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SubjectRe: [PATCH v2 1/1] printk: suppress rcu stall warnings caused by slow console devices
On Fri, Nov 12, 2021 at 11:58 AM Paul E. McKenney <paulmck@kernel.org> wrote:
>
> On Fri, Nov 12, 2021 at 11:42:39AM -0300, Wander Costa wrote:
> > On Thu, Nov 11, 2021 at 10:42 PM Sergey Senozhatsky
> > <senozhatsky@chromium.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
>
> So when we are having problems is when console_may_schedule == 0?
>
Exactly.
> Thanx, Paul
>

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