[PATCH v3 tip/core/rcu 9/9] documentation: Add verbiage on RCU-tasks stall warning messages

Paul E. McKenney
2014-07-31T21:55:42Z

From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>

This commit documents RCU-tasks stall warning messages and also describes
when to use the new cond_resched_rcu_qs() API.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
Documentation/RCU/stallwarn.txt | 33 ++++++++++++++++++++++++---------
1 file changed, 24 insertions(+), 9 deletions(-)

diff --git a/Documentation/RCU/stallwarn.txt b/Documentation/RCU/stallwarn.txt
index 68fe3ad27015..ef5a2fd4ff70 100644
--- a/Documentation/RCU/stallwarn.txt
+++ b/Documentation/RCU/stallwarn.txt
@@ -56,8 +56,20 @@ RCU_STALL_RAT_DELAY
two jiffies. (This is a cpp macro, not a kernel configuration
parameter.)

-When a CPU detects that it is stalling, it will print a message similar
-to the following:
+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
+ warnings. A positive value sets the stall-warning interval
+ in jiffies. An RCU-tasks stall warning starts wtih the line:
+
+ INFO: rcu_tasks detected stalls on tasks:
+
+ And continues with the output of sched_show_task() for each
+ task stalling the current RCU-tasks grace period.
+
+For non-RCU-tasks flavors of RCU, 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)

@@ -174,8 +186,12 @@ o A CPU looping with preemption disabled. This condition can
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 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 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
@@ -208,11 +224,10 @@ o A hardware failure. This is quite unlikely, but has occurred
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 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.
+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.
--
1.8.1.5