[PATCH tip/core/rcu 06/12] rcu: Make CPU-hotplug removal operations enable tick

[paulmck@xxxxxxxxxx]

From: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxx>

CPU-hotplug removal operations run the multi_cpu_stop() function, which
relies on the scheduler to gain control from whatever is running on the
various online CPUs, including any nohz_full CPUs running long loops in
kernel-mode code.  Lack of the scheduler-clock interrupt on such CPUs
can delay multi_cpu_stop() for several minutes and can also result in
RCU CPU stall warnings.  This commit therefore causes CPU-hotplug removal
operations to enable the scheduler-clock interrupt on all online CPUs.

[ paulmck: Apply Joel Fernandes TICK_DEP_MASK_RCU->TICK_DEP_BIT_RCU fix. ]
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxx>
---
 kernel/rcu/tree.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index f708d54..74bf5c65 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2091,6 +2091,7 @@ static void rcu_cleanup_dead_rnp(struct rcu_node *rnp_leaf)
  */
 int rcutree_dead_cpu(unsigned int cpu)
 {
+	int c;
 	struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
 	struct rcu_node *rnp = rdp->mynode;  /* Outgoing CPU's rdp & rnp. */
 
@@ -2101,6 +2102,10 @@ int rcutree_dead_cpu(unsigned int cpu)
 	rcu_boost_kthread_setaffinity(rnp, -1);
 	/* Do any needed no-CB deferred wakeups from this CPU. */
 	do_nocb_deferred_wakeup(per_cpu_ptr(&rcu_data, cpu));
+
+	// Stop-machine done, so allow nohz_full to disable tick.
+	for_each_online_cpu(c)
+		tick_dep_clear_cpu(c, TICK_DEP_BIT_RCU);
 	return 0;
 }
 
@@ -3074,6 +3079,7 @@ static void rcutree_affinity_setting(unsigned int cpu, int outgoing)
  */
 int rcutree_online_cpu(unsigned int cpu)
 {
+	int c;
 	unsigned long flags;
 	struct rcu_data *rdp;
 	struct rcu_node *rnp;
@@ -3087,6 +3093,10 @@ int rcutree_online_cpu(unsigned int cpu)
 		return 0; /* Too early in boot for scheduler work. */
 	sync_sched_exp_online_cleanup(cpu);
 	rcutree_affinity_setting(cpu, -1);
+
+	// Stop-machine done, so allow nohz_full to disable tick.
+	for_each_online_cpu(c)
+		tick_dep_clear_cpu(c, TICK_DEP_BIT_RCU);
 	return 0;
 }
 
@@ -3096,6 +3106,7 @@ int rcutree_online_cpu(unsigned int cpu)
  */
 int rcutree_offline_cpu(unsigned int cpu)
 {
+	int c;
 	unsigned long flags;
 	struct rcu_data *rdp;
 	struct rcu_node *rnp;
@@ -3107,6 +3118,10 @@ int rcutree_offline_cpu(unsigned int cpu)
 	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 
 	rcutree_affinity_setting(cpu, cpu);
+
+	// nohz_full CPUs need the tick for stop-machine to work quickly
+	for_each_online_cpu(c)
+		tick_dep_set_cpu(c, TICK_DEP_BIT_RCU);
 	return 0;
 }
 
-- 
2.9.5