documentation: Add verbiage on RCU-tasks stall warning messages

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>

diff --git a/Documentation/RCU/stallwarn.txt b/Documentation/RCU/stallwarn.txt
index 68fe3ad..ef5a2fd 100644 (file)
--- 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.