rcu: Document failing tick as cause of RCU CPU stall warning

One of lclaudio's systems was seeing RCU CPU stall warnings from idle.
These turned out to be caused by a bug that stopped scheduling-clock
tick interrupts from being sent to a given CPU for several hundred seconds.
This commit therefore updates the documentation to call this out as a
possible cause for RCU CPU stall warnings.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

1 files changed, 5 insertions, 0 deletions

diff --git a/Documentation/RCU/stallwarn.txt b/Documentation/RCU/stallwarn.txt
index 4e959208f736..f3e0625f4290 100644
--- a/Documentation/RCU/stallwarn.txt
+++ b/Documentation/RCU/stallwarn.txt
@@ -101,6 +101,11 @@ o	A CPU-bound real-time task in a CONFIG_PREEMPT_RT kernel that
         CONFIG_TREE_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=n kernels.
+
 o       A bug in the RCU implementation.
 
 o       A hardware failure.  This is quite unlikely, but has occurred