Re: [PATCH printk v2 16/18] printk: Provide threadprintk boot argument

[Petr Mladek <pmladek@suse.com>]

On Tue 2024-06-04 01:30:51, John Ogness wrote:
> For PREEMPT_RT, legacy console printing is performed in a dedicated
> kthread. However, this behavior can also be interesting for other
> preemption models as it minimizes the duration of printk() calls by
> deferring all printing.
> 
> Provide a new boot argument "threadprintk" that will create the
> dedicated kthread for legacy console printing for !PREEMPT_RT
> systems.
> 
> The implementation is the same as the "threadirqs" boot argument.
>
> Users should be aware that if this option is enabled, the shutdown,
> reboot, and panic messages probably will not be visible on the
> legacy consoles.

printk() is _heavily_ limited in this mode. Users would see the
messages only when the system is running well.

I think that this is a _big_ difference against "threadirqs".
It still allows to handle some critical IRQ handlers directly
by using IRQF_NO_THREAD.

> Non-legacy consoles (NBCON) already have their own dedicated kernel
> threads for printing and reliable shutdown, reboot, and panic
> printing. This option really only applies to legacy consoles.

OK, the NBCON consoles might make it a bit more useful. But there
will be only one at the beginning. And it won't work with boot
consoles.

All I want to say is that this mode has big limitations
and kind of weird semantic. This semantic is basically
needed only with PREEMPT_RT.

I would prefer to remove this patch for now. It would give us
more time to think about the threaded and sync modes.

Or I would at least use "force" in the name to make it more clear
that it forces some non-default (non-optimal) behavior.
I would call it "printk_thread_force" or "printk_legacy_thread_force".

Best Regards,
Petr