doc: Update stallwarn.txt to make causes more prominent

This commit rearranges the Documentation/RCU/stallwarn.txt file to
put the list of issues that can cause RCU CPU stall warnings near
the beginning of the document.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Documentation/RCU/stallwarn.txt

 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.
+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.
+
+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 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:
 
@@ -56,6 +149,9 @@ rcupdate.rcu_task_stall_timeout
 	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:
 
@@ -178,89 +274,3 @@ grace period is in flight.
 
 It is entirely possible to see stall warnings from normal and from
 expedited grace periods at about the same time from the same run.
-
-
-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().  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.
-
-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 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.