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Date: Sun, 12 Nov 2017 09:17:51 -0800 From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> To: Bruno Prémont <bonbons@...ophe.eu> Cc: Josh Triplett <josh@...htriplett.org>, Steven Rostedt <rostedt@...dmis.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Lai Jiangshan <jiangshanlai@...il.com>, linux-kernel@...r.kernel.org Subject: Re: RCU stall/SOFT-Lockup on 4.11.3/4.13.11 after multiple days uptime On Sun, Nov 12, 2017 at 12:09:28PM +0100, Bruno Prémont wrote: > On Sat, 11 November 2017 "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote: > > On Sat, Nov 11, 2017 at 08:38:32PM +0100, Bruno Prémont wrote: > > > Hi, > > > > > > On a single-CPU KVM-based virtual machine I'm suffering from RCU stall > > > and soft-lockup. 4.10.x kernels run fine (4.10.12) but starting with > > > 4.11.x (4.11.3, 4.13.11) I'm getting system freezes for no apparent > > > reason. > > > > > > All info I have is following console dump (from 4.13.11): > > > [526415.290012] INFO: rcu_sched self-detected stall on CPU > > > [526415.290012] o0-...: (745847 ticks this GP) idle=ba2/2/0 softirq=37393463/37393463 fqs=0 > > > [526415.290012] o (t=745854 jiffies g=23779976 c=23779975 q=32) > > > [526415.290012] rcu_sched kthread starved for 745854 jiffies! g23779976 c23779975 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x0 > > > > The above line says that the rcu_sched kthread asked to sleep for three > > jiffies, but ended up sleeping for more than 745,854 jiffies. > > > > If your system does not let the RCU's kernel threads run, RCU cannot > > help you much. > > > > The ->state of 0x0 indicates that the kthread is in fact runnable, but > > did not get a chance to run. Was the system heavily loaded to the > > point where you would expect a kthread to remain preempted for many > > minutes? > > > > I am guessing that the answer is no, given that CPU 0 is actually idle > > (idle=ba2/2/0). Seems unlikely, but I have to ask: Did you bind the > > kthread to a specific CPU? > > The system should be lightly loaded (about 5-10% CPU usage on average), so > plenty of time for RCU to do its work. > > I didn't bind processes (be it userspace process or kthread) to a specific > CPU, thus it's all auto-configured. > > I guess the question then is what is the system busy with or waiting for > that prevents RCU to get its work done... > Shouldn't the watchdog print a trace of where CPU#0 is stuck? If so I might need > to check at which log level and make sure that loglevel reaches console. > Nothing did hit the disk though. Do you have a high-speed interface to capture and store console output? (As in something that can handle, say, 50MB in a reasonable period of time.) Thanx, Paul > Thanks, > Bruno > > > Thanx, Paul > > > > > [526440.020015] watchdog: BUG: soft lockup - CPU#0 stuck for 22s! [swapper/0:0] > > > [526468.020005] watchdog: BUG: soft lockup - CPU#0 stuck for 22s! [swapper/0:0] > > > [526478.320009] INFO: rcu_sched self-detected stall on CPU > > > [526478.320009] o0-...: (752143 ticks this GP) idle=ba2/2/0 softirq=37393463/37393463 fqs=0 > > > [526478.320009] o (t=752157 jiffies g=23779976 c=23779975 q=32) > > > [526478.320009] rcu_sched kthread starved for 752157 jiffies! g23779976 c23779975 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x0 > > > [526504.020016] watchdog: BUG: soft lockup - CPU#0 stuck for 22s! [swapper/0:0] > > > [526532.020007] watchdog: BUG: soft lockup - CPU#0 stuck for 22s! [swapper/0:0] > > > ... > > > > > > Attached is kernel config (4.13.11). > > > > > > > > > The output obtained with 4.11.3 was: > > > [ 280.680010] INFO: rcu_sched self-detected stall on CPU > > > [ 280.680021] o0-...: (27312 ticks this GP) dile=b11/2/0 softirq=6119/6119 fqs=0 > > > [ 280.680021] o (t=27312 jiffies g=441 c=440 q=0) > > > [ 280.680021] rcu_sched_kthread starved for 27312 jiffies! g441 c440 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x0 > > > ... > > > > > > > > > As it's a remote VM for which I don't have access to the host I have little > > > options for further digging (can't trigger sysrq's). > > > > > > > > > Same kernel (4.13.11) seems to be running just fine on another KVM-base VM that > > > has two CPUs. > > > > > > > > > Does it ring a bell or is there some info that might be of any use, > > > assuming I can obtain it? > > > > > > Bruno > > > > >
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