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

From: Sergey Senozhatsky
Date: Thu Nov 11 2021 - 20:42:24 EST


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?