linux-kernel - Re: [PATCH] tty/serial/8250: make touch watchdog settable

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Message-ID: <X/WNeBikJYm8ncj6@jagdpanzerIV.localdomain>
Date:   Wed, 6 Jan 2021 19:14:16 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     "chenzhen (R)" <chenzhen105@...wei.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        gregkh@...uxfoundation.org, jirislaby@...nel.org,
        andriy.shevchenko@...ux.intel.com, lukas@...ner.de,
        Sergey.Semin@...kalelectronics.ru, vigneshr@...com,
        rose.chen@...wei.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tty/serial/8250: make touch watchdog settable

On (21/01/06 17:28), chenzhen (R) wrote:
> On 21/1/6 15:33, Sergey Senozhatsky wrote:
> > On (21/01/06 14:46), chenzhen wrote:
> >> Since 54f19b4a6(tty/serial/8250: Touch NMI watchdog in wait_for_xmitr), serial8250
> >> will always touch watchdog in write and wait_for_xmitr. However, serial8250 may
> >> become low speed thus take a long time to print. In this process, nmi and soft
> >> watchdog on current CPU will be invalid.
> >>
> >> To resolve this problem, add a cmdline option "tty_watchdog_enable" to control
> >> the touchdog in serial8250.
> > Sorry, I don't understand - what does this fix?
> >
> > 	-ss
> > .
> 
> It fixes that if serial8250 is low speed on some machine, when it
> writes for a long time, NMI/softlockup watchdog will never bark and
> potential rlock will not be detected. So an option to control the
> touchdog in serial8250 may help.

This is the intention. Suppose that serial8250 triggers the lockup
watchdog, how are you going to fix it? If you need to write NNNNN
bytes to the slow 8250 port then there is nothing you can do about
it; triggering watchdog from the 8250 write path is certainly not
going to help. If you have an excessive number of printk()-s then
reduce the number of spurious printk()-s (ratelimit, etc).

	-ss