rcu: improve the RCU CPU-stall warning documentation

The existing Documentation/RCU/stallwarn.txt has proven unhelpful, so
rework it a bit.  In particular, show how to interpret the stall-warning
messages.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 71 insertions, 23 deletions

diff --git a/Documentation/RCU/stallwarn.txt b/Documentation/RCU/stallwarn.txt
index 1423d2570d78..44c6dcc93d6d 100644
--- a/Documentation/RCU/stallwarn.txt
+++ b/Documentation/RCU/stallwarn.txt
@@ -3,35 +3,79 @@ Using RCU's CPU Stall Detector
 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().
@@ -39,20 +83,24 @@ o	For !CONFIG_PREEMPT kernels, a CPU looping anywhere in the kernel
 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.