[PATCH tip/core/rcu 12/21] rcu/nocb: Process batch locally as long as offloading isn't complete

[paulmck]

From: Frederic Weisbecker <frederic@xxxxxxxxxx>

This commit makes sure to process the callbacks locally (via either
RCU_SOFTIRQ or the rcuc kthread) whenever the segcblist isn't entirely
offloaded.  This ensures that callbacks are invoked one way or another
while a CPU is in the middle of a toggle operation.

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>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Inspired-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Tested-by: Boqun Feng <boqun.feng@xxxxxxxxx>
Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
 kernel/rcu/rcu_segcblist.h | 12 ++++++++++++
 kernel/rcu/tree.c          |  3 ++-
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/rcu_segcblist.h b/kernel/rcu/rcu_segcblist.h
index 28c9a52..afad6fc 100644
--- a/kernel/rcu/rcu_segcblist.h
+++ b/kernel/rcu/rcu_segcblist.h
@@ -95,6 +95,18 @@ static inline bool rcu_segcblist_is_offloaded(struct rcu_segcblist *rsclp)
 	return false;
 }
 
+static inline bool rcu_segcblist_completely_offloaded(struct rcu_segcblist *rsclp)
+{
+	int flags = SEGCBLIST_KTHREAD_CB | SEGCBLIST_KTHREAD_GP | SEGCBLIST_OFFLOADED;
+
+	if (IS_ENABLED(CONFIG_RCU_NOCB_CPU)) {
+		if ((rsclp->flags & flags) == flags)
+			return true;
+	}
+
+	return false;
+}
+
 /*
  * Are all segments following the specified segment of the specified
  * rcu_segcblist structure empty of callbacks?  (The specified
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 4ef59a5..ec14c01 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2700,6 +2700,7 @@ static __latent_entropy void rcu_core(void)
 	struct rcu_data *rdp = raw_cpu_ptr(&rcu_data);
 	struct rcu_node *rnp = rdp->mynode;
 	const bool offloaded = rcu_segcblist_is_offloaded(&rdp->cblist);
+	const bool do_batch = !rcu_segcblist_completely_offloaded(&rdp->cblist);
 
 	if (cpu_is_offline(smp_processor_id()))
 		return;
@@ -2729,7 +2730,7 @@ static __latent_entropy void rcu_core(void)
 	rcu_check_gp_start_stall(rnp, rdp, rcu_jiffies_till_stall_check());
 
 	/* If there are callbacks ready, invoke them. */
-	if (!offloaded && rcu_segcblist_ready_cbs(&rdp->cblist) &&
+	if (do_batch && rcu_segcblist_ready_cbs(&rdp->cblist) &&
 	    likely(READ_ONCE(rcu_scheduler_fully_active)))
 		rcu_do_batch(rdp);
 
-- 
2.9.5