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

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Message-ID: <f65f89b2-ce50-471f-b948-5869df69c197@paulmck-laptop>
Date: Tue, 18 Mar 2025 10:29:41 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: LKML <linux-kernel@...r.kernel.org>, Boqun Feng <boqun.feng@...il.com>,
	Joel Fernandes <joelagnelf@...dia.com>,
	Neeraj Upadhyay <neeraj.upadhyay@....com>,
	Uladzislau Rezki <urezki@...il.com>,
	Zqiang <qiang.zhang1211@...il.com>, rcu <rcu@...r.kernel.org>
Subject: Re: [PATCH] rcu/nocb: Dump gp state even if rdp gp itself is not
 offloaded

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@...nel.org>
> Signed-off-by: Frederic Weisbecker <frederic@...nel.org>

Much better, thank you!

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