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 11:42:39 -0300
From:   Wander Costa <wcosta@...hat.com>
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>,
        "Paul E . McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH v2 1/1] printk: suppress rcu stall warnings caused by slow
 console devices

On Thu, Nov 11, 2021 at 10:42 PM Sergey Senozhatsky
<senozhatsky@...omium.org> 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?
>
Hrm, console_unlock is called from vprintk_emit [0] with preemption
disabled. and it already has the logic implemented to call
cond_resched when possible [1].

[0] https://elixir.bootlin.com/linux/latest/source/kernel/printk/printk.c#L2244
[1] https://elixir.bootlin.com/linux/latest/source/kernel/printk/printk.c#L2719