[PATCH RFC tip/core/rcu 12/14] rcu/nohz: Force on tick when invoking lots of callbacks

["Paul E. McKenney" <paulmck@linux.ibm.com>]

Callback invocation can run for a significant time period, and within
CONFIG_NO_HZ_FULL=y kernels, this period will be devoid of scheduler-clock
interrupts.  In-kernel execution without such interrupts can cause all
manner of malfunction, with RCU CPU stall warnings being but one result.

This commit therefore forces scheduling-clock interrupts on whenever more
than a few RCU callbacks are invoked.  Because offloaded callback invocation
can be preempted, this forcing is withdrawn on each context switch.  This
in turn requires that the loop invoking RCU callbacks reiterate the forcing
periodically.

Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
---
 kernel/rcu/tree.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 71395e91b876..b80eb11c19e1 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2151,6 +2151,8 @@ static void rcu_do_batch(struct rcu_data *rdp)
 	rcu_nocb_unlock_irqrestore(rdp, flags);
 
 	/* Invoke callbacks. */
+	if (IS_ENABLED(CONFIG_NO_HZ_FULL))
+		tick_dep_set_task(current, TICK_DEP_MASK_RCU);
 	rhp = rcu_cblist_dequeue(&rcl);
 	for (; rhp; rhp = rcu_cblist_dequeue(&rcl)) {
 		debug_rcu_head_unqueue(rhp);
@@ -2217,6 +2219,8 @@ static void rcu_do_batch(struct rcu_data *rdp)
 	/* Re-invoke RCU core processing if there are callbacks remaining. */
 	if (!offloaded && rcu_segcblist_ready_cbs(&rdp->cblist))
 		invoke_rcu_core();
+	if (IS_ENABLED(CONFIG_NO_HZ_FULL))
+		tick_dep_clear_task(current, TICK_DEP_MASK_RCU);
 }
 
 /*
-- 
2.17.1