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

[Paul E. McKenney]

On Thu, Nov 11, 2021 at 04:59:04PM -0300, 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.
> 
> Signed-off-by: Wander Lairson Costa <wander@xxxxxxxxxx>

>From an RCU perspective:

Acked-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

> ---
>  kernel/printk/printk.c | 24 +++++++++++++++++++++++-
>  1 file changed, 23 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index cbc35d586afb..42b622454314 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -2588,6 +2588,20 @@ static inline int can_use_console(void)
>  	return cpu_online(raw_smp_processor_id()) || have_callable_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();
>  }
>  EXPORT_SYMBOL(console_unlock);
>  
> -- 
> 2.27.0
>