Re: [PATCH] printk: Allow to use the printk kthread immediately even for 1st nbcon

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

On 2025-06-04, Petr Mladek <pmladek@suse.com> wrote:
> The kthreads for nbcon consoles are created by nbcon_alloc() at
> the beginning of the console registration. But it currently works
> only for the 2nd or later nbcon console because the code checks
> @printk_kthreads_running.
>
> The kthread for the 1st registered nbcon console is created at the very
> end of register_console() by printk_kthreads_check_locked(). As a result,
> the entire log is replayed synchronously when the "enabled" message
> gets printed. It might block the boot for a long time with a slow serial
> console.
>
> Prevent the synchronous flush by creating the kthread even for the 1st
> nbcon console when it is safe (kthreads ready and no boot consoles).
>
> Also inform printk() to use the kthread by setting
> @printk_kthreads_running. Note that the kthreads already must be
> running when it is safe and this is not the 1st nbcon console.
>
> Symmetrically, clear @printk_kthreads_running when the last nbcon
> console was unregistered by nbcon_free(). This requires updating
> @have_nbcon_console before nbcon_free() gets called.
>
> Note that there is _no_ problem when the 1st nbcon console replaces boot
> consoles. In this case, the kthread will be started at the end
> of registration after the boot consoles are removed. But the console
> does not reply the entire log buffer in this case. Note that
> the flag CON_PRINTBUFFER is always cleared when the boot consoles are
> removed and vice versa.
>
> Closes: https://lore.kernel.org/r/20250514173514.2117832-1-mcobb@thegoodpenguin.co.uk
> Signed-off-by: Petr Mladek <pmladek@suse.com>

Reviewed-by: John Ogness <john.ogness@linutronix.de>

Thanks Petr! And thank you Michael for looking into this. I think this
will be a big improvement in boot times for a lot of developers.

John