locktorture: Add call_rcu_chains module parameter

When running locktorture on large systems, there will normally be
enough RCU activity to ensure that there is a grace period in flight
at all times.  However, on smaller systems, RCU might well be idle the
majority of the time.  This situation can be inconvenient in cases where
the RCU CPU stall warning is part of the debugging process.

This commit therefore adds an call_rcu_chains module parameter to
locktorture, allowing the user to specify the desired number of
self-propagating call_rcu() chains.  For good measure, immediately
before invoking call_rcu(), the self-propagating RCU callback invokes
start_poll_synchronize_rcu() to force the immediate start of a grace
period, with the call_rcu() forcing another to start shortly thereafter.

Booting with locktorture.call_rcu_chains=2 increases the probability
of a stuck locking primitive resulting in an RCU CPU stall warning from
about 25% to nearly 100%.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>

1 files changed, 7 insertions, 0 deletions

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 0a1731a0f0ef..300e2c30986c 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2913,6 +2913,13 @@
 			to extract confidential information from the kernel
 			are also disabled.
 
+	locktorture.call_rcu_chains= [KNL]
+			Specify the number of self-propagating call_rcu()
+			chains to set up.  These are used to ensure that
+			there is a high probability of an RCU grace period
+			in progress at any given time.	Defaults to 0,
+			which disables these call_rcu() chains.
+
 	locktorture.nreaders_stress= [KNL]
 			Set the number of locking read-acquisition kthreads.
 			Defaults to being automatically set based on the