Re: [PATCH RFC 0/2] netconsole: allow replaying the printk buffer on demand

[Petr Mladek <pmladek@suse.com>]

On Mon 2026-10-26 10:49:32, Breno Leitao wrote:
> A bit of context on the use case from our side.
> 
> In an ideal world netconsole is enabled from early boot through the
> kernel command line, but in some case this is impossible
> (linux as bootloaders - ipxe/sd-stubs/etc).
> 
> The other way to setup netconsole is to create a new target at runtime
> through configfs. Netconsole then only sends messages from the time the
> target is enabled.
> 
> For this second case, I would like to replay the whole kernel log on
> netconsole, such as previous messages. Something might have happened
> before the target was enabled, and we want visibility into that.

Just to be sure that I understand it correctly. 

Is the netconsole registered before it gets functional, please?

Is it possible to detect that the netconsole is not yet functional
and when it could start working?

I ask because there is the flag CON_PRINTBUFFER which should cause
re-playing all messages on the console during registration.

A solution might be to delay the console registration until
the netwok target is really working.

Another solution might be to temporary block/disable the console
until it gets operational. We already have console_suspend()/resume()
API. But we might need something new to distinguish it from
the system suspend/resume handling.

> That said, the "per-console counter" might work as well, since it would
> start as 0, thus, we know that netconsole target would be behind, and
> try to replay? (not sure how yet)

This is what the console_try_replay_all() does. The only difference
is that it resets the counter to the oldest available message,
see __console_rewind_all(). It is just a small optimization
when the ring buffer is already full and the oldest messages
are gone.

> That said, one way or another might work for our purposes ?!

Best Regards,
Petr