiescent systems taking anywhere from a few hours through to some six > >> days before showing the problem. > >> > >> More recent kernels seem to have richer stall and back trace > >> information so I was hoping that might shed some light on how they > >> might be occurring.. > >> > >> I usually only run a kernel till I get a new -rc release to test with, > >> so a run of 1 week is a typical cycle. > >> > >> I just wish I could find a sure fire method to trigger the problem!! > >> > >> I have included a few more traces of various kernels all showing the problem. > >> > >> Regards, > >> > >> Ross > >> > >> On Mon, Jan 4, 2016 at 5:15 AM, Paul E. McKenney > >> wrote: > >>> On Sun, Jan 03, 2016 at 07:27:17PM +1100, Ross Green wrote: > >>>> I would not describe the load on this test machine as high or real time. > >>>> > >>>> Apart from a number of standard daemons not much more is running at all! > >>>> > >>>> I normally build a release kernel as soon as possible and set it running. > >>>> Typically I run a series of benchmarks to confirm most things appear > >>>> to be working and then just leave it running. During a normal day i > >>>> will check on the machine 4/5 times just to see how its going! > >>>> Typically I will logon remotely via wifi network connection. > >>>> > >>>> just for your information i will include a few other stack traces from > >>>> previous kernels that may show some trend! > >>>> > >>>> > >>>> Please see the attachments. > >>> > >>> Thank you for the additional details. This does look similar to some > >>> problems I am seeing, though only in heavy rcutorture workloads with > >>> CPU hotplugging. > >>> > >>> I have some crude workarounds in progress, see for example > >>> 2da26818e515 (rcu: Awaken grace-period kthread when stalled) at > >>> git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git. > >>> This workaround kicks the RCU grace-period kthread on every stall warning. > >>> In my testing, this workaround results in slow but real forward progress. > >>> > >>> I have a better workaround in progress, however, please note: > >>> > >>> 1. I have no intention of sending these workarounds upstream. > >>> > >>> 2. The workarounds will splat when they take effect. In other words, > >>> the idea is not to paper over the problem, but instead to allow > >>> me to separate testing concerns. > >>> > >>> 3. A fix is needed for the underlying bug, wherever it might be. > >>> > >>> Thanx, Paul > >>> > >>>> Regards, > >>>> > >>>> Ross > >>>> > >>>> > >>>> > >>>> On Sun, Jan 3, 2016 at 5:17 PM, Paul E. McKenney > >>>> wrote: > >>>> > On Sun, Jan 03, 2016 at 04:29:11PM +1100, Ross Green wrote: > >>>> >> Still seeing these rcu_preempt stalls on kernels through to 4.4-rc7 > >>>> >> > >>>> >> Still have not found a sure fire method to evoke this stall, but have > >>>> >> found that it will normally occur within a week of running a kernel - > >>>> >> usually when it is quiet with light load. > >>>> >> > >>>> >> Have seen similar self detected stalls all the way back to 3.17. > >>>> >> Most recent kernels have included 4.4-rc5 4.4-rc6 and 4.4-rc7 > >>>> >> > >>>> >> Regards, > >>>> >> > >>>> >> Ross > >>>> >> > >>>> >> On Fri, Dec 11, 2015 at 10:17 PM, Ross Green wrote: > >>>> >> > I have been getting these stalls in kernels going back to 3.17. > >>>> >> > > >>>> >> > This stall occurs usually under light load but often requires several > >>>> >> > days to show itself. I have not found any simple way to trigger the > >>>> >> > stall. Indeed heavy workloads seems not to show the fault. > >>>> >> > > >>>> >> > Anyone have any thoughts here? > >>>> >> > > >>>> >> > The recent patch by peterz with kernel/sched/wait.c I thought might > >>>> >> > help the situation, but alas after a few days of running 4.4-rc4 the > >>>> >> > following turned up. > >>>> >> > > >>>> >> > [179922.003570] INFO: rcu_preempt self-detected stall on CPU > >>>> >> > [179922.008178] INFO: rcu_preempt detected stalls on CPUs/tasks: > >>>> >> > [179922.008178] 0-...: (1 ticks this GP) idle=a91/1/0 > >>>> > > >>>> > CPU 0 is non-idle from an RCU perspective. > >>>> > > >>>> >> > softirq=1296733/1296733 fqs=0 > >>>> >> > [179922.008178] > >>>> >> > [179922.008209] (detected by 1, t=8775 jiffies, g=576439, c=576438, q=102) > >>>> >> > [179922.008209] Task dump for CPU 0: > >>>> >> > [179922.008209] swapper/0 R [179922.008209] running [179922.008209] 0 0 0 0x00000000 > >>>> >> > [179922.008209] Backtrace: > >>>> >> > > >>>> >> > [179922.008239] Backtrace aborted due to bad frame pointer > >>>> > > >>>> > Can't have everything, I guess... > >>>> > > >>>> >> > [179922.008239] rcu_preempt kthread starved for 8775 jiffies! g576439 c576438 f0x0 s3 ->state=0x1 > >>>> > > >>>> > Something is keeping the rcu_preempt grace-period kthread from > >>>> > running. This far into the grace period, it should have a > >>>> > timer event waking it every few jiffies. It is currently > >>>> > in TASK_INTERRUPTIBLE state. > >>>> > > >>>> >> > [179922.060302] 0-...: (1 ticks this GP) idle=a91/1/0 softirq=1296733/1296733 fqs=0 > >>>> >> > [179922.068023] (t=8775 jiffies g=576439 c=576438 q=102) > >>>> >> > [179922.073913] rcu_preempt kthread starved for 8775 jiffies! g576439 c576438 f0x2 s3 ->state=0x100 > >>>> > > >>>> > Same story, same grace period, pretty much same time. Now there is an FQS > >>>> > request (f0x2) and the state is now TASK_WAKING (->state=0x100 == 256). > >>>> > > >>>> >> > [179922.083587] Task dump for CPU 0: > >>>> >> > [179922.087097] swapper/0 R running 0 0 0 0x00000000 > >>>> >> > [179922.093292] Backtrace: > >>>> >> > [179922.096313] [] (dump_backtrace) from [] (show_stack+0x18/0x1c) > >>>> >> > [179922.104675] r7:c0908514 r6:80080193 r5:00000000 r4:c090aca8 > >>>> >> > [179922.110809] [] (show_stack) from [] (sched_show_task+0xbc/0x110) > >>>> >> > [179922.119049] [] (sched_show_task) from [] (dump_cpu_task+0x40/0x44) > >>>> >> > [179922.127624] r5:c0917680 r4:00000000 > >>>> >> > [179922.131042] [] (dump_cpu_task) from [] (rcu_dump_cpu_stacks+0x9c/0xdc) > >>>> >> > [179922.140350] r5:c0917680 r4:00000001 > >>>> >> > [179922.143157] [] (rcu_dump_cpu_stacks) from [] (rcu_check_callbacks+0x504/0x8e4) > >>>> >> > [179922.153808] r9:c0908514 r8:c0917680 r7:00000066 r6:2eeab000 > >>>> >> > r5:c0904300 r4:ef7af300 > >>>> >> > [179922.161499] [] (rcu_check_callbacks) from [] (update_process_times+0x40/0x6c) > >>>> >> > [179922.170898] r10:c009a584 r9:00000001 r8:ef7abc4c r7:0000a3a3 > >>>> >> > r6:4ec3391c r5:00000000 > >>>> >> > [179922.179901] r4:c090aca8 > >>>> >> > [179922.182708] [] (update_process_times) from [] > >>>> >> > (tick_sched_handle+0x50/0x54) > >>>> >> > [179922.192108] r5:c0907f10 r4:ef7abe40 > >>>> >> > [179922.195983] [] (tick_sched_handle) from [] > >>>> >> > (tick_sched_timer+0x50/0x94) > >>>> >> > [179922.204895] [] (tick_sched_timer) from [] > >>>> >> > (__hrtimer_run_queues+0x110/0x1a0) > >>>> >> > [179922.214324] r7:00000000 r6:ef7abc40 r5:ef7abe40 r4:ef7abc00 > >>>> >> > [179922.220428] [] (__hrtimer_run_queues) from [] > >>>> >> > (hrtimer_interrupt+0xac/0x1f8) > >>>> >> > [179922.227111] r10:ef7abc78 r9:ef7abc98 r8:ef7abc14 r7:ef7abcb8 > >>>> >> > r6:ffffffff r5:00000003 > >>>> >> > [179922.238220] r4:ef7abc00 > >>>> >> > [179922.238220] [] (hrtimer_interrupt) from [] > >>>> >> > (twd_handler+0x38/0x48) > >>>> >> > [179922.238220] r10:c09084e8 r9:fa241100 r8:00000011 r7:ef028780 > >>>> >> > r6:c092574c r5:ef005cc0 > >>>> > > >>>> > All interrupt stack up to this point. > >>>> > > >>>> > It is quite possible that the stuff below here is at fault as well. > >>>> > That said, the grace-period should actually get to execute at some > >>>> > point. Do you have a heavy real-time load that might be starving > >>>> > things? > >>>> > > >>>> > Thanx, Paul > >>>> > > >>>> >> > [179922.257110] r4:00000001 > >>>> >> > [179922.257110] [] (twd_handler) from [] (handle_percpu_devid_irq+0x74/0x8c) > >>>> >> > [179922.269683] r5:ef005cc0 r4:ef7b1740 > >>>> >> > [179922.269683] [] (handle_percpu_devid_irq) from [] (generic_handle_irq+0x2c/0x3c) > >>>> >> > [179922.283233] r9:fa241100 r8:ef008000 r7:00000001 r6:00000000 > >>>> >> > r5:00000000 r4:c09013e8 > >>>> >> > [179922.290985] [] (generic_handle_irq) from [] (__handle_domain_irq+0x64/0xbc) > >>>> >> > [179922.300842] [] (__handle_domain_irq) from [] > >>>> >> > (gic_handle_irq+0x50/0x90) > >>>> >> > [179922.303222] r9:fa241100 r8:fa240100 r7:c0907f10 r6:fa24010c > >>>> >> > r5:c09087a8 r4:c0925748 > >>>> >> > [179922.315216] [] (gic_handle_irq) from [] > >>>> >> > (__irq_svc+0x54/0x90) > >>>> >> > [179922.319000] Exception stack(0xc0907f10 to 0xc0907f58) > >>>> >> > [179922.331542] 7f00: 00000000 > >>>> >> > ef7ab390 fe600000 00000000 > >>>> >> > [179922.331542] 7f20: c0906000 c090849c c0900364 c06a8124 c0907f80 > >>>> >> > c0944563 c09084e8 c0907f6c > >>>> >> > [179922.349029] 7f40: c0907f4c c0907f60 c00287ac c0010ba8 60080113 ffffffff > >>>> >> > [179922.349029] r9:c0944563 r8:c0907f80 r7:c0907f44 r6:ffffffff > >>>> >> > r5:60080113 r4:c0010ba8 > >>>> >> > [179922.357116] [] (arch_cpu_idle) from [] > >>>> >> > (default_idle_call+0x28/0x34) > >>>> >> > [179922.368926] [] (default_idle_call) from [] > >>>> >> > (cpu_startup_entry+0x114/0x18c) > >>>> >> > [179922.368926] [] (cpu_startup_entry) from [] > >>>> >> > (rest_init+0x90/0x94) > >>>> >> > [179922.385284] r7:ffffffff r4:00000002 > >>>> >> > [179922.393463] [] (rest_init) from [] > >>>> >> > (start_kernel+0x370/0x37c) > >>>> >> > [179922.400421] r5:c0947000 r4:00000000 > >>>> >> > [179922.400421] [] (start_kernel) from [<8000807c>] (0x8000807c) > >>>> >> > $ > >>>> > > >>>> > > >>> > >>> > >>>