Re: [PREEMPT_RT] 8250 IRQ lockup when flooding serial console (was Re: [ANNOUNCE] v5.4.28-rt19)

[Steven Rostedt]

On Thu, 23 Apr 2020 21:48:20 +0200 (CEST)
Jiri Kosina <jikos@xxxxxxxxxx> wrote:

> On Thu, 23 Apr 2020, Sebastian Andrzej Siewior wrote:
> 
> > The IRQ4 is edge and in charge of ttyS0. It is handled by 
> > handle_edge_irq() and after ->irq_ack(), the thread is woken up and then 
> > we get another ->handle_edge_irq() for IRQ4. With larger PASS_LIMIT the 
> > thread runs longer so note_interrupt() will make less IRQ_HANDLED based 
> > on ->threads_handled_last. If it observes 100 handled within 100000 
> > interrupts then the counters are reset again. On !RT it usually manages 
> > to get >100 per 100000 interrupts so it appears good. On RT it gets less 
> > and the interrupt gets disabled.
> > 
> > So it is not RT related, but RT triggers it more reliably (also the 
> > PASS_LIMIT change can vanish). I can't tell if this is a qemu bug in 
> > emulating the HW or not. I can't reproduce it real HW. I see a second 
> > edge interrupt only after the thread completed. I can't tell if this is 
> > because it is a real UART and the data is flowing slower or because the 
> > edge-IRQ is not triggered repeatedly.  
> 
> Yeah, it's all strange. In the hope of understanding the issue a little 
> bit better, I tried to disable IRQs in serial8250_handle_irq() to mimic 
> what !PREEMPT_RT spinlock would do; the idea was that this is some kind of 
> strange race / memory ordering (missed ack?) between the threaded irq4 
> handler and the do_IRQ() -> handle_edge_irq() -> ... path.
> 
> So I did this:
> 
> ---
>  drivers/tty/serial/8250/8250_port.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
> index e31217e8dce6..1a577305e174 100644
> --- a/drivers/tty/serial/8250/8250_port.c
> +++ b/drivers/tty/serial/8250/8250_port.c
> @@ -1813,12 +1813,13 @@ static bool handle_rx_dma(struct uart_8250_port *up, unsigned int iir)
>  int serial8250_handle_irq(struct uart_port *port, unsigned int iir)
>  {
>  	unsigned char status;
> -	unsigned long flags;
> +	unsigned long flags, f;
>  	struct uart_8250_port *up = up_to_u8250p(port);
>  
>  	if (iir & UART_IIR_NO_INT)
>  		return 0;
>  
> +	local_irq_save(f);
>  	spin_lock_irqsave(&port->lock, flags);

Note, this would break if there ever was a contention, as the spin lock is
a mutex and would sleep. And we don't want to do that with interrupts
disabled!

>  
>  	status = serial_port_in(port, UART_LSR);
> @@ -1833,6 +1834,7 @@ int serial8250_handle_irq(struct uart_port *port, unsigned int iir)
>  		serial8250_tx_chars(up);
>  
>  	uart_unlock_and_check_sysrq(port, flags);
> +	local_irq_restore(f);
>  	return 1;
>  }
> 
> But curiously enough, that exploded in the oposite order (so first there 
> was CPU stall, and only later the disabling of IRQ4 due to spurious 
> storm):


Now what may be interesting to try is to enable tracing and
ftrace_dump_on_opps, and set panic_on_warning, as well as
traceoff_on_warning.


 # echo 1 > /proc/sys/kernel/ftrace_dump_on_oops
 # echo 1 > /proc/sys/kernel/panic_on_warn
 # echo 1 > /proc/sys/kernel/traceoff_on_warning
 # echo 1 > /sys/kernel/tracing/events/enable

Enabling all events will include interrupt events and wake ups, and perhaps
give you an idea what interrupts are happening after the uart thread is
woken.

-- Steve