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.
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.
Best regards,
--
Miroslav Benes
SUSE Labs
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/kthread.h>
#include <linux/delay.h>
static struct task_struct *test_thread;
static int test_thread_fn(void *data)
{
while (1) {
if (kthread_should_stop())
break;
// msleep(1000);
}
return 0;
}
static int __init test_module_init(void)
{
test_thread = kthread_run(test_thread_fn, NULL, "test_thread");
return 0;
}
static void __exit test_module_cleanup(void)
{
kthread_stop(test_thread);
}
module_init(test_module_init);
module_exit(test_module_cleanup);
MODULE_LICENSE("GPL");
