docs: RCU: Convert stallwarn.txt to ReST

- Add a SPDX header;
- Adjust document and section titles;
- Fix list markups;
- Some whitespace fixes and new line breaks;
- Mark literal blocks as such;
- Add it to RCU/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

1 files changed, 329 insertions, 0 deletions

diff --git a/Documentation/RCU/stallwarn.rst b/Documentation/RCU/stallwarn.rst
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+.. SPDX-License-Identifier: GPL-2.0
+
+==============================
+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:
+
+-	A CPU looping in an RCU read-side critical section.
+
+-	A CPU looping with interrupts disabled.
+
+-	A CPU looping with preemption disabled.
+
+-	A CPU looping with bottom halves disabled.
+
+-	For !CONFIG_PREEMPT kernels, a CPU looping anywhere in the kernel
+	without invoking schedule().  If the looping in the kernel is
+	really expected and desirable behavior, you might need to add
+	some calls to cond_resched().
+
+-	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.
+
+-	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.
+
+-	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.
+
+-	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.
+
+	You can use the rcutree.kthread_prio kernel boot parameter to
+	increase the scheduling priority of RCU's kthreads, which can
+	help avoid this problem.  However, please note that doing this
+	can increase your system's context-switch rate and thus degrade
+	performance.
+
+-	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.
+
+-	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!
+
+-	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.
+
+-	A bug in the RCU implementation.
+
+-	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-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 seconds.  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=7075, 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 an rcu_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 "fqs=" 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 grace-period sequence number (7075), 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 dyntick_enabled: 1
+
+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(). "dyntick_enabled: 1" indicates that dyntick-idle
+processing is enabled.
+
+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! g7075 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x1 ->cpu=5
+
+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" number shows the current
+grace-period sequence number, 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, the "state" precedes value of the
+task_struct ->state field, and the "cpu" indicates that the grace-period
+kthread last ran on CPU 5.
+
+
+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.