[PATCH] rcu/nocb: Dump gp state even if rdp gp itself is not offloaded

[PATCH] rcu/nocb: Dump gp state even if rdp gp itself is not offloaded

[Frederic Weisbecker]

When a stall is detected, the state of each NOCB CPU is dumped along
with the state of each NOCB group. The latter part however is
incidentally ignored if the NOCB group leader happens not to be
offloaded itself.

Fix this to make sure related precious informations aren't lost over
a stall report.

Reported-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
---
 kernel/rcu/tree_nocb.h  | 3 +++
 kernel/rcu/tree_stall.h | 3 +--
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index 4a954ecf1c36..56baa78c6e85 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -1564,6 +1564,9 @@ static void show_rcu_nocb_state(struct rcu_data *rdp)
 	if (rdp->nocb_gp_rdp == rdp)
 		show_rcu_nocb_gp_state(rdp);
 
+	if (!rcu_segcblist_is_offloaded(&rdp->cblist))
+		return;
+
 	nocb_next_rdp = list_next_or_null_rcu(&rdp->nocb_gp_rdp->nocb_head_rdp,
 					      &rdp->nocb_entry_rdp,
 					      typeof(*rdp),
diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
index 56b21219442b..f96235d51642 100644
--- a/kernel/rcu/tree_stall.h
+++ b/kernel/rcu/tree_stall.h
@@ -927,8 +927,7 @@ void show_rcu_gp_kthreads(void)
 	for_each_possible_cpu(cpu) {
 		rdp = per_cpu_ptr(&rcu_data, cpu);
 		cbs += data_race(READ_ONCE(rdp->n_cbs_invoked));
-		if (rcu_segcblist_is_offloaded(&rdp->cblist))
-			show_rcu_nocb_state(rdp);
+		show_rcu_nocb_state(rdp);
 	}
 	pr_info("RCU callbacks invoked since boot: %lu\n", cbs);
 	show_rcu_tasks_gp_kthreads();
-- 
2.48.1

Re: [PATCH] rcu/nocb: Dump gp state even if rdp gp itself is not offloaded

[Paul E. McKenney]

On Tue, Mar 18, 2025 at 10:23:36AM +0100, Frederic Weisbecker wrote:
> When a stall is detected, the state of each NOCB CPU is dumped along
> with the state of each NOCB group. The latter part however is
> incidentally ignored if the NOCB group leader happens not to be
> offloaded itself.
> 
> Fix this to make sure related precious informations aren't lost over
> a stall report.
> 
> Reported-by: Paul E. McKenney <paulmck@kernel.org>
> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>

Much better, thank you!

Reviewed-by: Paul E. McKenney <paulmck@kernel.org>

> ---
>  kernel/rcu/tree_nocb.h  | 3 +++
>  kernel/rcu/tree_stall.h | 3 +--
>  2 files changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index 4a954ecf1c36..56baa78c6e85 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -1564,6 +1564,9 @@ static void show_rcu_nocb_state(struct rcu_data *rdp)
>  	if (rdp->nocb_gp_rdp == rdp)
>  		show_rcu_nocb_gp_state(rdp);
>  
> +	if (!rcu_segcblist_is_offloaded(&rdp->cblist))
> +		return;
> +
>  	nocb_next_rdp = list_next_or_null_rcu(&rdp->nocb_gp_rdp->nocb_head_rdp,
>  					      &rdp->nocb_entry_rdp,
>  					      typeof(*rdp),
> diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
> index 56b21219442b..f96235d51642 100644
> --- a/kernel/rcu/tree_stall.h
> +++ b/kernel/rcu/tree_stall.h
> @@ -927,8 +927,7 @@ void show_rcu_gp_kthreads(void)
>  	for_each_possible_cpu(cpu) {
>  		rdp = per_cpu_ptr(&rcu_data, cpu);
>  		cbs += data_race(READ_ONCE(rdp->n_cbs_invoked));
> -		if (rcu_segcblist_is_offloaded(&rdp->cblist))
> -			show_rcu_nocb_state(rdp);
> +		show_rcu_nocb_state(rdp);
>  	}
>  	pr_info("RCU callbacks invoked since boot: %lu\n", cbs);
>  	show_rcu_tasks_gp_kthreads();
> -- 
> 2.48.1
>