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>

Documentation/RCU/index.rst

@@ -20,6 +20,7 @@ RCU concepts
    rculist_nulls
    rcuref
    torture
+   stallwarn
    listRCU
    NMI-RCU
    UP

Documentation/RCU/stallwarn.txt → Documentation/RCU/stallwarn.rst

+.. 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
@@ -7,39 +11,40 @@ 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.
+-	A CPU looping in an RCU read-side critical section.
 
-o	A CPU looping with interrupts disabled.
+-	A CPU looping with interrupts disabled.
 
-o	A CPU looping with preemption disabled.
+-	A CPU looping with preemption disabled.
 
-o	A CPU looping with bottom halves disabled.
+-	A CPU looping with bottom halves disabled.
 
-o	For !CONFIG_PREEMPT kernels, a CPU looping anywhere in the kernel
+-	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().
 
-o	Booting Linux using a console connection that is too slow to
+-	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.
+-	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,
+	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
+-	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,
@@ -48,7 +53,7 @@ o	A CPU-bound real-time task in a CONFIG_PREEMPT kernel, which might
 	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
+-	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
@@ -63,7 +68,7 @@ o	A CPU-bound real-time task in a CONFIG_PREEMPT_RT kernel that
 	can increase your system's context-switch rate and thus degrade
 	performance.
 
-o	A periodic interrupt whose handler takes longer than the time
+-	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
@@ -71,20 +76,20 @@ o	A periodic interrupt whose handler takes longer than the time
 	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
+-	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
+-	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.
+-	A bug in the RCU implementation.
 
-o	A hardware failure.  This is quite unlikely, but has occurred
+-	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
@@ -109,6 +114,7 @@ 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
@@ -118,6 +124,7 @@ 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
@@ -137,6 +144,7 @@ CONFIG_RCU_CPU_STALL_TIMEOUT
 	/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
@@ -145,6 +153,7 @@ RCU_STALL_DELAY_DELTA
 	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.
@@ -155,6 +164,7 @@ RCU_STALL_RAT_DELAY
 	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
@@ -168,9 +178,10 @@ rcupdate.rcu_task_stall_timeout
 
 
 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:
+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
@@ -223,7 +234,7 @@ 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:
+for each CPU::
 
 	0: (64628 ticks this GP) idle=dd5/3fffffffffffffff/0 softirq=82/543 last_accelerate: a345/d342 dyntick_enabled: 1
 
@@ -235,7 +246,7 @@ 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:
+the following::
 
 	INFO: Stall ended before state dump start
 
@@ -248,7 +259,7 @@ 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:
+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
 
@@ -261,7 +272,7 @@ which is way less than 23807.  Finally, the root rcu_node structure's
 
 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:
+the following additional line is printed::
 
 	kthread starved for 23807 jiffies! g7075 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x1 ->cpu=5
 
@@ -276,6 +287,7 @@ 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
@@ -285,9 +297,10 @@ 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:
+like the following in dmesg::
 
 	INFO: rcu_sched detected expedited stalls on CPUs/tasks: { 7-... } 21119 jiffies s: 73 root: 0x2/.
 

kernel/rcu/tree_stall.h

@@ -468,7 +468,7 @@ static void print_other_cpu_stall(unsigned long gp_seq, unsigned long gps)
 
 	/*
 	 * OK, time to rat on our buddy...
-	 * See Documentation/RCU/stallwarn.txt for info on how to debug
+	 * See Documentation/RCU/stallwarn.rst for info on how to debug
 	 * RCU CPU stall warnings.
 	 */
 	pr_err("INFO: %s detected stalls on CPUs/tasks:\n", rcu_state.name);
@@ -535,7 +535,7 @@ static void print_cpu_stall(unsigned long gps)
 
 	/*
 	 * OK, time to rat on ourselves...
-	 * See Documentation/RCU/stallwarn.txt for info on how to debug
+	 * See Documentation/RCU/stallwarn.rst for info on how to debug
 	 * RCU CPU stall warnings.
 	 */
 	pr_err("INFO: %s self-detected stall on CPU\n", rcu_state.name);