Re: RCU stall/SOFT-Lockup on 4.11.3/4.13.11 after multiple days uptime

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Sun, Nov 12, 2017 at 12:09:28PM +0100, Bruno Prémont wrote:
> On Sat, 11 November 2017 "Paul E. McKenney" <paulmck@linux.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  
> > 
> > 
>