linux-kernel - Re: [PATCH v4 0/2] Add support to dump printk buffer to console via sysrq

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Date: Wed, 14 Feb 2024 16:00:43 +0530
From: Sreenath Vijayan <sreenath.vijayan@...y.com>
To: Petr Mladek <pmladek@...e.com>
Cc: john.ogness@...utronix.de, corbet@....net, gregkh@...uxfoundation.org,
        jirislaby@...nel.org, rdunlap@...radead.org, rostedt@...dmis.org,
        senozhatsky@...omium.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org,
        taichi.shimoyashiki@...y.com, daniel.palmer@...y.com,
        anandakumar.balasubramaniam@...y.com
Subject: Re: [PATCH v4 0/2] Add support to dump printk buffer to console via
 sysrq

On Wed, Feb 07, 2024 at 03:12:09PM +0100, Petr Mladek wrote:
> Hi,
> 
> first, I am sorry for joining the game so late. I was sick
> and have had a lot of pending tasks after Christmas's holidays
> and the sickness.
> 
> On Thu 2024-02-01 13:12:39, Sreenath Vijayan wrote:
> > Hi,
> > 
> > This patch series enables one to dump the messages in printk ring
> > buffer unless all CPUs are locked up. This is useful to view the
> > kernel messages when terminal is unresponsive to enter commands
> > like dmesg and syslog services are also disabled, especially on
> > embedded targets.
> 
> What is the exact scenario for this feature, please?
> 

Sometimes, we do some testing without connecting display or consoles
and when we connect back to console, we find the system in a hung
state and unable to enter any commands. The system maynot be
completely locked up. In this scenario, we use this feature to view
the printk buffer to get some idea about why the system is hung.

Basically, this can be used in any scenario where you are unable
to enter commands due to some reason, but atleast one cpu is in
a condition to process the unbound workqueue. We have found this
useful during nfs hangs also.

> IMHO, rewinding the entire log on an unresponsive terminal
> has a questionable value. Most messages would scroll down
> quickly and only the last messages would stay visible.
> 
> Also this code would rewind all consoles, including
> (slow) serial ones. I wonder if rewind on these consoles
> would be useful as well.
> 
> That said, I am not completely against this feature.
> I just want to be sure that it does what you expect.
> 
> Best Regards,
> Petr

We mostly use this to display the kernel messages on serial
consoles and have found it to be working fine. In this case,
we use some terminal emulator and can scroll through the output.

Regards,
Sreenath