Messages in this thread | | | Date | Thu, 11 Dec 2014 08:50:10 -0800 | From | "Paul E. McKenney" <> | Subject | Re: rcu_sched stall detected, but no state dump |
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On Thu, Dec 11, 2014 at 10:35:15AM +0100, Miroslav Benes wrote: > On Wed, 10 Dec 2014, Paul E. McKenney wrote: > > > On Wed, Dec 10, 2014 at 01:52:02PM +0100, Miroslav Benes wrote: > > > > > > Hi, > > > > > > today I came across RCU stall which was correctly detected, but there is > > > no state dump. This is a bit suspicious, I think. > > > > > > This is the output in serial console: > > > > > > [ 105.727003] INFO: rcu_sched detected stalls on CPUs/tasks: > > > [ 105.727003] (detected by 0, t=21002 jiffies, g=3269, c=3268, q=138) > > > [ 105.727003] INFO: Stall ended before state dump start > > > [ 168.732006] INFO: rcu_sched detected stalls on CPUs/tasks: > > > [ 168.732006] (detected by 0, t=84007 jiffies, g=3269, c=3268, q=270) > > > [ 168.732006] INFO: Stall ended before state dump start > > > [ 231.737003] INFO: rcu_sched detected stalls on CPUs/tasks: > > > [ 231.737003] (detected by 0, t=147012 jiffies, g=3269, c=3268, q=388) > > > [ 231.737003] INFO: Stall ended before state dump start > > > [ 294.742003] INFO: rcu_sched detected stalls on CPUs/tasks: > > > [ 294.742003] (detected by 0, t=210017 jiffies, g=3269, c=3268, q=539) > > > [ 294.742003] INFO: Stall ended before state dump start > > > [ 357.747003] INFO: rcu_sched detected stalls on CPUs/tasks: > > > [ 357.747003] (detected by 0, t=273022 jiffies, g=3269, c=3268, q=693) > > > [ 357.747003] INFO: Stall ended before state dump start > > > [ 420.752003] INFO: rcu_sched detected stalls on CPUs/tasks: > > > [ 420.752003] (detected by 0, t=336027 jiffies, g=3269, c=3268, q=806) > > > [ 420.752003] INFO: Stall ended before state dump start > > > ... > > > > > > It can be reproduced by trivial code attached to this mail (infinite > > > loop in kernel thread created in kernel module). I have CONFIG_PREEMPT=n. > > > The kernel thread is scheduled on the same CPU which causes soft lockup > > > (reliably detected when lockup detector is on). There is certainly RCU > > > stall, but I would expect a state dump. Is this an expected behaviour? > > > Maybe I overlooked some config option, don't know. > > > > Definitely not expected behavior! Unless you have only one CPU, but in > > that case you should be running tiny RCU, not tree RCU. > > So indeed I messed up my configs somehow and run the code on uniprocessor > with SMP=y and tree RCU. With more processors RCU stall is detected and > correct state is dumped. On uniprocessor with SMP=n and tiny RCU > softlockup is detected, but no RCU stall in the log (is this correct?). So > I'm really sorry for the noise. > > Anyway I still think that running SMP kernel with tree RCU on > uniprocessor is possible option (albeit suboptimal and maybe improbable). > Should I proceed with your patch below and bisection or am I mistaken > completely and we can leave it because there is no problem?
Not a problem, there have been some interesting RCU CPU stall warnings recently, and your data did add some insight.
So the combination SMP=n PREEMPT=y can happen straightforwardly via kbuild. The combination SMP=n PREEMPT=n can happen (somewhat less) straightforwardly by running an SMP=y PREEMPT=n kernel on a single-CPU system. In both cases, what can happen is that RCU's grace-period kthreads are starved, which can result in those reports.
And these reports are confusing. I am considering attempting to improve the diagnostics. If I do, would you be willing to test the resulting patches?
Thanx, Paul
> Thanks, > Miroslav > > > > I tested 3.18 and also next-20141210. If it is improper behaviour I could > > > try to find a good kernel release and bisect it. > > > > Please! Could you also please try the (untested) diagnostic patch below > > on either 3.18 or -next? It should print messages covering all your > > CPUs, and the CPU that your kernel module's kthread is running on should > > show up as a one bit in the corresponding "mask" printout. > > > > Could you also please check what CPU the rcu_sched kthread is running on? > > One possibility is that this kthread is for some reason pinned on the > > same CPU that is running your kthread. > > > > Thanx, Paul > > > > ------------------------------------------------------------------------ > > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > > index 884e0ff020f1..d4018c025ac6 100644 > > --- a/kernel/rcu/tree.c > > +++ b/kernel/rcu/tree.c > > @@ -1129,6 +1129,7 @@ static void print_other_cpu_stall(struct rcu_state *rsp) > > print_cpu_stall_info_begin(); > > rcu_for_each_leaf_node(rsp, rnp) { > > raw_spin_lock_irqsave(&rnp->lock, flags); > > + pr_err("[ CPUs %d-%d mask %#lx ]\n", rnp->grplo, rnp->grphi, rnp->qsmask); > > ndetected += rcu_print_task_stall(rnp); > > if (rnp->qsmask != 0) { > > for (cpu = 0; cpu <= rnp->grphi - rnp->grplo; cpu++) > > >
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