linux-kernel - Re: [PATCH 1/2] rcu: Delete a redundant check in rcu_check_gp_kthread

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Date:   Mon, 10 Jul 2023 12:03:35 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Zhen Lei <thunder.leizhen@...wei.com>
Cc:     Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Josh Triplett <josh@...htriplett.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Zqiang <qiang.zhang1211@...il.com>, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] rcu: Delete a redundant check in
 rcu_check_gp_kthread_starvation()

On Wed, Jul 05, 2023 at 03:30:19PM +0800, Zhen Lei wrote:
> The above condition "if (gpk)" already ensures that gp_kthread is created,
> so the local variable 'cpu' cannot be negative here.
> 
> Signed-off-by: Zhen Lei <thunder.leizhen@...wei.com>
> ---
>  kernel/rcu/tree_stall.h | 12 +++++-------
>  1 file changed, 5 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
> index b10b8349bb2a48b..dcfaa3d5db2cbc7 100644
> --- a/kernel/rcu/tree_stall.h
> +++ b/kernel/rcu/tree_stall.h
> @@ -537,13 +537,11 @@ static void rcu_check_gp_kthread_starvation(void)
>  			pr_err("\tUnless %s kthread gets sufficient CPU time, OOM is now expected behavior.\n", rcu_state.name);
>  			pr_err("RCU grace-period kthread stack dump:\n");
>  			sched_show_task(gpk);
> -			if (cpu >= 0) {

I am not quite this trusting of the relation between the relationship
between the existence of the grace-period khread and its CPU number
being in range.  Let's please start with something like this:

			if (!WARN_ON_ONCE(cpu < 0)) {

Please note that this is not just me.  See for example the use of the
cpumask_check() function, albeit the opposite concern.

> -				if (cpu_is_offline(cpu)) {
> -					pr_err("RCU GP kthread last ran on offline CPU %d.\n", cpu);
> -				} else  {
> -					pr_err("Stack dump where RCU GP kthread last ran:\n");
> -					dump_cpu_task(cpu);
> -				}
> +			if (cpu_is_offline(cpu)) {
> +				pr_err("RCU GP kthread last ran on offline CPU %d.\n", cpu);
> +			} else  {
> +				pr_err("Stack dump where RCU GP kthread last ran:\n");
> +				dump_cpu_task(cpu);
>  			}
>  			wake_up_process(gpk);
>  		}
> -- 
> 2.25.1
>