[PATCH RFC 5/7] rcu: Add rcutree.boost_disable to disable priority boosting at runtime

[Joel Fernandes <joelagnelf@nvidia.com>]

CONFIG_RCU_BOOST is now enabled automatically for preemptible RCU and
cannot be turned off, so rcutorture's BUSTED-BOOST scenario, which is
meant to show that rcutorture detects broken priority boosting, can no
longer break it.  Add a boot parameter that makes rcu_initiate_boost()
do nothing, giving that scenario a way to break boosting.

Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
---
 kernel/rcu/tree_plugin.h | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index ae5d21bae7bc..04a58dd003f4 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1270,6 +1270,9 @@ static int rcu_boost_kthread(void *arg)
 	return 0;
 }
 
+static bool boost_disable;
+module_param(boost_disable, bool, 0444);
+
 /*
  * Check to see if it is time to start boosting RCU readers that are
  * blocking the current grace period, and, if so, tell the per-rcu_node
@@ -1284,7 +1287,7 @@ static void rcu_initiate_boost(struct rcu_node *rnp, unsigned long flags)
 	__releases(rnp->lock)
 {
 	raw_lockdep_assert_held_rcu_node(rnp);
-	if (!rnp->boost_kthread_task ||
+	if (boost_disable || !rnp->boost_kthread_task ||
 	    (!rcu_preempt_blocked_readers_cgp(rnp) && !rnp->exp_tasks)) {
 		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 		return;
-- 
2.54.0