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Subject[PATCH tip/core/rcu 3/9] doc: Update stallwarn.txt to make causes more prominent
Date
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 | 190 +++++++++++++++++++++-------------------
1 file changed, 100 insertions(+), 90 deletions(-)

diff --git a/Documentation/RCU/stallwarn.txt b/Documentation/RCU/stallwarn.txt
index e93d04133fe7..96a3d81837e1 100644
--- a/Documentation/RCU/stallwarn.txt
+++ b/Documentation/RCU/stallwarn.txt
@@ -1,9 +1,102 @@
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.
--
2.5.2
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