Re: [PATCH v8 01/13] rcu: Fix missing nocb gp wake on rcu_barrier()

[Frederic Weisbecker <frederic@kernel.org>]

On Fri, Oct 14, 2022 at 08:46:06AM -0700, Paul E. McKenney wrote:
> On Fri, Oct 14, 2022 at 11:19:28AM -0400, Joel Fernandes wrote:
> > I agree with the discussion, though if all CBs are in the bypass list,
> > the patch will also save 2 jiffies.
> > 
> > So just commit messages that need rework then? This one can be taken instead:
> > https://lore.kernel.org/rcu/21ECDA9F-81B1-4D22-8B03-020FB5DADA4F@joelfernandes.org/T/#m14d21fbce23539a521693a4184b28ddc55d7d2c5
> 
> This one looks plausible to me.

With the following modified diff (passed 25 hours of TREE01):

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 96d678c9cfb6..7f1f6f792240 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3914,6 +3914,8 @@ static void rcu_barrier_entrain(struct rcu_data *rdp)
 {
 	unsigned long gseq = READ_ONCE(rcu_state.barrier_sequence);
 	unsigned long lseq = READ_ONCE(rdp->barrier_seq_snap);
+	bool wake_nocb = false;
+	bool was_alldone = false;
 
 	lockdep_assert_held(&rcu_state.barrier_lock);
 	if (rcu_seq_state(lseq) || !rcu_seq_state(gseq) || rcu_seq_ctr(lseq) != rcu_seq_ctr(gseq))
@@ -3922,7 +3924,14 @@ static void rcu_barrier_entrain(struct rcu_data *rdp)
 	rdp->barrier_head.func = rcu_barrier_callback;
 	debug_rcu_head_queue(&rdp->barrier_head);
 	rcu_nocb_lock(rdp);
+	/*
+	 * Flush bypass and wakeup rcuog if we add callbacks to an empty regular
+	 * queue. This way we don't wait for bypass timer that can reach seconds
+	 * if it's fully lazy.
+	 */
+	was_alldone = rcu_rdp_is_offloaded(rdp) && !rcu_segcblist_pend_cbs(&rdp->cblist);
 	WARN_ON_ONCE(!rcu_nocb_flush_bypass(rdp, NULL, jiffies));
+	wake_nocb = was_alldone && rcu_segcblist_pend_cbs(&rdp->cblist);
 	if (rcu_segcblist_entrain(&rdp->cblist, &rdp->barrier_head)) {
 		atomic_inc(&rcu_state.barrier_cpu_count);
 	} else {
@@ -3930,6 +3939,8 @@ static void rcu_barrier_entrain(struct rcu_data *rdp)
 		rcu_barrier_trace(TPS("IRQNQ"), -1, rcu_state.barrier_sequence);
 	}
 	rcu_nocb_unlock(rdp);
+	if (wake_nocb)
+		wake_nocb_gp(rdp, false);
 	smp_store_release(&rdp->barrier_seq_snap, gseq);
 }
 
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index d4a97e40ea9c..925dd98f8b23 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -439,6 +439,7 @@ static void zero_cpu_stall_ticks(struct rcu_data *rdp);
 static struct swait_queue_head *rcu_nocb_gp_get(struct rcu_node *rnp);
 static void rcu_nocb_gp_cleanup(struct swait_queue_head *sq);
 static void rcu_init_one_nocb(struct rcu_node *rnp);
+static bool wake_nocb_gp(struct rcu_data *rdp, bool force);
 static bool rcu_nocb_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
 				  unsigned long j);
 static bool rcu_nocb_try_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index f77a6d7e1356..094fd454b6c3 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -1558,6 +1558,11 @@ static void rcu_init_one_nocb(struct rcu_node *rnp)
 {
 }
 
+static bool wake_nocb_gp(struct rcu_data *rdp, bool force)
+{
+	return false;
+}
+
 static bool rcu_nocb_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
 				  unsigned long j)
 {