Using RCU's CPU Stall Detector

  The rcu_cpu_stall_suppress module parameter enables RCU's CPU stall
  detector, which detects conditions that unduly delay RCU grace periods.
  This module parameter enables CPU stall detection by default, but
  may be overridden via boot-time parameter or at runtime via sysfs.
  The stall detector's idea of what constitutes "unduly delayed" is
  controlled by a set of kernel configuration variables and cpp macros:

  CONFIG_RCU_CPU_STALL_TIMEOUT

  	This kernel configuration parameter defines the period of time
  	that RCU will wait from the beginning of a grace period until it
  	issues an RCU CPU stall warning.  This time period is normally
  	ten seconds.

  
  RCU_SECONDS_TILL_STALL_RECHECK
  
  	This macro defines the period of time that RCU will wait after

  	issuing a stall warning until it issues another stall warning

  	for the same stall.  This time period is normally set to three
  	times the check interval plus thirty seconds.

  
  RCU_STALL_RAT_DELAY

  	The CPU stall detector tries to make the offending CPU print its
  	own warnings, as this often gives better-quality stack traces.
  	However, if the offending CPU does not detect its own stall in
  	the number of jiffies specified by RCU_STALL_RAT_DELAY, then
  	some other CPU will complain.  This delay is normally set to
  	two jiffies.

  When a CPU detects that it is stalling, it will print a message similar
  to the following:
  
  INFO: rcu_sched_state detected stall on CPU 5 (t=2500 jiffies)
  
  This message indicates that CPU 5 detected that it was causing a stall,
  and that the stall was affecting RCU-sched.  This message will normally be
  followed by a stack dump of the offending CPU.  On TREE_RCU kernel builds,
  RCU and RCU-sched are implemented by the same underlying mechanism,
  while on TREE_PREEMPT_RCU kernel builds, RCU is instead implemented
  by rcu_preempt_state.
  
  On the other hand, if the offending CPU fails to print out a stall-warning
  message quickly enough, some other CPU will print a message similar to
  the following:
  
  INFO: rcu_bh_state detected stalls on CPUs/tasks: { 3 5 } (detected by 2, 2502 jiffies)
  
  This message indicates that CPU 2 detected that CPUs 3 and 5 were both
  causing stalls, and that the stall was affecting RCU-bh.  This message
  will normally be followed by stack dumps for each CPU.  Please note that
  TREE_PREEMPT_RCU builds can be stalled by tasks as well as by CPUs,
  and that the tasks will be indicated by PID, for example, "P3421".
  It is even possible for a rcu_preempt_state stall to be caused by both
  CPUs -and- tasks, in which case the offending CPUs and tasks will all
  be called out in the list.
  
  Finally, if the grace period ends just as the stall warning starts
  printing, there will be a spurious stall-warning message:
  
  INFO: rcu_bh_state detected stalls on CPUs/tasks: { } (detected by 4, 2502 jiffies)
  
  This is rare, but does happen from time to time in real life.
  
  So your kernel printed an RCU CPU stall warning.  The next question is
  "What caused it?"  The following problems can result in RCU CPU stall
  warnings:

  
  o	A CPU looping in an RCU read-side critical section.
  	

  o	A CPU looping with interrupts disabled.  This condition can
  	result in RCU-sched and RCU-bh stalls.

  o	A CPU looping with preemption disabled.  This condition can
  	result in RCU-sched stalls and, if ksoftirqd is in use, RCU-bh
  	stalls.
  
  o	A CPU looping with bottom halves disabled.  This condition can
  	result in RCU-sched and RCU-bh stalls.

  
  o	For !CONFIG_PREEMPT kernels, a CPU looping anywhere in the kernel
  	without invoking schedule().

  o	A CPU-bound real-time task in a CONFIG_PREEMPT kernel, which might
  	happen to preempt a low-priority task in the middle of an RCU
  	read-side critical section.   This is especially damaging if
  	that low-priority task is not permitted to run on any other CPU,
  	in which case the next RCU grace period can never complete, which
  	will eventually cause the system to run out of memory and hang.
  	While the system is in the process of running itself out of
  	memory, you might see stall-warning messages.
  
  o	A CPU-bound real-time task in a CONFIG_PREEMPT_RT kernel that
  	is running at a higher priority than the RCU softirq threads.
  	This will prevent RCU callbacks from ever being invoked,
  	and in a CONFIG_TREE_PREEMPT_RCU kernel will further prevent
  	RCU grace periods from ever completing.  Either way, the
  	system will eventually run out of memory and hang.  In the
  	CONFIG_TREE_PREEMPT_RCU case, you might see stall-warning
  	messages.

  o	A hardware or software issue shuts off the scheduler-clock
  	interrupt on a CPU that is not in dyntick-idle mode.  This
  	problem really has happened, and seems to be most likely to
  	result in RCU CPU stall warnings for CONFIG_NO_HZ=n kernels.

  o	A bug in the RCU implementation.
  
  o	A hardware failure.  This is quite unlikely, but has occurred

  	at least once in real life.  A CPU failed in a running system,

  	becoming unresponsive, but not causing an immediate crash.
  	This resulted in a series of RCU CPU stall warnings, eventually
  	leading the realization that the CPU had failed.

  The RCU, RCU-sched, and RCU-bh implementations have CPU stall warning.
  SRCU does not have its own CPU stall warnings, but its calls to
  synchronize_sched() will result in RCU-sched detecting RCU-sched-related
  CPU stalls.  Please note that RCU only detects CPU stalls when there is
  a grace period in progress.  No grace period, no CPU stall warnings.

  
  To diagnose the cause of the stall, inspect the stack traces.
  The offending function will usually be near the top of the stack.
  If you have a series of stall warnings from a single extended stall,
  comparing the stack traces can often help determine where the stall
  is occurring, which will usually be in the function nearest the top of
  that portion of the stack which remains the same from trace to trace.
  If you can reliably trigger the stall, ftrace can be quite helpful.

  
  RCU bugs can often be debugged with the help of CONFIG_RCU_TRACE.