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SubjectRCU CPU stall console spews leads to soft lockup disabled is reasonable ?
Hi,

On my x86_64 qemu virtual machine, RCU CPU stall console spews may
lead to soft lockup disabled.

If softlockup_thresh > rcu_cpu_stall_timeout (softlockup_thresh = 2 * watchdog_thresh):

/ #
/ # busybox cat /sys/module/rcupdate/parameters/rcu_cpu_stall_timeout
21
/ # echo 60 > /proc/sys/kernel/watchdog_thresh
/ # busybox insmod softlockup_test.ko
[ 44.959044] INFO: rcu_preempt detected stalls on CPUs/tasks: {} (detected by 0, t=21002 jiffies, g=85, c=84, q=4)
[ 44.959044] INFO: Stall ended before state dump start
[ 107.964045] INFO: rcu_preempt detected stalls on CPUs/tasks: {} (detected by 0, t=84007 jiffies, g=85, c=84, q=4)
[ 107.964045] INFO: Stall ended before state dump start
[ 170.969060] INFO: rcu_preempt detected stalls on CPUs/tasks: {} (detected by 0, t=147012 jiffies, g=85, c=84, q=4)
[ 170.969060] INFO: Stall ended before state dump start
[ 233.974057] INFO: rcu_preempt detected stalls on CPUs/tasks: {} (detected by 0, t=210017 jiffies, g=85, c=84, q=4)
[ 233.974057] INFO: Stall ended before state dump start
[ 296.979059] INFO: rcu_preempt detected stalls on CPUs/tasks: {} (detected by 0, t=273022 jiffies, g=85, c=84, q=4)
[ 296.979059] INFO: Stall ended before state dump start
[ 359.984058] INFO: rcu_preempt detected stalls on CPUs/tasks: {} (detected by 0, t=336027 jiffies, g=85, c=84, q=4)
[ 359.984058] INFO: Stall ended before state dump start
[ 422.989059] INFO: rcu_preempt detected stalls on CPUs/tasks: {} (detected by 0, t=399032 jiffies, g=85, c=84, q=4)
[ 422.989059] INFO: Stall ended before state dump start
[ 485.994056] INFO: rcu_preempt detected stalls on CPUs/tasks: {} (detected by 0, t=462037 jiffies, g=85, c=84, q=4)
[ 485.994056] INFO: Stall ended before state dump start
[ 548.999059] INFO: rcu_preempt detected stalls on CPUs/tasks: {} (detected by 0, t=525042 jiffies, g=85, c=84, q=4)
[ 548.999059] INFO: Stall ended before state dump start
[ 612.004061] INFO: rcu_preempt detected stalls on CPUs/tasks: {} (detected by 0, t=588047 jiffies, g=85, c=84, q=4)
[ 612.004061] INFO: Stall ended before state dump start
[ 675.009058] INFO: rcu_preempt detected stalls on CPUs/tasks: {} (detected by 0, t=651052 jiffies, g=85, c=84, q=4)
[ 675.009058] INFO: Stall ended before state dump start

If softlockup_thresh < rcu_cpu_stall_timeout:

/ #
/ # busybox cat /sys/module/rcupdate/parameters/rcu_cpu_stall_timeout
21
/ # echo 5 > /proc/sys/kernel/watchdog_thresh
/ # busybox insmod softlockup_test.ko
[ 38.450061] BUG: soft lockup - CPU#0 stuck for 11s! [busybox:53]
[ 52.450061] BUG: soft lockup - CPU#0 stuck for 11s! [busybox:53]
[ 66.450073] BUG: soft lockup - CPU#0 stuck for 11s! [busybox:53]
[ 80.450060] BUG: soft lockup - CPU#0 stuck for 11s! [busybox:53]
[ 94.450061] BUG: soft lockup - CPU#0 stuck for 11s! [busybox:53]

The softlockup_test.ko source code is:
//
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/spinlock.h>
#include <linux/slab.h>

static int hello_start(void)
{
DEFINE_SPINLOCK(hello_lock);
spin_lock_init(&hello_lock);
spin_lock(&hello_lock);
spin_lock(&hello_lock);
return 0;
}

static int __init test_init(void)
{
hello_start();

printk(KERN_INFO "Module init\n");
return 0;
}

static void __exit test_exit(void)
{
printk(KERN_INFO "Module exit!\n");
}

module_init(test_init)
module_exit(test_exit)
MODULE_LICENSE("GPL");
//

My kernel version is v3.10.63, and i checked the kernel source code,

update_process_times
-> run_local_timers
-> hrtimer_run_queues
-> __run_hrtimer
-> watchdog_timer_fn
-> is_softlockup

-> rcu_check_callbacks
-> __rcu_pending
-> check_cpu_stall
-> print_cpu_stall

If softlockup_thresh > rcu_cpu_stall_timeout, print_cpu_stall will print log to serial port.

The 8250 serial driver will call serial8250_console_write => touch_nmi_watchdog() which reset
watchdog_touch_ts to 0. So the softlockup will not be triggered.

Is this reasonable? Why?
If it is not reasonable, we should adjust the printk loglevel from *KERN_ERR* to *KERN_INFO*
in print_cpu_stall.




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