Messages in this thread Patch in this message | | | Date | Wed, 10 Dec 2014 08:28:23 -0800 | From | "Paul E. McKenney" <> | Subject | Re: rcu_sched stall detected, but no state dump |
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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.
> 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
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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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