linux-kernel - Re: [PATCH v2 1/1] printk: suppress rcu stall warnings caused by slow console devices

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Date:   Fri, 12 Nov 2021 10:41:55 +0900
From:   Sergey Senozhatsky <senozhatsky@...omium.org>
To:     Wander Lairson Costa <wander@...hat.com>
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        John Ogness <john.ogness@...utronix.de>,
        open list <linux-kernel@...r.kernel.org>,
        "Paul E . McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH v2 1/1] printk: suppress rcu stall warnings caused by
 slow console devices

On (21/11/11 16:59), Wander Lairson Costa wrote:
> 
> If we have a reasonable large dataset to flush in the printk ring
> buffer in the presence of a slow console device (like a serial port
> with a low baud rate configured), the RCU stall detector may report
> warnings.
> 
> This patch suppresses RCU stall warnings while flushing the ring buffer
> to the console.
> 
[..]
> +extern int rcu_cpu_stall_suppress;
> +
> +static void rcu_console_stall_suppress(void)
> +{
> +	if (!rcu_cpu_stall_suppress)
> +		rcu_cpu_stall_suppress = 4;
> +}
> +
> +static void rcu_console_stall_unsuppress(void)
> +{
> +	if (rcu_cpu_stall_suppress == 4)
> +		rcu_cpu_stall_suppress = 0;
> +}
> +
>  /**
>   * console_unlock - unlock the console system
>   *
> @@ -2634,6 +2648,9 @@ void console_unlock(void)
>  	 * and cleared after the "again" goto label.
>  	 */
>  	do_cond_resched = console_may_schedule;
> +
> +	rcu_console_stall_suppress();
> +
>  again:
>  	console_may_schedule = 0;
>  
> @@ -2645,6 +2662,7 @@ void console_unlock(void)
>  	if (!can_use_console()) {
>  		console_locked = 0;
>  		up_console_sem();
> +		rcu_console_stall_unsuppress();
>  		return;
>  	}
>  
> @@ -2716,8 +2734,10 @@ void console_unlock(void)
>  
>  		handover = console_lock_spinning_disable_and_check();
>  		printk_safe_exit_irqrestore(flags);
> -		if (handover)
> +		if (handover) {
> +			rcu_console_stall_unsuppress();
>  			return;
> +		}
>  
>  		if (do_cond_resched)
>  			cond_resched();
> @@ -2738,6 +2758,8 @@ void console_unlock(void)
>  	retry = prb_read_valid(prb, next_seq, NULL);
>  	if (retry && console_trylock())
>  		goto again;
> +
> +	rcu_console_stall_unsuppress();
>  }

May be we can just start touching watchdogs from printing routine?