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

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Date:   Thu, 11 Nov 2021 18:23:36 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Sergey Senozhatsky <senozhatsky@...omium.org>
Cc:     Wander Lairson Costa <wander@...hat.com>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        John Ogness <john.ogness@...utronix.de>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/1] printk: suppress rcu stall warnings caused by
 slow console devices

On Fri, Nov 12, 2021 at 10:41:55AM +0900, Sergey Senozhatsky wrote:
> 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?

You could invoke cond_resched() periodically and keep RCU happy.

But if you also get stall warnings in PREEMPT=y kernels, something
more is required.

							Thanx, Paul