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    SubjectRe: [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@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

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