Using RCU's CPU Stall Detector

This document first discusses what sorts of issues RCU's CPU stall
detector can locate, and then discusses kernel parameters and Kconfig
options that can be used to fine-tune the detector's operation.  Finally,
this document explains the stall detector's "splat" format.


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.

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().  Note that cond_resched()
	does not necessarily prevent RCU CPU stall warnings.  Therefore,
	if the looping in the kernel is really expected and desirable
	behavior, you might need to replace some of the cond_resched()
	calls with calls to cond_resched_rcu_qs().

o	Booting Linux using a console connection that is too slow to
	keep up with the boot-time console-message rate.  For example,
	a 115Kbaud serial console can be -way- too slow to keep up
	with boot-time message rates, and will frequently result in
	RCU CPU stall warning messages.  Especially if you have added
	debug printk()s.

o	Anything that prevents RCU's grace-period kthreads from running.
	This can result in the "All QSes seen" console-log message.
	This message will include information on when the kthread last
	ran and how often it should be expected to run.  It can also
	result in the "rcu_.*kthread starved for" console-log message,
	which will include additional debugging information.

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_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_PREEMPT_RCU case, you might see stall-warning
	messages.

o	A periodic interrupt whose handler takes longer than the time
	interval between successive pairs of interrupts.  This can
	prevent RCU's kthreads and softirq handlers from running.
	Note that certain high-overhead debugging options, for example
	the function_graph tracer, can result in interrupt handler taking
	considerably longer than normal, which can in turn result in
	RCU CPU stall warnings.

o	Testing a workload on a fast system, tuning the stall-warning
	timeout down to just barely avoid RCU CPU stall warnings, and then
	running the same workload with the same stall-warning timeout on a
	slow system.  Note that thermal throttling and on-demand governors
	can cause a single system to be sometimes fast and sometimes slow!

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_COMMON=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, RCU-bh, and RCU-tasks implementations have CPU stall
warning.  Note that SRCU does -not- have CPU stall warnings.  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.  For information on RCU's event tracing,
see include/trace/events/rcu.h.


Fine-Tuning the RCU CPU Stall Detector

The rcuupdate.rcu_cpu_stall_suppress module parameter disables 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
	21 seconds.

	This configuration parameter may be changed at runtime via the
	/sys/module/rcupdate/parameters/rcu_cpu_stall_timeout, however
	this parameter is checked only at the beginning of a cycle.
	So if you are 10 seconds into a 40-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/rcupdate/parameters/rcu_cpu_stall_suppress.

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.  (This is a cpp
	macro, not a kernel configuration parameter.)

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.  (This is a cpp macro, not a kernel configuration
	parameter.)

rcupdate.rcu_task_stall_timeout

	This boot/sysfs parameter controls the RCU-tasks stall warning
	interval.  A value of zero or less suppresses RCU-tasks stall
	warnings.  A positive value sets the stall-warning interval
	in jiffies.  An RCU-tasks stall warning starts with the line:

		INFO: rcu_tasks detected stalls on tasks:

	And continues with the output of sched_show_task() for each
	task stalling the current RCU-tasks grace period.


Interpreting RCU's CPU Stall-Detector "Splats"

For non-RCU-tasks flavors of RCU, when a CPU detects that it is stalling,
it will print a message similar to the following:

	INFO: rcu_sched detected stalls on CPUs/tasks:
	2-...: (3 GPs behind) idle=06c/0/0 softirq=1453/1455 fqs=0
	16-...: (0 ticks this GP) idle=81c/0/0 softirq=764/764 fqs=0
	(detected by 32, t=2603 jiffies, g=7073, c=7072, q=625)

This message indicates that CPU 32 detected that CPUs 2 and 16 were both
causing stalls, and that the stall was affecting RCU-sched.  This message
will normally be followed by stack dumps for each CPU.  Please note that
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.

CPU 2's "(3 GPs behind)" indicates that this CPU has not interacted with
the RCU core for the past three grace periods.  In contrast, CPU 16's "(0
ticks this GP)" indicates that this CPU has not taken any scheduling-clock
interrupts during the current stalled grace period.

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 non-negative number if in the idle loop (as shown above) and a
very large positive number otherwise.

The "softirq=" portion of the message tracks the number of RCU softirq
handlers that the stalled CPU has executed.  The number before the "/"
is the number that had executed since boot at the time that this CPU
last noted the beginning of a grace period, which might be the current
(stalled) grace period, or it might be some earlier grace period (for
example, if the CPU might have been in dyntick-idle mode for an extended
time period.  The number after the "/" is the number that have executed
since boot until the current time.  If this latter number stays constant
across repeated stall-warning messages, it is possible that RCU's softirq
handlers are no longer able to execute on this CPU.  This can happen if
the stalled CPU is spinning with interrupts are disabled, or, in -rt
kernels, if a high-priority process is starving RCU's softirq handler.

The "fps=" shows the number of force-quiescent-state idle/offline
detection passes that the grace-period kthread has made across this
CPU since the last time that this CPU noted the beginning of a grace
period.

The "detected by" line indicates which CPU detected the stall (in this
case, CPU 32), how many jiffies have elapsed since the start of the
grace period (in this case 2603), the number of the last grace period
to start and to complete (7073 and 7072, respectively), and an estimate
of the total number of RCU callbacks queued across all CPUs (625 in
this case).

In kernels with CONFIG_RCU_FAST_NO_HZ, more information is printed
for each CPU:

	0: (64628 ticks this GP) idle=dd5/3fffffffffffffff/0 softirq=82/543 last_accelerate: a345/d342 nonlazy_posted: 25 .D

The "last_accelerate:" prints the low-order 16 bits (in hex) of the
jiffies counter when this CPU last invoked rcu_try_advance_all_cbs()
from rcu_needs_cpu() or last invoked rcu_accelerate_cbs() from
rcu_prepare_for_idle().  The "nonlazy_posted:" prints the number
of non-lazy callbacks posted since the last call to rcu_needs_cpu().
Finally, an "L" indicates that there are currently no non-lazy callbacks
("." is printed otherwise, as shown above) and "D" indicates that
dyntick-idle processing is enabled ("." is printed otherwise, for example,
if disabled via the "nohz=" kernel boot parameter).

If the grace period ends just as the stall warning starts printing,
there will be a spurious stall-warning message, which will include
the following:

	INFO: Stall ended before state dump start

This is rare, but does happen from time to time in real life.  It is also
possible for a zero-jiffy stall to be flagged in this case, depending
on how the stall warning and the grace-period initialization happen to
interact.  Please note that it is not possible to entirely eliminate this
sort of false positive without resorting to things like stop_machine(),
which is overkill for this sort of problem.

If all CPUs and tasks have passed through quiescent states, but the
grace period has nevertheless failed to end, the stall-warning splat
will include something like the following:

	All QSes seen, last rcu_preempt kthread activity 23807 (4297905177-4297881370), jiffies_till_next_fqs=3, root ->qsmask 0x0

The "23807" indicates that it has been more than 23 thousand jiffies
since the grace-period kthread ran.  The "jiffies_till_next_fqs"
indicates how frequently that kthread should run, giving the number
of jiffies between force-quiescent-state scans, in this case three,
which is way less than 23807.  Finally, the root rcu_node structure's
->qsmask field is printed, which will normally be zero.

If the relevant grace-period kthread has been unable to run prior to
the stall warning, as was the case in the "All QSes seen" line above,
the following additional line is printed:

	kthread starved for 23807 jiffies! g7073 c7072 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x1

Starving the grace-period kthreads of CPU time can of course result
in RCU CPU stall warnings even when all CPUs and tasks have passed
through the required quiescent states.  The "g" and "c" numbers flag the
number of the last grace period started and completed, respectively,
the "f" precedes the ->gp_flags command to the grace-period kthread,
the "RCU_GP_WAIT_FQS" indicates that the kthread is waiting for a short
timeout, and the "state" precedes value of the task_struct ->state field.


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.


Stall Warnings for Expedited Grace Periods

If an expedited grace period detects a stall, it will place a message
like the following in dmesg:

	INFO: rcu_sched detected expedited stalls on CPUs/tasks: { 7-... } 21119 jiffies s: 73 root: 0x2/.

This indicates that CPU 7 has failed to respond to a reschedule IPI.
The three periods (".") following the CPU number indicate that the CPU
is online (otherwise the first period would instead have been "O"),
that the CPU was online at the beginning of the expedited grace period
(otherwise the second period would have instead been "o"), and that
the CPU has been online at least once since boot (otherwise, the third
period would instead have been "N").  The number before the "jiffies"
indicates that the expedited grace period has been going on for 21,119
jiffies.  The number following the "s:" indicates that the expedited
grace-period sequence counter is 73.  The fact that this last value is
odd indicates that an expedited grace period is in flight.  The number
following "root:" is a bitmask that indicates which children of the root
rcu_node structure correspond to CPUs and/or tasks that are blocking the
current expedited grace period.  If the tree had more than one level,
additional hex numbers would be printed for the states of the other
rcu_node structures in the tree.

As with normal grace periods, 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 entirely possible to see stall warnings from normal and from
expedited grace periods at about the same time during the same run.
