rcu: improve the RCU CPU-stall warning documentation

The CONFIG_RCU_CPU_STALL_DETECTOR kernel config parameter enables

RCU's CPU stall detector, which detects conditions that unduly delay

RCU grace periods.  The stall detector's idea of what constitutes

"unduly delayed" is controlled by a pair of C preprocessor macros:

"unduly delayed" is controlled by a set of C preprocessor macros:

RCU_SECONDS_TILL_STALL_CHECK

	This macro defines the period of time that RCU will wait from

	the beginning of a grace period until it issues an RCU CPU

	stall warning.	It is normally ten seconds.

	stall warning.	This time period is normally ten seconds.

RCU_SECONDS_TILL_STALL_RECHECK

	This macro defines the period of time that RCU will wait after

	issuing a stall warning until it issues another stall warning.

	It is normally set to thirty seconds.

	issuing a stall warning until it issues another stall warning

	for the same stall.  This time period is normally set to thirty

	seconds.

RCU_STALL_RAT_DELAY

	The CPU stall detector tries to make the offending CPU rat on itself,

	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 other CPUs will

	complain.  This is normally set to two jiffies.

	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.

The following problems can result in an RCU CPU stall warning:

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.

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 interrupts disabled.  This condition can

	result in RCU-sched and RCU-bh stalls.

o	A CPU looping with preemption 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().

o	A bug in the RCU implementation.

o	A hardware failure.  This is quite unlikely, but has occurred

	at least once in a former life.  A CPU failed in a running system,

	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 do so directly, but its calls to synchronize_sched() will

result in RCU-sched detecting any CPU stalls that might be occurring.

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 the stack that stays the same from

trace to trace.

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.