[PATCH 2/6] rcu/nocb: Prepare nocb_cb_wait() to start with a non-offloaded rdp

[Frederic Weisbecker]

In order to be able to toggle the offloaded state from cpusets, a nocb
kthread will need to be created for all possible CPUs whenever the
"rcu_nocbs=" or "nohz_full=" parameters are passed.

Therefore nocb_cb_wait() kthread must prepare to start running on a
de-offloaded rdp. Simply move the sleeping condition in the beginning
of the kthread callback is enough to prevent from running callbacks
before the rdp ever becomes offloaded.

Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
Cc: Neeraj Upadhyay <quic_neeraju@xxxxxxxxxxx>
Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
Cc: Uladzislau Rezki <urezki@xxxxxxxxx>
Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
---
 kernel/rcu/tree_nocb.h | 23 ++++++++++++-----------
 1 file changed, 12 insertions(+), 11 deletions(-)

diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index cc1165559177..e1cb06840454 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -797,6 +797,18 @@ static void nocb_cb_wait(struct rcu_data *rdp)
 	bool can_sleep = true;
 	struct rcu_node *rnp = rdp->mynode;
 
+	do {
+		swait_event_interruptible_exclusive(rdp->nocb_cb_wq,
+						    nocb_cb_wait_cond(rdp));
+
+		// VVV Ensure CB invocation follows _sleep test.
+		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));
+
+
 	local_irq_save(flags);
 	rcu_momentary_dyntick_idle();
 	local_irq_restore(flags);
@@ -849,17 +861,6 @@ static void nocb_cb_wait(struct rcu_data *rdp)
 
 	if (needwake_state)
 		swake_up_one(&rdp->nocb_state_wq);
-
-	do {
-		swait_event_interruptible_exclusive(rdp->nocb_cb_wq,
-						    nocb_cb_wait_cond(rdp));
-
-		// VVV Ensure CB invocation follows _sleep test.
-		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));
 }
 
 /*
-- 
2.25.1