linux-kernel - Re: [RFC 0/1] serial: 8250: nbcon_atomic_flush_pending() might trigger watchdog warnigns

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Message-ID: <CALqELGycYx7GSVWwoBaWipoF84EG6iYu37F0pWif1MFVEkotaQ@mail.gmail.com>
Date: Mon, 29 Sep 2025 09:50:30 +0100
From: Andrew Murray <amurray@...goodpenguin.co.uk>
To: Petr Mladek <pmladek@...e.com>
Cc: John Ogness <john.ogness@...utronix.de>, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Jiri Slaby <jirislaby@...nel.org>, 
	Sergey Senozhatsky <senozhatsky@...omium.org>, Steven Rostedt <rostedt@...dmis.org>, 
	Thomas Gleixner <tglx@...utronix.de>, Esben Haabendal <esben@...nix.com>, linux-serial@...r.kernel.org, 
	linux-kernel@...r.kernel.org, 
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>, Arnd Bergmann <arnd@...db.de>, 
	Tony Lindgren <tony@...mide.com>, Niklas Schnelle <schnelle@...ux.ibm.com>, 
	Serge Semin <fancer.lancer@...il.com>
Subject: Re: [RFC 0/1] serial: 8250: nbcon_atomic_flush_pending() might
 trigger watchdog warnigns

On Wed, 24 Sept 2025 at 16:32, Petr Mladek <pmladek@...e.com> wrote:
>
> Added Andred Murray into Cc.
>

Thanks for the CC.


>
> Well, releasing the console context ownership after each record
> might solve also some other problems [*]
>
> I am going to try implementing the 3rd solution and see how
> complicated  it would be.
>
> It would be possible to change it two 2nd easily just by
> using a global counter and updating it in emergency_enter/exit API.
>
>
> [*] Andrew Murray is trying to do similar thing with console_lock
>     and the legacy_kthread, see
>     https://lore.kernel.org/r/20250915-printk_legacy_thread_console_lock-v1-0-f34d42a9bcb3@thegoodpenguin.co.uk
>
>     He told me off-list that he saw similar problems also with nbcon_thread.
>     I am not sure but it will likely be related to
>     __nbcon_atomic_flush_pending_con() blocking a nbcon console context
>     for too long.

I replicated my 'slow serial causes delays boot via contention at
filp_open call in console_on_rootfs' test, both with and without the
last patch [1] in your "printk/nbcon: Prevent hardlockup reports
caused by atomic nbcon flush" series. Though I found no obvious
improvement from your patch. I found without the patch the delay at
filp_open (with 8250 nbcon and with PREEMPT_RT) would range from 1.6
second through to 40 seconds, and with the patch from 0.07 through to
26 seconds. Potentially promising, but the numbers are all over the
place, so there must be something else going on. I'll continue to
investigate. In any case, that's not the problem your series was
trying to solve.

[1] https://lore.kernel.org/all/20250926124912.243464-4-pmladek@suse.com/


>
> Best Regards,
> Petr

Thanks,

Andrew Murray