rcu: upgrade stallwarn.txt documentation for CPU-bound RT processes
CPU-bound real-time processes can cause RCU CPU stall warnings, and
much other trouble as well. Document the fact that they can cause
RCU CPU stall warnings.
Suggested-by: Darren Hart <dvhltc@us.ibm.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
diff --git a/Documentation/RCU/stallwarn.txt b/Documentation/RCU/stallwarn.txt
index 44c6dcc..862c08e 100644
--- a/Documentation/RCU/stallwarn.txt
+++ b/Documentation/RCU/stallwarn.txt
@@ -80,6 +80,24 @@
o For !CONFIG_PREEMPT kernels, a CPU looping anywhere in the kernel
without invoking schedule().
+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_TREE_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_TREE_PREEMPT_RCU case, you might see stall-warning
+ messages.
+
o A bug in the RCU implementation.
o A hardware failure. This is quite unlikely, but has occurred