rcu: Make CPU-hotplug removal operations enable tick

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. ]
[ paulmck: Apply simplifications suggested by Frederic Weisbecker. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tree.c

@@ -2101,6 +2101,9 @@ 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.
+	tick_dep_clear(TICK_DEP_BIT_RCU);
 	return 0;
 }
 
@@ -3085,6 +3088,9 @@ 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.
+	tick_dep_clear(TICK_DEP_BIT_RCU);
 	return 0;
 }
 
@@ -3105,6 +3111,9 @@ 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
+	tick_dep_set(TICK_DEP_BIT_RCU);
 	return 0;
 }