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
	sixty seconds.

	This configuration parameter may be changed at runtime via the
	/sys/module/rcutree/parameters/rcu_cpu_stall_timeout, however
	this parameter is checked only at the beginning of a cycle.
	So if you are 30 seconds into a 70-second stall, setting this
	sysfs parameter to (say) five will shorten the timeout for the
	-next- stall, or the following warning for the current stall
	(assuming the stall lasts long enough).  It will not affect the
	timing of the next warning for the current stall.

	Stall-warning messages may be enabled and disabled completely via
	/sys/module/rcutree/parameters/rcu_cpu_stall_suppress.

CONFIG_RCU_CPU_STALL_VERBOSE

	This kernel configuration parameter causes the stall warning to
	also dump the stacks of any tasks that are blocking the current
	RCU-preempt grace period.

RCU_CPU_STALL_INFO

	This kernel configuration parameter causes the stall warning to
	print out additional per-CPU diagnostic information, including
	information on scheduling-clock ticks and RCU's idle-CPU tracking.

RCU_STALL_DELAY_DELTA

	Although the lockdep facility is extremely useful, it does add
	some overhead.  Therefore, under CONFIG_PROVE_RCU, the
	RCU_STALL_DELAY_DELTA macro allows five extra seconds before
	giving an RCU CPU stall warning message.

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.

If the CONFIG_RCU_CPU_STALL_INFO kernel configuration parameter is set,
more information is printed with the stall-warning message, for example:

	INFO: rcu_preempt detected stall on CPU
	0: (63959 ticks this GP) idle=241/3fffffffffffffff/0
	   (t=65000 jiffies)

In kernels with CONFIG_RCU_FAST_NO_HZ, even more information is
printed:

	INFO: rcu_preempt detected stall on CPU
	0: (64628 ticks this GP) idle=dd5/3fffffffffffffff/0 drain=0 . timer not pending
	   (t=65000 jiffies)

The "(64628 ticks this GP)" indicates that this CPU has taken more
than 64,000 scheduling-clock interrupts during the current stalled
grace period.  If the CPU was not yet aware of the current grace
period (for example, if it was offline), then this part of the message
indicates how many grace periods behind the CPU is.

The "idle=" portion of the message prints the dyntick-idle state.
The hex number before the first "/" is the low-order 12 bits of the
dynticks counter, which will have an even-numbered value if the CPU is
in dyntick-idle mode and an odd-numbered value otherwise.  The hex
number between the two "/"s is the value of the nesting, which will
be a small positive number if in the idle loop and a very large positive
number (as shown above) otherwise.

For CONFIG_RCU_FAST_NO_HZ kernels, the "drain=0" indicates that the CPU is
not in the process of trying to force itself into dyntick-idle state, the
"." indicates that the CPU has not given up forcing RCU into dyntick-idle
mode (it would be "H" otherwise), and the "timer not pending" indicates
that the CPU has not recently forced RCU into dyntick-idle mode (it
would otherwise indicate the number of microseconds remaining in this
forced state).


Multiple Warnings From One Stall

If a stall lasts long enough, multiple stall-warning messages will be
printed for it.  The second and subsequent messages are printed at
longer intervals, so that the time between (say) the first and second
message will be about three times the interval between the beginning
of the stall and the first message.


What Causes RCU CPU Stall Warnings?

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
and with RCU's event tracing.
