[PATCH 06/16] rcu/nocb: Don't deoffload an offline CPU with pending work

[Frederic Weisbecker]

Offlining offloaded CPUs don't migrate their callbacks just like
non-offloaded CPUs do. It's up to their CB/GP kthread to handle what
remains.

Therefore we can't afford to de-offload an offline CPU that still has
pending work to do, or the callbacks would be ignored.

NOTE: The long term solution will be to wait for all pending callbacks
to be processed before completing a CPU down operation.

Suggested-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx>
Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
Cc: Neeraj Upadhyay <neeraju@xxxxxxxxxxxxxx>
---
 kernel/rcu/tree_plugin.h | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 09caf319a4a9..33e9d53d2181 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -2228,6 +2228,14 @@ static int __rcu_nocb_rdp_deoffload(struct rcu_data *rdp)
 	printk("De-offloading %d\n", rdp->cpu);
 
 	rcu_nocb_lock_irqsave(rdp, flags);
+	/*
+	 * If there are still pending work offloaded, the offline
+	 * CPU won't help much handling them.
+	 */
+	if (cpu_is_offline(rdp->cpu) && !rcu_segcblist_empty(&rdp->cblist)) {
+		rcu_nocb_unlock_irqrestore(rdp, flags);
+		return -EBUSY;
+	}
 	raw_spin_lock_rcu_node(rnp);
 	rcu_segcblist_offload(cblist, false);
 	raw_spin_unlock_rcu_node(rnp);
-- 
2.25.1