rcu/nocb: De-offloading CB kthread

To de-offload callback processing back onto a CPU, it is necessary to
clear SEGCBLIST_OFFLOAD and notify the nocb CB kthread, which will then
clear its own bit flag and go to sleep to stop handling callbacks.  This
commit makes that change.  It will also be necessary to notify the nocb
GP kthread in this same way, which is the subject of a follow-on commit.

Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Neeraj Upadhyay <neeraju@codeaurora.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Inspired-by: Paul E. McKenney <paulmck@kernel.org>
Tested-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
[ paulmck: Add export per kernel test robot feedback. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

1 files changed, 112 insertions, 18 deletions

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 7e291ce0a1d6..1b870d0d2445 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -2081,16 +2081,29 @@ static int rcu_nocb_gp_kthread(void *arg)
 	return 0;
 }
 
+static inline bool nocb_cb_can_run(struct rcu_data *rdp)
+{
+	u8 flags = SEGCBLIST_OFFLOADED | SEGCBLIST_KTHREAD_CB;
+	return rcu_segcblist_test_flags(&rdp->cblist, flags);
+}
+
+static inline bool nocb_cb_wait_cond(struct rcu_data *rdp)
+{
+	return nocb_cb_can_run(rdp) && !READ_ONCE(rdp->nocb_cb_sleep);
+}
+
 /*
  * Invoke any ready callbacks from the corresponding no-CBs CPU,
  * then, if there are no more, wait for more to appear.
  */
 static void nocb_cb_wait(struct rcu_data *rdp)
 {
+	struct rcu_segcblist *cblist = &rdp->cblist;
+	struct rcu_node *rnp = rdp->mynode;
+	bool needwake_state = false;
+	bool needwake_gp = false;
 	unsigned long cur_gp_seq;
 	unsigned long flags;
-	bool needwake_gp = false;
-	struct rcu_node *rnp = rdp->mynode;
 
 	local_irq_save(flags);
 	rcu_momentary_dyntick_idle();
@@ -2100,32 +2113,50 @@ static void nocb_cb_wait(struct rcu_data *rdp)
 	local_bh_enable();
 	lockdep_assert_irqs_enabled();
 	rcu_nocb_lock_irqsave(rdp, flags);
-	if (rcu_segcblist_nextgp(&rdp->cblist, &cur_gp_seq) &&
+	if (rcu_segcblist_nextgp(cblist, &cur_gp_seq) &&
 	    rcu_seq_done(&rnp->gp_seq, cur_gp_seq) &&
 	    raw_spin_trylock_rcu_node(rnp)) { /* irqs already disabled. */
 		needwake_gp = rcu_advance_cbs(rdp->mynode, rdp);
 		raw_spin_unlock_rcu_node(rnp); /* irqs remain disabled. */
 	}
-	if (rcu_segcblist_ready_cbs(&rdp->cblist)) {
-		rcu_nocb_unlock_irqrestore(rdp, flags);
-		if (needwake_gp)
-			rcu_gp_kthread_wake();
-		return;
-	}
 
-	trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("CBSleep"));
 	WRITE_ONCE(rdp->nocb_cb_sleep, true);
+
+	if (rcu_segcblist_test_flags(cblist, SEGCBLIST_OFFLOADED)) {
+		if (rcu_segcblist_ready_cbs(cblist))
+			WRITE_ONCE(rdp->nocb_cb_sleep, false);
+	} else {
+		/*
+		 * De-offloading. Clear our flag and notify the de-offload worker.
+		 * We won't touch the callbacks and keep sleeping until we ever
+		 * get re-offloaded.
+		 */
+		WARN_ON_ONCE(!rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_CB));
+		rcu_segcblist_clear_flags(cblist, SEGCBLIST_KTHREAD_CB);
+		if (!rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_GP))
+			needwake_state = true;
+	}
+
+	if (rdp->nocb_cb_sleep)
+		trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("CBSleep"));
+
 	rcu_nocb_unlock_irqrestore(rdp, flags);
 	if (needwake_gp)
 		rcu_gp_kthread_wake();
-	swait_event_interruptible_exclusive(rdp->nocb_cb_wq,
-				 !READ_ONCE(rdp->nocb_cb_sleep));
-	if (!smp_load_acquire(&rdp->nocb_cb_sleep)) { /* VVV */
+
+	if (needwake_state)
+		swake_up_one(&rdp->nocb_state_wq);
+
+	do {
+		swait_event_interruptible_exclusive(rdp->nocb_cb_wq,
+						    nocb_cb_wait_cond(rdp));
+
 		/* ^^^ Ensure CB invocation follows _sleep test. */
-		return;
-	}
-	WARN_ON(signal_pending(current));
-	trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("WokeEmpty"));
+		if (smp_load_acquire(&rdp->nocb_cb_sleep)) {
+			WARN_ON(signal_pending(current));
+			trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("WokeEmpty"));
+		}
+	} while (!nocb_cb_can_run(rdp));
 }
 
 /*
@@ -2187,6 +2218,67 @@ static void do_nocb_deferred_wakeup(struct rcu_data *rdp)
 		do_nocb_deferred_wakeup_common(rdp);
 }
 
+static int __rcu_nocb_rdp_deoffload(struct rcu_data *rdp)
+{
+	struct rcu_segcblist *cblist = &rdp->cblist;
+	bool wake_cb = false;
+	unsigned long flags;
+
+	printk("De-offloading %d\n", rdp->cpu);
+
+	rcu_nocb_lock_irqsave(rdp, flags);
+	rcu_segcblist_offload(cblist, false);
+
+	if (rdp->nocb_cb_sleep) {
+		rdp->nocb_cb_sleep = false;
+		wake_cb = true;
+	}
+	rcu_nocb_unlock_irqrestore(rdp, flags);
+
+	if (wake_cb)
+		swake_up_one(&rdp->nocb_cb_wq);
+
+	swait_event_exclusive(rdp->nocb_state_wq,
+			      !rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_CB));
+
+	return 0;
+}
+
+static long rcu_nocb_rdp_deoffload(void *arg)
+{
+	struct rcu_data *rdp = arg;
+
+	WARN_ON_ONCE(rdp->cpu != raw_smp_processor_id());
+	return __rcu_nocb_rdp_deoffload(rdp);
+}
+
+int rcu_nocb_cpu_deoffload(int cpu)
+{
+	struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
+	int ret = 0;
+
+	if (rdp == rdp->nocb_gp_rdp) {
+		pr_info("Can't deoffload an rdp GP leader (yet)\n");
+		return -EINVAL;
+	}
+	mutex_lock(&rcu_state.barrier_mutex);
+	cpus_read_lock();
+	if (rcu_segcblist_is_offloaded(&rdp->cblist)) {
+		if (cpu_online(cpu)) {
+			ret = work_on_cpu(cpu, rcu_nocb_rdp_deoffload, rdp);
+		} else {
+			ret = __rcu_nocb_rdp_deoffload(rdp);
+		}
+		if (!ret)
+			cpumask_clear_cpu(cpu, rcu_nocb_mask);
+	}
+	cpus_read_unlock();
+	mutex_unlock(&rcu_state.barrier_mutex);
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(rcu_nocb_cpu_deoffload);
+
 void __init rcu_init_nohz(void)
 {
 	int cpu;
@@ -2229,7 +2321,8 @@ void __init rcu_init_nohz(void)
 		rdp = per_cpu_ptr(&rcu_data, cpu);
 		if (rcu_segcblist_empty(&rdp->cblist))
 			rcu_segcblist_init(&rdp->cblist);
-		rcu_segcblist_offload(&rdp->cblist);
+		rcu_segcblist_offload(&rdp->cblist, true);
+		rcu_segcblist_set_flags(&rdp->cblist, SEGCBLIST_KTHREAD_CB);
 	}
 	rcu_organize_nocb_kthreads();
 }
@@ -2239,6 +2332,7 @@ static void __init rcu_boot_init_nocb_percpu_data(struct rcu_data *rdp)
 {
 	init_swait_queue_head(&rdp->nocb_cb_wq);
 	init_swait_queue_head(&rdp->nocb_gp_wq);
+	init_swait_queue_head(&rdp->nocb_state_wq);
 	raw_spin_lock_init(&rdp->nocb_lock);
 	raw_spin_lock_init(&rdp->nocb_bypass_lock);
 	raw_spin_lock_init(&rdp->nocb_gp_lock);