Re: Serial console is causing system lock-up

[John Ogness <john.ogness@linutronix.de>]

On 2019-03-07, Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> wrote:
>>> Sounds like another aurgment for the new printk design.
>> 
>> Assuming the bad checksum messages are considered an emergency (for
>> example, at least loglevel KERN_WARN), then the new printk design
>> would print those messages synchronously to the slow serial line in
>> the context of the driver as the driver is producing them.
>
> I wonder if we have several CPU doing the checksum verification.
> New sync printk mechanism has a Big-Konsole-spin-Lock, all CPUs that
> want to printk() should line up and wait, in whatever context they
> currently are.
>
>> There wouldn't be a lock-up, but it would definitely slow down the
>> driver. The situation of "messages being produced faster than the
>> kernel can write them" would never exist because the printk() call
>> will only return after the writing is completed.
>
> If the serial console is a really slow one and we have a CPU in atomic
> context spinnig on prb_lock, while the prb_lock is always acquired by
> other CPUs, then it may look like a lock-up scenario.

When the console is constantly printing messages, I wouldn't say that
looks like a lock-up scenario. It looks like the system is busy printing
critical information to the console (which it is).

John Ogness